second Kunitz domain of WAP, Kazal, immunoglobulin, Kunitz and NTR domain-containing proteins; This subfamily includes WAP, Kazal, immunoglobulin, Kunitz and NTR domain-containing protein 1 (WFIKKN1, WFKN1), WFIKKN2 (WFKN2), and similar proteins. WFIKKN proteins are protease inhibitors that contain two distinct Kunitz-type protease inhibitor domains. They may have serine protease- and metalloprotease-inhibitor activity. This model represents the second Kunitz (KU2) domain, which has been shown to inhibit trypsin, but not chymotrypsin, elastase, plasmin, pancreatic kallikrein, lung tryptase, plasma kallikrein, thrombin, urokinase or tissue plasminogen activator. However, the inhibition constant of this domain for bovine trypsin is about five orders of magnitudes lower than that of bovine pancreatic trypsin inhibitor (BPTI) for trypsin. This could be due to unfavorable side-chain conformation of a tryptophan at P2' site which is incompatible with a trypsin complex; typical trypsin inhibitors of the Kunitz family feature a tyrosine residue or other less bulky residues at this site. The structure of KU2 is similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|...
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMCP 338
Cdd:cd22606     1 ICSLPAVQGPCKAWEPRWAYNSLLKQCQSFVYGGCEGNENNFESKEACEDACP 53

NTR_like domain; a beta barrel with an oligosaccharide/oligonucleotide-binding fold found in netrins, complement proteins, tissue inhibitors of metalloproteases (TIMP), and procollagen C-proteinase enhancers (PCOLCE), amongst others. In netrins, the domain plays a role in controlling axon branching in neural development, while the common function of these modules in TIMPs appears to be binding to metzincins. A subset of this family is also known as the C345C domain because it occurs as a C-terminal domain in complement C3, C4 and C5. In C5, the domain interacts with various partners during the formation of the membrane attack complex.

                          10        20        30
                  ....*....|....*....|....*....|....*
gi 1304835748 355 VTSFCKSDFAILGRIIELTEDQDSGLALVSVEDIL 389
Cdd:cd03575     1 VPSLCRSDFAIVGRLTELTEELDSGHARVTLEEVL 35

second Kunitz domain of WAP, Kazal, immunoglobulin, Kunitz and NTR domain-containing proteins; This subfamily includes WAP, Kazal, immunoglobulin, Kunitz and NTR domain-containing protein 1 (WFIKKN1, WFKN1), WFIKKN2 (WFKN2), and similar proteins. WFIKKN proteins are protease inhibitors that contain two distinct Kunitz-type protease inhibitor domains. They may have serine protease- and metalloprotease-inhibitor activity. This model represents the second Kunitz (KU2) domain, which has been shown to inhibit trypsin, but not chymotrypsin, elastase, plasmin, pancreatic kallikrein, lung tryptase, plasma kallikrein, thrombin, urokinase or tissue plasminogen activator. However, the inhibition constant of this domain for bovine trypsin is about five orders of magnitudes lower than that of bovine pancreatic trypsin inhibitor (BPTI) for trypsin. This could be due to unfavorable side-chain conformation of a tryptophan at P2' site which is incompatible with a trypsin complex; typical trypsin inhibitors of the Kunitz family feature a tyrosine residue or other less bulky residues at this site. The structure of KU2 is similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|...
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMCP 338
Cdd:cd22606     1 ICSLPAVQGPCKAWEPRWAYNSLLKQCQSFVYGGCEGNENNFESKEACEDACP 53

Kunitz/Bovine pancreatic trypsin inhibitor domain; Indicative of a protease inhibitor, usually a serine protease inhibitor. Structure is a disulfide rich alpha+beta fold. BPTI (bovine pancreatic trypsin inhibitor) is an extensively studied model structure. Certain family members are similar to the tick anticoagulant peptide (TAP). This is a highly selective inhibitor of factor Xa in the blood coagulation pathways. TAP molecules are highly dipolar, and are arranged to form a twisted two- stranded antiparallel beta-sheet followed by an alpha helix.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:pfam00014   2 CSLPPDSGPCKASIPRWYYNPTTGTCEPFTYGGCGGNANNFESLEECESTC 52

Immunoglobulin I-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748 115 PSLVNHPAHKTVSVGDTVSFHCDVTGKPKPEITWEKevegeDNVVMRPNHVVANLVVTNIAQLVIYNAQIQDAGIYTCTA 194
Cdd:pfam07679   1 PKFTQKPKDVEVQEGESARFTCTVTGTPDPEVSWFK-----DGQPLRSSDRFKVTYEGGTYTLTISNVQPDDSGKYTCVA 75

                          90
                  ....*....|....*
gi 1304835748 195 KNAGGLIKVNFPLSV 209
Cdd:pfam07679  76 TNSAGEAEASAELTV 90

NTR domain, WFIKKN subfamily; WFIKKN proteins contain a C-terminal NTR domain and are putative secreted proteins which may be multivalent protease inhibitors that act on serine proteases as well as metalloproteases. Human WFIKKN and a related protein sharing the same domain architecture were observed to have distinct tissue expression patterns. WFIKKN is also referred to as growth and differentiation factor-associated serum protein-1 (GASP-1). It inhibits the activity of mature myostatin, a specific regulator of skeletal muscle mass and a member of the TGFbeta superfamily.

                          10        20        30
                  ....*....|....*....|....*....|....*
gi 1304835748 355 VTSFCKSDFAILGRIIELTEDQDSGLALVSVEDIL 389
Cdd:cd03575     1 VPSLCRSDFAIVGRLTELTEELDSGHARVTLEEVL 35

Kunitz/Bovine pancreatic trypsin inhibitor domain; Indicative of a protease inhibitor, usually a serine protease inhibitor. Structure is a disulfide rich alpha+beta fold. BPTI (bovine pancreatic trypsin inhibitor) is an extensively studied model structure. Certain family members are similar to the tick anticoagulant peptide (TAP). This is a highly selective inhibitor of factor Xa in the blood coagulation pathways. TAP molecules are highly dipolar, and are arranged to form a twisted two- stranded antiparallel beta-sheet followed by an alpha helix.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|..
gi 1304835748 229 CLKPPVSDDCGEEQTRWYFDAKKNNCFIFVYGNCNINLNHFDTYESCMTTCM 280
Cdd:pfam00014   2 CSLPPDSGPCKASIPRWYYNPTTGTCEPFTYGGCGGNANNFESLEECESTCR 53

Kazal type serine protease inhibitors; Kazal type serine protease inhibitors and follistatin-like domains.

                           10        20        30        40
                   ....*....|....*....|....*....|....*....|
gi 1304835748   39 CRDRCEKEPSFTCASDGLTYYNKCFMDAEACTKAISLTVV 78
Cdd:smart00280   2 CPEACPREYDPVCGSDGVTYSNECHLCKAACESGKSIEVK 41

second Kunitz domain of WAP, Kazal, immunoglobulin, Kunitz and NTR domain-containing proteins; This subfamily includes WAP, Kazal, immunoglobulin, Kunitz and NTR domain-containing protein 1 (WFIKKN1, WFKN1), WFIKKN2 (WFKN2), and similar proteins. WFIKKN proteins are protease inhibitors that contain two distinct Kunitz-type protease inhibitor domains. They may have serine protease- and metalloprotease-inhibitor activity. This model represents the second Kunitz (KU2) domain, which has been shown to inhibit trypsin, but not chymotrypsin, elastase, plasmin, pancreatic kallikrein, lung tryptase, plasma kallikrein, thrombin, urokinase or tissue plasminogen activator. However, the inhibition constant of this domain for bovine trypsin is about five orders of magnitudes lower than that of bovine pancreatic trypsin inhibitor (BPTI) for trypsin. This could be due to unfavorable side-chain conformation of a tryptophan at P2' site which is incompatible with a trypsin complex; typical trypsin inhibitors of the Kunitz family feature a tyrosine residue or other less bulky residues at this site. The structure of KU2 is similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|...
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMCP 338
Cdd:cd22606     1 ICSLPAVQGPCKAWEPRWAYNSLLKQCQSFVYGGCEGNENNFESKEACEDACP 53

Kunitz/Bovine pancreatic trypsin inhibitor (BPTI) domain; This family contains the Kunitz domain which is a common structural fold found in a family of reversible serine protease inhibitors. This domain is thought to have evolved over 500 million years and is ubiquitous in all kingdoms of life and has been incorporated into many different genes. In general, each domain is encoded by a single exon. Some genes encode proteins with a single Kunitz domain, e.g. bovine pancreatic trypsin inhibitor (BPTI), trophoblast Kunitz domain protein (TKDP), amyloid beta-protein precursor (ABPP), as well as Kunitz-type venom peptides such as dendrotoxin. Genes that encode multiple Kunitz domains include hepatocyte growth factor activator inhibitors HAI1 and HAI2 (two domains), tissue factor pathway inhibitor TFPI1 and TFPI2 (three domains) and Caenorhabditis elegans papilin (eleven domains). In addition, the Kunitz domain has been integrated into multi-domain proteins, e.g. the collagen alpha3(VI), alpha1(VII) and alpha1(XXVIII) chains, WFIKKN1 (containing WAP, Follistatin/Kazal, Immunoglobulin, two Kunitz and NTR domains) and papilin. Furthermore, each domain within a multi-Kunitz domain protein may exhibit different protease activity, such as for the three tandemly repeated domains within both tissue factor pathway inhibitors 1 and 2. The Kunitz domain is a representative of alpha/beta proteins with irregular secondary structure stabilized by three disulfide bonds and presenting three peptide loops that can be varied without introducing much destabilization to the scaffold. Protease inhibitors meet the scaffold criteria in that they are small, stable and capable of evolving the binding activity of exposed peptide loops through targeted randomization to construct combinatorial libraries. Kunitz domain-based scaffolds have been successfully utilized to construct and select a library of protease inhibitors with the potential for therapeutic application.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd00109     1 CLLPPDPGPCRAYFPRWYYNSETGQCEEFIYGGCGGNANNFETKEECEATC 51

BPTI/Kunitz family of serine protease inhibitors; Serine protease inhibitors. One member of the family is encoded by an alternatively-spliced form of Alzheimer's amyloid beta-protein.

                           10        20        30        40        50
                   ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748  287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:smart00131   3 CLLPPDTGPCGGSIPRYYYDPETGTCEPFTYGGCGGNANNFESLEECERTC 53

Immunoglobulin domain; This family contains immunoglobulin-like domains.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748 114 PPSLVNHPAHKTVSVGDTVSFHCDVTGKPKPEITWEKevegeDNVVMRPNHVVANLVVTNIAQLVIYNAQIQDAGIYTCT 193
Cdd:pfam13927   1 KPVITVSPSSVTVREGETVTLTCEATGSPPPTITWYK-----NGEPISSGSTRSRSLSGSNSTLTISNVTRSDAGTYTCV 75

                  ...
gi 1304835748 194 AKN 196
Cdd:pfam13927  76 ASN 78

Kunitz domain 2 of hepatocyte growth factor activator inhibitor-1 (HAI1); This model includes Kunitz domain 2 (KD2) of hepatocyte growth factor activator inhibitor type 1 (HAI-1 or HAI1, also known as Kunitz-type protease inhibitor 1), a membrane-bound multidomain protein essential to the integrity of the basement membrane during placental development. HAI-1 contains an extracellular region and several internal domains that include two Kunitz domains separated in sequence but spatially closed to each other, and their interdomain interactions have evolved to stimulate the inhibitory activity of an integrated Kunitz. While the Kunitz domain 1 (KD1) is the major inhibitory domain of HAI-1 and involved in auto-inhibition of the extracellular region via steric blockage of its active site in the HAI-1 compact tertiary structure, studies show that deletion of HAI-1 Kunitz domain 2 (KD2) and the extracellular region enhanced inhibition of matriptase. HAI-1 KD2 has been shown to have potent inhibitory activity against trypsin, but it cannot inhibit hepatocyte growth factor activator (HGFA), and matriptase. HAI-1 is also important in maintaining postnatal homeostasis in many tissues, including keratinization of the epidermis, hair development, colonic epithelium integrity, proliferation and cell fate of neural progenitor cells, and tissue injury and repair. The interaction between HAI-1 and matriptase is critical for tissue morphogenesis and cellular biology. HAI-1:matriptase ratio imbalance results in tumorigenesis; slight overexpression of matriptase relative to HAI-1 causes spontaneous squamous cell carcinoma, a phenotype that can be effectively reversed back to wild type by additional expression of HAI-1, indicating the need for a tight functional relationship between the two to maintain homeostasis. The structure of KD2 is similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22624     2 CTEPPVTGPCRASFTRWYYDPLSRKCHRFTYGGCDGNENNFETEDECMETC 52

Pseudocerastes persicus trypsin inhibitor (PPTI), Kunitz-type serine protease inhibitor bitisilin, and similar proteins; This group contains Pseudocerastes persicus trypsin inhibitor (PPTI), Bitis gabonica Kunitz-type serine protease inhibitor bitisilin-1 (BG-11), -2 (BG-15) and -3 (two-Kunitz protease inhibitor), Oxyuranus scutellatus scutellatus taicatoxin, and serine protease inhibitor component (TSPI, also called venom protease inhibitor 1 or venom protease inhibitor 2), among others. PPTI from P. persicus venom shows inhibitory effect against trypsin proteolytic activity and has similarities to dendrotoxins (DTXs), with corresponding functionally important residues. Studies have shown the ability of PPTI to inhibit voltage-gated potassium channels, and consequently have dual functionality. Bitilisins 1, 2, and 3 are serine protease inhibitors expressed in snake venom glands; bitsilin-3 consists of two Kunitz protease inhibitor domains. Taicatoxin inhibits trypsin, tissue kallikrein, elastase, plasmin and factor Xa, and is also known to block the voltage-dependent L-type calcium channels from the heart, and the small conductance calcium-activated potassium channels (KCa) in chromaffin cells and in the brain. The structures of these Kunitz-type proteins are similar to other Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22608     4 CYLPADPGPCKAYIPRFYYNSASNKCQQFIYGGCKGNANNFETKDECRYTC 54

Kunitz-type serine protease inhibitor 6 (BmTI-6), A (BmTI-A), and similar proteins; This group includes Kunitz-type serine protease inhibitors 6 (BmTI-6) and A (BmTI-A), both of which inhibit bovine trypsin, bovine chymotrypsin, human plasmin, human plasma kallikrein and human neutrophil elastase, but not bovine thrombin, human factor Xa or porcine pancreatic kallikrein. They may play a role in blocking blood coagulation during the larvae fixation on cattle. This subfamily also includes Rhipicephalus microplus protease inhibitor carrapatin. These proteins are similar to Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|....
gi 1304835748 284 VTLCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22604     3 EKQCSPTADSGPCFAYFPMWWYNVKTGQCEEFIYGGCQGNDNRYETEEECEKTC 56

Kunitz domain of epididymal protease inhibitor eppin and similar proteins; This subfamily includes the Kunitz inhibitor domain protein eppin (also called Cancer/testis antigen 71 or CT71, epididymal protease inhibitor, protease inhibitor WAP7, serine protease inhibitor-like with Kunitz and WAP domains 1, or WAP four-disulfide core domain protein 7) as well as WAP four-disulfide core domain proteins 6A and 6B in mice, and similar proteins. Eppin is a serine protease inhibitor that plays an essential role in male reproduction and fertility. It modulates the hydrolysis of seminal fluid protein semenogelin 1 (SEMG1) by the serine protease kallikrein-related peptidase 3 (KLK3, PSA), provides antimicrobial protection for spermatozoa in the ejaculate coagulum, and binds SEMG1, thereby inhibiting sperm motility. Thus, eppin could potentially be used as a target for male contraception. These domains are similar to Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|...
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMCP 338
Cdd:cd22611     2 VCSLPKESGPCMAYFPRWWYDKETNTCSKFIYGGCQGNNNNFQSEAICQNICK 54

Kunitz domain 1 (KD1) of tissue factor pathway inhibitor 2 (TFPI2) and similar proteins; This model represents the Kunitz-type domain 1 (KD1) of tissue factor pathway inhibitor 2 (TFPI2 or TFPI-2) and similar proteins. TFPI2 exhibits inhibitory activity primarily toward trypsin, plasmin, and factor VIIa (FVIIa)/tissue factor (TF) via its KD1. It is believed to be the major inhibitor of plasmin in the extracellular matrix (ECM) but has little inhibitory activity toward urokinase-type plasminogen activator, tissue-type plasminogen activator, or thrombin. TFPI2 specifically inhibits the proteases via the P1 arginine residue in KD1. The TFPI2 domains KD2 and KD3 appear to have no discernible inhibitory activity and may serve to bind to nearby proteins to localize TFPI2 in the ECM. Structure studies of KD1 complexed with proteases may help in the development of specific and potent KD1 domain protein that may have a large pharmacologic impact in preventing tumor metastasis, retinal degeneration, and degradation of collagen in the ECM. The structure of this domain is similar to that of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22616     5 CLLPPDEGPCRALIPRYYYDRYTQTCREFSYGGCEGNANNFESLEDCEKTC 55

Immunoglobulin like; IG domains that cannot be classified into one of IGv1, IGc1, IGc2, IG.

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748  121 PAHKTVSVGDTVSFHCDVTGKPKPEITWEKE----VEGEDNVVMRPNHvvanlvvtNIAQLVIYNAQIQDAGIYTCTAKN 196
Cdd:smart00410   1 PPSVTVKEGESVTLSCEASGSPPPEVTWYKQggklLAESGRFSVSRSG--------STSTLTISNVTPEDSGTYTCAATN 72

                           90
                   ....*....|...
gi 1304835748  197 AGGLIKVNFPLSV 209
Cdd:smart00410  73 SSGSASSGTTLTV 85

Kunitz-type domain 2 (KD2) of hepatocyte growth factor activator inhibitor type 2 (HAI-2), and similar proteins; This model includes Kunitz domain 2 (KD2) of hepatocyte growth factor activator inhibitor type 2 (HAI-2 or HAI2, also known as placental bikunin or Kunitz-type protease inhibitor 2). HAI-2 is composed of two Kunitz domains that strongly inhibit many serine proteases with sub-nanomolar affinities. It has been found to be a natural tumor suppressor in renal cell carcinoma, breast cancer and prostate cancer, the loss of which leads to tumor growth and progression attributable at least in part to increased MET signaling. HAI-2 is a specific substrate of mesotrypsin which is up-regulated with progression in prostate cancers and shown to contribute to invasion and metastasis; these activities of mesotrypsin may in part be mediated through cleavage and inactivation of HAI-2, resulting in increases in hetatocyte growth factor/scatter factor (HGF/SF) activation and MET signaling. HAI-2 is a physiological inhibitor of hepsin and matriptase, two type II transmembrane serine proteases that, like HGF activator, can convert latent pro-HGF/SF into the two-chain active signaling heterodimer. KD2 is similar to KD1, whose structure is similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|..
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22622     2 YCAAPRVTGPCRAAFPRWYYDPESQSCKEFIYGGCRGNKNNYLSEEECMDRC 53

Kunitz-type actitoxins such as Anemonia viridis U-actitoxin-Avd3l, and similar proteins; This model includes the Kunitz-type actitoxins such as Anemonia viridis U-actitoxin-Avd3l (also called U-AITX-Avd3l or AsKC9), Anthopleura elegantissima KappaPI-actitoxin-Ael3a (also called KappaPI-AITX-Ael3a or Kunitz-type serine protease inhibitor APEKTx1) and Anthopleura aff. xanthogrammica PI-actitoxin-Axm2b (also called PI-AITX-Axm2b or Kunitz-type proteinase inhibitor AXPI-II). U-AITX-Avd3l and KappaPI-AITX-Ael3a are dual-function toxins that inhibit both the serine protease trypsin and voltage-gated potassium channels Kv1.2/KCNA2. These proteins are similar to Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|..
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22633     4 ICLLPKDVGGCRARFPRYYYNSSTRRCEKFRYGGCGGNANNFHTLEECEKVC 55

Immunoglobulin (Ig)-like ectodomain of the LRIG1 (Leucine-rich Repeats And Immunoglobulin-like Domains Protein 1) and similar proteins; member of the I-set of IgSF domains; The members here are composed of subgroup of the immunoglobulin (Ig) domain found in the Ig superfamily. The Ig superfamily is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. A predominant feature of most Ig domains is a disulfide bridge connecting the two beta-sheets with a tryptophan residue packed against the disulfide bond. The ectodomain of LRIG1 has two distinct regions: the proposed 15 LRRs and three Ig-like domains closer to the membrane. LRIG1 has been reported to interact with many receptor tyrosine kinases, GDNF/c-Ret, E-cadherin, JAK/STAT, c-Met, and the EGFR family signaling systems. Immunoglobulin Superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. The structure of the LRIG1 extracellular Ig domain lacks a C" strand and thus is better described as a member of the I-set of IgSF domains.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748 116 SLVNHPAHKTVSVGDTVSFHCDVTGKPKPEITWEKEvEGEDNVVMRPNHVvanLVVTNIAQLVIYNAQIQDAGIYTCTAK 195
Cdd:cd05763     1 SFTKTPHDITIRAGSTARLECAATGHPTPQIAWQKD-GGTDFPAARERRM---HVMPEDDVFFIVDVKIEDTGVYSCTAQ 76

                          90
                  ....*....|....
gi 1304835748 196 NAGGLIKVNFPLSV 209
Cdd:cd05763    77 NSAGSISANATLTV 90

chymotrypsin inhibitor SCI-I_III-like; This model includes the Kunitz-type chymotrypsin inhibitors SCI-III and SCI-I, and similar proteins in insects. SCI-III and SCI-I inhibit chymotrypsin, avoiding the accidental chymotrypsin-mediated activation of prophenoloxidase. This enzyme is required by the insect immune system to produce melanin which is used to engulf foreign objects. This subfamily also includes Kunitz-type male accessory gland peptide with protease inhibitory activity, synthesized and secreted by male accessory glands of Drosophila funebris; it may play a role as an acrosin inhibitor involved in reproduction. These proteins are similar to Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40
                  ....*....|....*....|....*....|....*....|....
gi 1304835748 294 GPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22634    14 ISCFAYIPSWSYNPDKNECEEFIYGGCGGNDNRFSTKAECEQKC 57

Kunitz protease inhibitor (KPI) domain 3 (KPI-3 or K3) of tissue factor pathway inhibitor (TFPI) and TFPI2, and similar proteins; This model represents the third Kunitz-type domain (K3 or KPI-3) of tissue factor pathway inhibitor (TFPI or TFPI1), also known as extrinsic pathway inhibitor (EPI) or lipoprotein-associated coagulation inhibitor (LACI), and of TFPI2 (or TFPI-2). TFPI1 down-regulates the extrinsic coagulation pathway via inhibition of activated factor X (FXa or Xa) and FVIIa (VIIa). It inhibits activated FXa via a "slow-tight binding mechanism", i.e. rapid formation of a loose FXa-TFPI1 complex that then slowly isomerizes to a tight FXa-TFPI1* complex. Subsequent inhibition of FVIIa is facilitated by the presence of tissue factor (TF) and FXa, which together rapidly and efficiently form a quaternary FXa-TFPI1-TF-FVIIa complex in which the activity of FXa and FVIIa are inhibited. TFPI1 consists of 3 Kunitz-type protease inhibitor (KPI) domains in a tandem arrangement; while the K1 domain of TFPI has been shown to bind and inhibit FVIIa and the K2 domain similarly inhibits FXa, the K3 domain has no known inhibitory function. However, Protein S, which functions as a cofactor for TFPI to efficiently enhance TFPI inhibition of FXa and FXa activated TF-VIIa, is dependent on direct interactions with two important residues within K3, a Glutamate and an Arginine. This model also includes TFPI2 Kunitz domain 3 (KD3). TFPI2 exhibits inhibitory activity primarily toward trypsin, plasmin, and factor VIIa (FVIIa)/tissue factor (TF) via its KD1. It is believed to be the major inhibitor of plasmin in the extracellular matrix (ECM) but has little inhibitory activity toward urokinase-type plasminogen activator, tissue-type plasminogen activator, or thrombin. While TFPI2 specifically inhibits the proteases via the P1 arginine residue in KD1, domains KD2 and KD3 appear to have no discernible inhibitory activity and may serve to bind to nearby proteins to localize TFPI2 in the ECM. The structure of this domain is similar to that of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|..
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22615     3 FCLSPKDEGLCSASVTRYYYNSATKTCEPFNYTGCGGNNNNFTSKKDCLRVC 54

venom Kunitz-type proteins such as textilinin, BF9 and PILP; This group includes toxins isolated from snake venoms, such as textilinin, vestiginin, spermatin, mulgin, venom basic protease inhibitor IX (BF9), and protease inhibitor-like protein (PILP), among others. Pseudonaja textilis textilinin-1 is a Kunitz-type serine protease inhibitor that binds to and blocks the activity of a range of serine proteases, including plasmin and trypsin. Ability of testilinin to inhibit plasmin, a protease involved in fibrinolysis, raises the possibility that it may be used as an alternative to aprotinin (Trasylol), which is a systemic antibleeding agent in surgery. Also included is the Bungarus fasciatus fraction IX (BF9), a chymotrypsin inhibitor that binds chymotrypsin but not trypsin. Protease inhibitor-like proteins PILP-1 and PILP-2 show weak binding and inhibition of matrix metalloproteinase-2 (MMP-2) and show an activity in inhibiting migration and invasion of neuroblastoma; they do not inhibit chymotrypsin or trypsin. The structures of these toxins are similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|..
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22594     4 FCELPADPGPCNAYKPAFYYNPASHKCLEFIYGGCGGNANNFKTIDECHRTC 55

first Kunitz domain of Carboxypeptidase Inhibitor SmCI and similar domains; This group includes Sabellastarte magnifica carboxypeptidase inhibitor (SmCI), a tri-domain BPTI-Kunitz inhibitor capable of inhibiting serine proteases and A-like metallocarboxypeptidases. While the BPTI-Kunitz family of proteins includes voltage gated channel blockers and inhibitors of serine proteases, SmCI is the only BPTI-Kunitz protein capable of inhibiting metallocarboxypeptidases. Binding studies show that SmCI is able to bind three trypsin molecules under saturating conditions, but only one elastase interacts with the inhibitor. Additionally, SmCI can bind serine proteases and carboxypeptidases at the same time (at least in the ratio 1:1:1), thus becoming the first protease inhibitor that simultaneously blocks these two mechanistic classes of enzymes. This model contains the first Kunitz domain of SmCI, which has a structure similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|..
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22601     3 VCDLPADRGPCTAYIPRWFYNKTTKKCEKFVYGGCQGNKNRFETKDDCLANC 54

Third immunoglobulin (Ig)-like domain of fibroblast growth factor receptor (FGFR); member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the third immunoglobulin (Ig)-like domain of fibroblast growth factor receptor (FGFR). Fibroblast growth factors (FGFs) participate in morphogenesis, development, angiogenesis, and wound healing. These FGF-stimulated processes are mediated by four FGFR tyrosine kinases (FGRF1-4). FGFRs are comprised of an extracellular portion consisting of three Ig-like domains, a transmembrane helix, and a cytoplasmic portion having protein tyrosine kinase activity. The highly conserved Ig-like domains 2 and 3, and the linker region between D2 and D3 define a general binding site for FGFs. This group belongs to the I-set of IgSF domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand, but lack a C" strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748 115 PSLV-NHPAHKTVSVGDTVSFHCDVTGKPKPEITWEKEVE------GEDNVV-MRPNHVVANLVVTNIAQLVIYNAQIQD 186
Cdd:cd04974     1 PILQaGLPANQTVVLGSDVEFHCKVYSDAQPHIQWLKHVEvngskyGPDGLPyVTVLKVAGVNTTGEENTLTISNVTFDD 80

                          90       100
                  ....*....|....*....|..
gi 1304835748 187 AGIYTCTAKNAGGLIKVNFPLS 208
Cdd:cd04974    81 AGEYICLAGNSIGLSFHSAWLT 102

Kunitz-type domain from the alpha6 chain of human type VI collagen, and similar proteins; This model includes the Kunitz-type domain from the alpha6 chain of type VI collagen (collagen alpha 6(VI) chain), encoded by COL6A6 gene, and similar proteins. Collagen VI is a widely expressed member of the triple helix-containing protein superfamily of collagens and forms beaded microfibrils that anchor large interstitial structures. Immediately after fibril formation, the Kunitz domain can be cleaved off. This domain is similar to that of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22630     3 CSLDQDEGECQNYVLKWYYDQEQKECSQFWYGGCGGNKNRFETQEECEALC 53

Kunitz/Bovine pancreatic trypsin inhibitor (BPTI) domain; This family contains the Kunitz domain which is a common structural fold found in a family of reversible serine protease inhibitors. This domain is thought to have evolved over 500 million years and is ubiquitous in all kingdoms of life and has been incorporated into many different genes. In general, each domain is encoded by a single exon. Some genes encode proteins with a single Kunitz domain, e.g. bovine pancreatic trypsin inhibitor (BPTI), trophoblast Kunitz domain protein (TKDP), amyloid beta-protein precursor (ABPP), as well as Kunitz-type venom peptides such as dendrotoxin. Genes that encode multiple Kunitz domains include hepatocyte growth factor activator inhibitors HAI1 and HAI2 (two domains), tissue factor pathway inhibitor TFPI1 and TFPI2 (three domains) and Caenorhabditis elegans papilin (eleven domains). In addition, the Kunitz domain has been integrated into multi-domain proteins, e.g. the collagen alpha3(VI), alpha1(VII) and alpha1(XXVIII) chains, WFIKKN1 (containing WAP, Follistatin/Kazal, Immunoglobulin, two Kunitz and NTR domains) and papilin. Furthermore, each domain within a multi-Kunitz domain protein may exhibit different protease activity, such as for the three tandemly repeated domains within both tissue factor pathway inhibitors 1 and 2. The Kunitz domain is a representative of alpha/beta proteins with irregular secondary structure stabilized by three disulfide bonds and presenting three peptide loops that can be varied without introducing much destabilization to the scaffold. Protease inhibitors meet the scaffold criteria in that they are small, stable and capable of evolving the binding activity of exposed peptide loops through targeted randomization to construct combinatorial libraries. Kunitz domain-based scaffolds have been successfully utilized to construct and select a library of protease inhibitors with the potential for therapeutic application.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 229 CLKPPVSDDCGEEQTRWYFDAKKNNCFIFVYGNCNINLNHFDTYESCMTTC 279
Cdd:cd00109     1 CLLPPDPGPCRAYFPRWYYNSETGQCEEFIYGGCGGNANNFETKEECEATC 51

Immunoglobulin domain; The members here are composed of the immunoglobulin (Ig) domain found in the Ig superfamily. The Ig superfamily is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. Members of this group are components of immunoglobulin, neuroglia, cell surface glycoproteins, including T-cell receptors, CD2, CD4, CD8, and membrane glycoproteins, including butyrophilin and chondroitin sulfate proteoglycan core protein. A predominant feature of most Ig domains is a disulfide bridge connecting the two beta-sheets with a tryptophan residue packed against the disulfide bond. Ig superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Typically, the V-set domains have A, B, E, and D strands in one sheet and A', G, F, C, C' and C" in the other. The structures in C1-set are smaller than those in the V-set; they have one beta sheet that is formed by strands A, B, E, and D and the other by strands G, F, C, and C'. Moreover, a C1-set Ig domain contains a short C' strand (three residues) and lacks A' and C" strand. Unlike other Ig domain sets, C2-set structures do not have a D strand. Like the V-set Ig domains, members of the I-set have a discontinuous A strand, but lack a C" strand.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 1304835748 132 VSFHCDVTGKPKPEITWEKevegeDNVVMRPNHVVANLVVTNIAQLVIYNAQIQDAGIYTCTAKNAGG 199
Cdd:cd00096     1 VTLTCSASGNPPPTITWYK-----NGKPLPPSSRDSRRSELGNGTLTISNVTLEDSGTYTCVASNSAG 63

Kunitz protease inhibitor (KPI) domain 2 (KPI-2 or K2) of tissue factor pathway inhibitor (TFPI); This model represents the second Kunitz-type domain (K2 or KPI-2) of tissue factor pathway inhibitor (TFPI or TFPI1), also known as extrinsic pathway inhibitor (EPI) or lipoprotein-associated coagulation inhibitor (LACI). TFPI down-regulates the extrinsic coagulation pathway via inhibition of activated factor X (FXa or Xa) and FVIIa (VIIa). It inhibits activated FXa via a "slow-tight binding mechanism", i.e. rapid formation of a loose FXa-TFPI complex that then slowly isomerizes to a tight FXa-TFPI* complex. Subsequent inhibition of FVIIa is facilitated by the presence of tissue factor (TF) and FXa, which together rapidly and efficiently form a quaternary FXa-TFPI-TF-FVIIa complex in which the activity of FXa and FVIIa are inhibited. TFPI consists of 3 Kunitz-type protease inhibitor (KPI) domains in a tandem arrangement; the K2 domain is exposed on functionally active TFPI pools in circulation in blood, in platelets, and attached to the endothelium. While the K1 (or KPI-1) domain of TFPI has been shown to bind and inhibit FVIIa, the K2 domain inhibits FXa by binding directly to the active site and forming a FXa:TFPI complex. A close interaction between the TFPI K2 domain and the FXa active site is essential for the FXa inhibitory action of TFPI and for the formation of an inactive TF/FVIIa/FXa/TFPI complex which then prevents FXa generation. Thus, blockage of K2 would prevent TFPI binding to both FXa and FVIIa/TF, and fully abolish TFPI inhibition of the coagulation cascade. The structure of the K2 domain is similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40
                  ....*....|....*....|....*....|....*....|....
gi 1304835748 294 GPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22614    12 GICRGLITRYFYNNQSKQCERFKYGGCLGNQNNFESLEECQNTC 55

trophoblast Kunitz domain protein (TKDP) and similar proteins; This model contains the trophoblast Kunitz domain protein 1 (TKDP-1) and splice variant TKDP-4, among others, which are Kunitz inhibitor domain proteins. TKDP-1 is expressed in the trophectoderm which forms the outer epithelial layer of the trophoblast, and may play a role in mediating maternal-conceptus interactions in the immediate preimplantation period. However, it does not appear to have proteinase inhibitory activity. These domains are similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|..
gi 1304835748 286 LCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22609     1 FCLEPKVVGVCKASMTRYFYNAQTGHCEQFVYGGCGGNRNNFLTLEDCMKTC 52

Kunitz-type domain from the alpha6 chain of fish type VI collagen, and similar proteins; This model includes the Kunitz-type domain from the alpha6 chain of type VI collagen (collagen alpha 6(VI) chain) and similar proteins. Collagen VI is a widely expressed member of the triple helix-containing protein superfamily of collagens and forms beaded microfibrils that anchor large interstitial structures. Immediately after fibril formation, the Kunitz domain can be cleaved off. This domain is similar to that of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22631     1 CLLGQDAGSCQNYTMMWFFDSKQGRCSRFWYGGCGGNANRFETQEECENLC 51

Kunitz domain 2 of hepatocyte growth factor activator inhibitor-1 (HAI1); This model includes Kunitz domain 2 (KD2) of hepatocyte growth factor activator inhibitor type 1 (HAI-1 or HAI1, also known as Kunitz-type protease inhibitor 1), a membrane-bound multidomain protein essential to the integrity of the basement membrane during placental development. HAI-1 contains an extracellular region and several internal domains that include two Kunitz domains separated in sequence but spatially closed to each other, and their interdomain interactions have evolved to stimulate the inhibitory activity of an integrated Kunitz. While the Kunitz domain 1 (KD1) is the major inhibitory domain of HAI-1 and involved in auto-inhibition of the extracellular region via steric blockage of its active site in the HAI-1 compact tertiary structure, studies show that deletion of HAI-1 Kunitz domain 2 (KD2) and the extracellular region enhanced inhibition of matriptase. HAI-1 KD2 has been shown to have potent inhibitory activity against trypsin, but it cannot inhibit hepatocyte growth factor activator (HGFA), and matriptase. HAI-1 is also important in maintaining postnatal homeostasis in many tissues, including keratinization of the epidermis, hair development, colonic epithelium integrity, proliferation and cell fate of neural progenitor cells, and tissue injury and repair. The interaction between HAI-1 and matriptase is critical for tissue morphogenesis and cellular biology. HAI-1:matriptase ratio imbalance results in tumorigenesis; slight overexpression of matriptase relative to HAI-1 causes spontaneous squamous cell carcinoma, a phenotype that can be effectively reversed back to wild type by additional expression of HAI-1, indicating the need for a tight functional relationship between the two to maintain homeostasis. The structure of KD2 is similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|.
gi 1304835748 229 CLKPPVSDDCGEEQTRWYFDAKKNNCFIFVYGNCNINLNHFDTYESCMTTC 279
Cdd:cd22624     2 CTEPPVTGPCRASFTRWYYDPLSRKCHRFTYGGCDGNENNFETEDECMETC 52

scorpion venom Kunitz-type toxin (KTT) such as LmKTT-1a, BmKTT-1, and BmKTT-2; This model includes scorpion Kunitz-type toxin (KTT) such as Lychas mucronatus LmKTT-1a (also called Delta-KTx 2.1 or SdPII), Mesobuthus martensii BmKTT-1 (also called Delta-KTx 2.4) and BmKTT-2 (also called Delta-KTx 3.1), all expressed by the venom gland. LmKTT-1a, BmKTT-1 and BmKTT-2 are all dual-function toxins that completely inhibit trypsin activity but have no effect on chymotrypsin or elastase. They also inhibit mKv1.3/KCNA3 potassium channel currents. The structures of these domains are similar to those of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor); however, they lack the conserved CysII-CysIV disulfide bond but contains 2 cysteine residues at the C-terminus that generate a new disulfide bond.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|....
gi 1304835748 287 CSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCedmcpfpknqkCKVCKP 350
Cdd:cd22620     3 CQLPSDTGRGKASFTRYYYNEESGKCETFIYGGVGGNSNNFLTKEDC-----------CKECAQ 55

Third immunoglobulin (Ig)-like domain in Robo (roundabout) receptors; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the third immunoglobulin (Ig)-like domain in Robo (roundabout) receptors. Robo receptors play a role in the development of the central nervous system (CNS), and are receptors of Slit protein. Slit is a repellant secreted by the neural cells in the midline. Slit acts through Robo to prevent most neurons from crossing the midline from either side. Three mammalian Robo homologs (Robo1, Robo2, Robo3), and three mammalian Slit homologs (Slit-1,Slit-2, Slit-3), have been identified. Commissural axons, which cross the midline, express low levels of Robo; longitudinal axons, which avoid the midline, express high levels of Robo. Robo1, Robo2, and Robo3 are expressed by commissural neurons in the vertebrate spinal cord and Slit-1, Slit-2, and Slit-3 are expressed at the ventral midline. Robo-3 is a divergent member of the Robo family which instead of being a positive regulator of Slit responsiveness, antagonizes Slit responsiveness in precrossing axons. The Slit-Robo interaction is mediated by the second leucine-rich repeat (LRR) domain of Slit and the two N-terminal Ig domains of Robo, Ig1 and Ig2. The primary Robo binding site for Slit2 has been shown by surface plasmon resonance experiments and mutational analysis to be the Ig1 domain, while the Ig2 domain has been proposed to harbor a weak secondary binding site. This group belongs to the I-set of IgSF domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand, but lack a C" strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748 118 VNHPAHKTVSVGDTVSFHCDVTGKPKPEITWEKEvEGE---DNVVMRPNHvvanlvvtniaQLVIYNAQIQDAGIYTCTA 194
Cdd:cd05725     1 VKRPQNQVVLVDDSAEFQCEVGGDPVPTVRWRKE-DGElpkGRYEILDDH-----------SLKIRKVTAGDMGSYTCVA 68

                          90
                  ....*....|....*
gi 1304835748 195 KNAGGLIKVNFPLSV 209
Cdd:cd05725    69 ENMVGKIEASATLTV 83

Immunoglobulin (Ig)-like domain of titin and similar proteins; The members here are composed of the immunoglobulin (Ig)-like domain found in titin-like proteins and similar proteins. Titin (also called connectin) is a fibrous sarcomeric protein specifically found in vertebrate striated muscle. Titin is a giant protein; depending on isoform composition, it ranges from 2970 to 3700 kDa, and is of a length that spans half a sarcomere. Titin largely consists of multiple repeats of Ig-like and fibronectin type 3 (FN-III)-like domains. Titin connects the ends of myosin thick filaments to Z disks and extends along the thick filament to the H zone. It appears to function similarly to an elastic band, keeping the myosin filaments centered in the sarcomere during muscle contraction or stretching. Within the sarcomere, titin is also attached to or is associated with myosin binding protein C (MyBP-C). MyBP-C appears to contribute to the generation of passive tension by titin and like titin has repeated Ig-like and FN-III domains. Also included in this group are worm twitchin and insect projectin, thick filament proteins of invertebrate muscle which also have repeated Ig-like and FN-III domains.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*..
gi 1304835748 123 HKTVSVGDTVSFHCDVTGKPKPEITWEKEvegeDNVVMRPNHVVANLVVTNiAQLVIYNAQIQDAGIYTCTAKNAGG 199
Cdd:cd05748     1 TIVVRAGESLRLDIPIKGRPTPTVTWSKD----GQPLKETGRVQIETTASS-TSLVIKNAKRSDSGKYTLTLKNSAG 72

Immunoglobulin (Ig)-like domain of myotilin, palladin, and myopalladin; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig)-like domain in myotilin, palladin, and myopalladin. Myotilin, palladin, and myopalladin function as scaffolds that regulate actin organization. Myotilin and myopalladin are most abundant in skeletal and cardiac muscle; palladin is ubiquitously expressed in the organs of developing vertebrates and plays a key role in cellular morphogenesis. The three family members each interact with specific molecular partners with all three binding to alpha-actinin; In addition, palladin also binds to vasodilator-stimulated phosphoprotein (VASP) and ezrin, myotilin binds to filamin and actin, and myopalladin also binds to nebulin and cardiac ankyrin repeat protein (CARP). This group belongs to the I-set of IgSF domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand, but lack a C" strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748 115 PSLVNHPAHKTVSVGDTVSFHCDVTGKPKPEITWEKevegeDNVVMRPNHVVANLV-VTNIAQLVIYNAQIQDAGIYTCT 193
Cdd:cd05744     1 PHFLQAPGDLEVQEGRLCRFDCKVSGLPTPDLFWQL-----NGKPVRPDSAHKMLVrENGRHSLIIEPVTKRDAGIYTCI 75

                          90
                  ....*....|....*.
gi 1304835748 194 AKNAGGLIKVNFPLSV 209
Cdd:cd05744    76 ARNRAGENSFNAELVV 91

agrin-responsive first immunoglobulin-like domains (Ig1) of the MuSK ectodomain; a member of the I-set of IgSF domains; The members here are composed of the first immunoglobulin-like domains (Ig1) of the Muscle-specific kinase (MuSK). MuSK is a receptor tyrosine kinase specifically expressed in skeletal muscle, where it plays a central role in the formation and maintenance of the neuromuscular junction (NMJ). MuSK is activated by agrin, a neuron-derived heparan sulfate proteoglycan. The activation of MUSK in myotubes regulates the formation of NMJs through the regulation of different processes including the specific expression of genes in subsynaptic nuclei, the reorganization of the actin cytoskeleton and the clustering of the acetylcholine receptors (AChR) in the postsynaptic membrane. The Ig superfamily (IgSF) is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. IgSF domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Unlike the V-set, one of the distinctive features of I-set domains is the lack of a C" strand. The structure of the MuSK lacks this strand and thus it belongs to the I-set of the IgSF. I-set domains are found in several cell adhesion molecules (such as VCAM, ICAM, and MADCAM), and are also present in numerous other diverse protein families, including several tyrosine-protein kinase receptors, the hemolymph protein hemolin, the muscle proteins titin, telokin, and twitchin, the neuronal adhesion molecule axonin-1, and the signaling molecule semaphorin 4D that is involved in axonal guidance, immune function and angiogenesis.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748 116 SLVNHPAH---KTVSVGDTVSFHCDVTGKPKPEITWEKE----VEGEDNVVMRpnhvvanlvvTNIAQLVIYNAQIQDAG 188
Cdd:cd20970     1 PVISTPQPsftVTAREGENATFMCRAEGSPEPEISWTRNgnliIEFNTRYIVR----------ENGTTLTIRNIRRSDMG 70

                          90       100
                  ....*....|....*....|..
gi 1304835748 189 IYTCTAKN-AGGLIKVNFPLSV 209
Cdd:cd20970    71 IYLCIASNgVPGSVEKRITLQV 92

dendrotoxins I, K, B and similar proteins; This group includes toxins isolated from snake venoms, such as dendrotoxins (DTXs) I, K and B, mambaquaretin-1 (MQ-1) and calcicludine. The dendrotoxins have little or no anti-protease activity but have been shown to block certain subtypes of voltage dependent potassium channels in neurons. Dendroaspis angusticeps (green mamba) alpha-dendrotoxin is a neurotoxin that enhances acetylcholine release at neuromuscular junctions. Studies with cloned K(+) channels show that this toxin blocks Kv1.1, Kv1.2 and Kv1.6 channels in the nanomolar range, whereas Dendroaspis polylepis (black mamba) dendrotoxin K preferentially blocks Kv1.1 channels. Also, structural analogs of dendrotoxins have facilitated defining the molecular recognition properties of different types of K(+) channels, and therefore, dendrotoxins are widely used as probes for studying the function of K(+) channels in physiology and pathophysiology. The structures of these toxins are similar to that of Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.

                          10        20        30        40        50
                  ....*....|....*....|....*....|....*....|....*....|...
gi 1304835748 285 TLCSYPALQGPCKAYEPRWAYNRLLGQCQSFVYGGCGGNENNFESKETCEDMC 337
Cdd:cd22595     2 KFCKLPVRPGPCKAFISAFYYNWKAKKCHPFTYSGCGGNANRFKTIEECRRTC 54

Fourth immunoglobulin domain of the Drosophila melanogaster Dscam protein, and similar domains; a member of the I-set of IgSF domains; The members here are composed of the fourth immunoglobulin domain of the Drosophila melanogaster Down syndrome cell adhesion molecule (DSCAM) protein and similar proteins. Down syndrome cell adhesion molecule (DSCAM) is a cell adhesion molecule that plays critical roles in neural development, including axon guidance and branching, axon target recognition, self-avoidance and synaptic formation. DSCAM belongs to the immunoglobulin superfamily and contributes to defects in the central nervous system in Down syndrome patients. Vertebrate DSCAMs differ from Drosophila Dscam1 in that they lack the extensive alternative splicing that occurs in the insect gene. Drosophila melanogaster Dscam has 38,016 isoforms generated by the alternative splicing of four variable exon clusters, which allows every neuron in the fly to display a distinctive set of Dscam proteins on its cell surface. Drosophila Dscam1 is a cell-surface protein that plays important roles in neural development and axon tiling of neurons. It is shown that thousands of isoforms bind themselves through specific homophilic (self-binding) interactions, a process which mediates cellular self-recognition. Drosophila Dscam2 is also alternatively spliced and plays a key role in the development of two visual system neurons, monopolar cells L1 and L2. This group is a member of the I-set Ig domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand but lack a C" strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1304835748 114 PPSLVNHPAHKTVSVGDTVSFHCDVTGKPKPEITWEKevegeDNVVMRPNH-VVANLVVTNIAQLV----IYNAQIQDAG 188
Cdd:cd20956     1 APVLLETFSEQTLQPGPSVSLKCVASGNPLPQITWTL-----DGFPIPESPrFRVGDYVTSDGDVVsyvnISSVRVEDGG 75

                          90
                  ....*....|...
gi 1304835748 189 IYTCTAKNAGGLI 201
Cdd:cd20956    76 EYTCTATNDVGSV 88