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        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 389 GPPVISSDPIQYAVRGERGEVKCYIVSTPPPDKIMWAWNENVWGD---ERYTVEPSlLPGGAVLSTLFITSVMDSDFHSP 465
Cdd:cd05898     1 GPPIISSEQVQYAVRGERGKVKCFIGSTPPPDRIAWAWKENVLESgtlERYTVERT-STGSGVLSTLTINNIMEADFQTH 79

                          90
                  ....*....|....*....
gi 1658177163 466 YNCTAWNHFGPGSMVIYLQ 484
Cdd:cd05898    80 YNCTAWNSFGSGTAIIQLE 98

Immunoglobulin domain; This family contains immunoglobulin-like domains.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 1658177163 306 PLLLAEPTPVTADIETDVTLRCKWSGNPPPTLTWTKKG---------SSVVLSNNAQLYLRSVSQSDAGQYICKA 371
Cdd:pfam13927   2 PVITVSPSSVTVREGETVTLTCEATGSPPPTITWYKNGepissgstrSRSLSGSNSTLTISNVTRSDAGTYTCVA 76

Immunoglobulin domain; This family contains immunoglobulin-like domains.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 221 PIVTLSVEPRSVLQGDKVTFTCHASANPPVMgYRWAKGGILLEGARESVFVTTADHSFFT 280
Cdd:pfam13927   2 PVITVSPSSVTVREGETVTLTCEATGSPPPT-ITWYKNGEPISSGSTRSRSLSGSNSTLT 60

Fifth immunoglobulin (Ig)-like domain of Kirrel (kin of irregular chiasm-like) 3 protein; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the fifth immunoglobulin (Ig)-like domain of Kirrel (kin of irregular chiasm-like) 3 protein (also known as Neph2). This protein has five Ig-like domains, one transmembrane domain, and a cytoplasmic tail. Included in this group is mammalian Kirrel (Neph1). These proteins contain multiple Ig domains, have properties of cell adhesion molecules, and are important in organ development. Neph1 and 2 may mediate axonal guidance and synapse formation in certain areas of the CNS. In the kidney they participate in the formation of the slit diaphragm.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 389 GPPVISSDPIQYAVRGERGEVKCYIVSTPPPDKIMWAWNENVWGD---ERYTVEPSlLPGGAVLSTLFITSVMDSDFHSP 465
Cdd:cd05898     1 GPPIISSEQVQYAVRGERGKVKCFIGSTPPPDRIAWAWKENVLESgtlERYTVERT-STGSGVLSTLTINNIMEADFQTH 79

                          90
                  ....*....|....*....
gi 1658177163 466 YNCTAWNHFGPGSMVIYLQ 484
Cdd:cd05898    80 YNCTAWNSFGSGTAIIQLE 98

Immunoglobulin domain; This family contains immunoglobulin-like domains.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 1658177163 306 PLLLAEPTPVTADIETDVTLRCKWSGNPPPTLTWTKKG---------SSVVLSNNAQLYLRSVSQSDAGQYICKA 371
Cdd:pfam13927   2 PVITVSPSSVTVREGETVTLTCEATGSPPPTITWYKNGepissgstrSRSLSGSNSTLTISNVTRSDAGTYTCVA 76

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        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 323 VTLRCKWSGNPPPTLTWTKKGSSV---------VLSNNAQLYLRSVSQSDAGQYICKAIVPQIGVGEREV 383
Cdd:cd00096     1 VTLTCSASGNPPPTITWYKNGKPLppssrdsrrSELGNGTLTISNVTLEDSGTYTCVASNSAGGSASASV 70

Immunoglobulin I-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163  20 RFSQEPADQSVILGQRAVLSCVVfnySG----IVQWTKDGLalgigdDLRAWPRYRVLRgmEMGQYDLEILSAELSDDSL 95
Cdd:pfam07679   2 KFTQKPKDVEVQEGESARFTCTV---TGtpdpEVSWFKDGQ------PLRSSDRFKVTY--EGGTYTLTISNVQPDDSGK 70

                          90       100
                  ....*....|....*....|
gi 1658177163  96 YECQATEEA-LRSRRAKLTV 114
Cdd:pfam07679  71 YTCVATNSAgEAEASAELTV 90

First immunoglobulin (Ig)-like domain in neogenin, and similar domains; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the first immunoglobulin (Ig)-like domain in neogenin and related proteins. Neogenin is a cell surface protein which is expressed in the developing nervous system of vertebrate embryos in the growing nerve cells. It is also expressed in other embryonic tissues and may play a general role in developmental processes such as cell migration, cell-cell recognition, and tissue growth regulation. Included in this group is the tumor suppressor protein DCC which is deleted in colorectal carcinoma. DCC and neogenin each have four Ig-like domains followed by six fibronectin type III domains, a transmembrane domain, and an intracellular domain. 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 1658177163  20 RFSQEPADQSVILGQRAVLSCVVFNYSGIV-QWTKDGLALGIGDDLRawpRYRVLRGMEMGQydlEILSAEL--SDDSLY 96
Cdd:cd05722     3 YFLSEPSDIVAMRGGPVVLNCSAESDPPPKiEWKKDGVLLNLVSDER---RQQLPNGSLLIT---SVVHSKHnkPDEGFY 76

                          90       100
                  ....*....|....*....|.
gi 1658177163  97 ECQATEEALR---SRRAKLTV 114
Cdd:cd05722    77 QCVAQNESLGsivSRTARVTV 97

Immunoglobulin domain; This family contains immunoglobulin-like domains.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 221 PIVTLSVEPRSVLQGDKVTFTCHASANPPVMgYRWAKGGILLEGARESVFVTTADHSFFT 280
Cdd:pfam13927   2 PVITVSPSSVTVREGETVTLTCEATGSPPPT-ITWYKNGEPISSGSTRSRSLSGSNSTLT 60

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain of histocompatibility antigen (HLA) DQ and related proteins; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) DQ. MHC class II molecules are encoded by three different loci, HLA-DR, -DQ, and -DP, which are about 70% similar to each other. HLA-DQ (DQ) is a cell surface receptor protein found on antigen presenting cells. It is an alphabeta heterodimer of type MHC class II. The alpha and beta chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. A person often produces two alpha-chain and two beta chain variants and thus 4 isoforms of DQ. Two autoimmune diseases in which HLA-DQ is involved are celiac disease and diabetes mellitus type 1. DQ is one of several antigens involved in rejection of organ transplants. DQ8 is a split antigen of the DQ3 broad antigen. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|..
gi 1658177163 221 PIVTlsVEPRS-VLQGDKVTFTCHA-SANPPVMGYRWAKGG-ILLEGARESVFVTTADHSFF 279
Cdd:cd21008     3 PEVT--VFPKSpVTLGQPNTLICLVdNIFPPVINITWLSNGhSVTEGVSETSFLSKSDHSFL 62

Fifth immunoglobulin (Ig)-like domain of Kirrel (kin of irregular chiasm-like) 3 protein; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the fifth immunoglobulin (Ig)-like domain of Kirrel (kin of irregular chiasm-like) 3 protein (also known as Neph2). This protein has five Ig-like domains, one transmembrane domain, and a cytoplasmic tail. Included in this group is mammalian Kirrel (Neph1). These proteins contain multiple Ig domains, have properties of cell adhesion molecules, and are important in organ development. Neph1 and 2 may mediate axonal guidance and synapse formation in certain areas of the CNS. In the kidney they participate in the formation of the slit diaphragm.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 389 GPPVISSDPIQYAVRGERGEVKCYIVSTPPPDKIMWAWNENVWGD---ERYTVEPSlLPGGAVLSTLFITSVMDSDFHSP 465
Cdd:cd05898     1 GPPIISSEQVQYAVRGERGKVKCFIGSTPPPDRIAWAWKENVLESgtlERYTVERT-STGSGVLSTLTINNIMEADFQTH 79

                          90
                  ....*....|....*....
gi 1658177163 466 YNCTAWNHFGPGSMVIYLQ 484
Cdd:cd05898    80 YNCTAWNSFGSGTAIIQLE 98

Fifth immunoglobulin (Ig)-like domain of Kirrel (kin of irregular chiasm-like) 3, and similar domains; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the fifth immunoglobulin (Ig)-like domain of Kirrel (kin of irregular chiasm-like) 3 (also known as Neph2). This protein has five Ig-like domains, one transmembrane domain, and a cytoplasmic tail. Included in this group is mammalian Kirrel (also known as Neph1), Kirrel2 (also known as Neph3), and Drosophila RST (also known as irregular chiasm C-roughest) protein. These proteins contain multiple Ig domains, have properties of cell adhesion molecules, and are important in organ development.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 389 GPPVISSDPIQYAVRGERGEVKCYIVSTPPPDKIMWAWNENV-----WGdeRYTVEPSLLPGGaVLSTLFITSVMDSDFH 463
Cdd:cd05758     1 GPPIITAEATQPAILGEKARLECLVFSSPPPDRIVWSWDEGFlesgsSG--RFSVETFPTEPG-VISVLHISGTQRSDFQ 77

                          90       100
                  ....*....|....*....|.
gi 1658177163 464 SPYNCTAWNHFGPGSMVIYLQ 484
Cdd:cd05758    78 TSFNCSAWNRFGEGTAIVSLG 98

CD80-like C2-set immunoglobulin domain; These domains belong to the immunoglobulin superfamily.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 125 EGPQVLLRAGEA-YNLSCVSRGAKPPARIHWEKDGMPLDGGFSVTEVLADRKRTTTRSFLPILPVESDTGRNITCVSSNA 203
Cdd:pfam08205   4 EPPASLLEGEGPeVVATCSSAGGKPAPRITWYLDGKPLEAAETSSEQDPESGLVTVTSELKLVPSRSDHGQSLTCQVSYG 83

                  .
gi 1658177163 204 A 204
Cdd:pfam08205  84 A 84

Immunoglobulin I-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 312 PTPVTADIETDVTLRCKWSGNPPPTLTWTKKGSSVVLS---------NNAQLYLRSVSQSDAGQYICKAIVPqigVGERE 382
Cdd:pfam07679   7 PKDVEVQEGESARFTCTVTGTPDPEVSWFKDGQPLRSSdrfkvtyegGTYTLTISNVQPDDSGKYTCVATNS---AGEAE 83

                  ....*
gi 1658177163 383 VTLTV 387
Cdd:pfam07679  84 ASAEL 88

Immunoglobulin domain; This domain contains immunoglobulin-like domains.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 306 PLLLAEPTPVTadIETDVTLRCKWSGNPPPTLTWTKKGSsvVLSNNAQLYLRSVSQSDAGQYICKAIVPQIGVGEREVTL 385
Cdd:pfam13895   2 PVLTPSPTVVT--EGEPVTLTCSAPGNPPPSYTWYKDGS--AISSSPNFFTLSVSAEDSGTYTCVARNGRGGKVSNPVEL 77

                  ..
gi 1658177163 386 TV 387
Cdd:pfam13895  78 TV 79

Second immunoglobulin (Ig)-like domain of the nectin-like molecules Necl-1 - Necl-4; member of the I-set of Ig superfamily domains; The members here are composed of the second immunoglobulin (Ig)-like domain of the nectin-like molecules Necl-1 (also known as cell adhesion molecule 3 or CADM3), Necl-2 (also known as CADM1), Necl-3 (also known as CADM2) and Necl-4 (also known as CADM4). These nectin-like molecules have similar domain structures to those of nectins. At least five nectin-like molecules have been identified (Necl-1 through Necl-5). These have an extracellular region containing three Ig-like domains, one transmembrane region, and one cytoplasmic region. The N-terminal Ig-like domain of the extracellular region belongs to the V-type subfamily of Ig domains, is essential to cell-cell adhesion, and plays a part in the interaction with the envelope glycoprotein D of various viruses. Necl-1 and Necl-2 have Ca(2+)-independent homophilic and heterophilic cell-cell adhesion activity. Necl-1 is specifically expressed in neural tissue and is important to the formation of synapses, axon bundles, and myelinated axons. Necl-2 is expressed in a wide variety of tissues, and is a putative tumour suppressor gene, which is downregulated in aggressive neuroblastoma. Necl-3 has been shown to accumulate in tissues of the central and peripheral nervous system, where it is expressed in ependymal cells and myelinated axons. It is observed at the interface between the axon shaft and the myelin sheath. Necl-4 is expressed on Schwann cells, and plays a key part in initiating peripheral nervous system (PNS) myelination. Necl-4 participates in cell-cell adhesion and is proposed to play a role in tumor suppression.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 115 LVPPGDPVI--YEGPqvlLRAGEAYNLSCVSRGAKPPARIHWEKDGMPLDGgfsVTEVLAD--RKRTTTRSFLPILPVES 190
Cdd:cd05761     1 LGVPEKPVItgFTSP---VVEGDEITLTCTTSGSKPAADIRWFKNDKELKG---VKEVQESgaGKTFTVTSTLRFRVDRD 74

                          90       100
                  ....*....|....*....|....*...
gi 1658177163 191 DTGRNITC-VSSNAAAPTGKYATVSLNV 217
Cdd:cd05761    75 DDGVAVICrVDHESLTSTPKQTQQVLEV 102

Immunoglobulin I-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163  20 RFSQEPADQSVILGQRAVLSCVVfnySG----IVQWTKDGLalgigdDLRAWPRYRVLRgmEMGQYDLEILSAELSDDSL 95
Cdd:pfam07679   2 KFTQKPKDVEVQEGESARFTCTV---TGtpdpEVSWFKDGQ------PLRSSDRFKVTY--EGGTYTLTISNVQPDDSGK 70

                          90       100
                  ....*....|....*....|
gi 1658177163  96 YECQATEEA-LRSRRAKLTV 114
Cdd:pfam07679  71 YTCVATNSAgEAEASAELTV 90

Fourth immunoglobulin (Ig)-like domain in Robo (roundabout) receptors; member of the I-set of Ig superfamily (IgSF) domains; Members here are composed the fourth 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
                  ....*....|....*....|....*....|....*....|....*....|....*....|....
gi 1658177163 323 VTLRCKWSGNPPPTLTWTKKGSSVVL--------------SNNAQLYLRSVSQSDAGQYICKAI 372
Cdd:cd05726    17 VTFQCETKGNPQPAIFWQKEGSQNLLfpyqppqpssrfsvSPTGDLTITNVQRSDVGYYICQAL 80

Fourth immunoglobulin (Ig)-like domain of hemolin, and similar domains; a member of the I-set of IgSF domains; The members here are composed of the fourth immunoglobulin (Ig)-like domain of hemolin and similar proteins. Hemolin, an insect immunoglobulin superfamily (IgSF) member containing four Ig-like domains, is a lipopolysaccharide-binding immune protein induced during bacterial infection. Hemolin shares significant sequence similarity with the first four Ig-like domains of the transmembrane cell adhesion molecules (CAMs) of the L1 family. 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 fourth Ig-like domain of hemolin 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 domains, members of the I-set have a discontinuous A strand but lack a C" strand. 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 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 1658177163 311 EPTPVTADIETDVTLRCKWSGNPPPTLTWTKKGSSVVLSN------NAQLYLRSVSQSDAGQYICKAiVPQIGVGEREVT 384
Cdd:cd20978     7 PEKNVVVKGGQDVTLPCQVTGVPQPKITWLHNGKPLQGPMeratveDGTLTIINVQPEDTGYYGCVA-TNEIGDIYTETL 85

                  ...
gi 1658177163 385 LTV 387
Cdd:cd20978    86 LHV 88

Third immunoglobulin-like domain of the human WFIKKN (WAP, follistatin, immunoglobulin, Kunitz and NTR domain-containing protein), and similar domains; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the third immunoglobulin-like domain of the human WFIKKN (WAP, follistatin, immunoglobulin, Kunitz and NTR domain-containing protein) and similar proteins. WFIKKN is a secreted protein that consists of multiple types of protease inhibitory modules, including two tandem Kunitz-type protease inhibitor-domains. The Ig superfamily is a heterogenous group of proteins built on a common fold comprised of a sandwich of two beta sheets. Members of the Ig superfamily are components of immunoglobulin, neuroglia, cell surface glycoproteins, such as T-cell receptors, CD2, CD4, CD8, and membrane glycoproteins, such as 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. 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 1658177163 306 PLLLAEPTPVTADIETDVTLRCKWSGNPPPTLTWTKKG--------------SSVVLSNNAQLYLRSVSQSDAGQYICKA 371
Cdd:cd05765     1 PALVNSPTHQTVKVGETASFHCDVTGRPQPEITWEKQVpgkenlimrpnhvrGNVVVTNIGQLVIYNAQPQDAGLYTCTA 80

Immunoglobulin (Ig)-like I-set domain of Neural Cell Adhesion Molecule 1 (NCAM-1); The members here are composed of the fourth Ig domain of Neural Cell Adhesion Molecule 1(NCAM-1). NCAM plays important roles in the development and regeneration of the central nervous system, in synaptogenesis and neural migration. NCAM mediates cell-cell and cell-substratum recognition and adhesion via homophilic (NCAM-NCAM) and heterophilic (NCAM-non-NCAM) interactions. NCAM is expressed as three major isoforms having different intracellular extensions. The extracellular portion of NCAM has five N-terminal Ig-like domains and two fibronectin type III domains. The double zipper adhesion complex model for NCAM homophilic binding involves Ig1, Ig2, and Ig3. By this model, Ig1 and Ig2 mediate dimerization of NCAM molecules situated on the same cell surface (cis interactions), and Ig3 domains mediate interactions between NCAM molecules expressed on the surface of opposing cells (trans interactions), through binding to the Ig1 and Ig2 domains. The adhesive ability of NCAM is modulated by the addition of polysialic acid chains to the fifth Ig-like domain. One of the unique features of I-set domains is the lack of a C" strand. The structures of this group show that the Ig domain lacks this strand and thus is a member of the I-set of Ig domains.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 318 DIETDVTLRCKWSGNPPPTLTWtkKGSSVVLSNNAQ----------------LYLRSVSQSDAGQYICKAIVPqIGVGER 381
Cdd:cd05869    15 ELEEQITLTCEASGDPIPSITW--RTSTRNISSEEKtldghivvrsharvssLTLKYIQYTDAGEYLCTASNT-IGQDSQ 91

                  ....*.
gi 1658177163 382 EVTLTV 387
Cdd:cd05869    92 SMYLEV 97

First immunoglobulin (Ig)-like domain in neogenin, and similar domains; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the first immunoglobulin (Ig)-like domain in neogenin and related proteins. Neogenin is a cell surface protein which is expressed in the developing nervous system of vertebrate embryos in the growing nerve cells. It is also expressed in other embryonic tissues and may play a general role in developmental processes such as cell migration, cell-cell recognition, and tissue growth regulation. Included in this group is the tumor suppressor protein DCC which is deleted in colorectal carcinoma. DCC and neogenin each have four Ig-like domains followed by six fibronectin type III domains, a transmembrane domain, and an intracellular domain. 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 1658177163  20 RFSQEPADQSVILGQRAVLSCVVFNYSGIV-QWTKDGLALGIGDDLRawpRYRVLRGMEMGQydlEILSAEL--SDDSLY 96
Cdd:cd05722     3 YFLSEPSDIVAMRGGPVVLNCSAESDPPPKiEWKKDGVLLNLVSDER---RQQLPNGSLLIT---SVVHSKHnkPDEGFY 76

                          90       100
                  ....*....|....*....|.
gi 1658177163  97 ECQATEEALR---SRRAKLTV 114
Cdd:cd05722    77 QCVAQNESLGsivSRTARVTV 97

Second 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 second 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 the 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, and 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, 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 Slit-2 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 1658177163 311 EPTPVTADIETDVTLRCK--WsGNPPPTLTWTKKGSSVVLSNN-------AQLYLRSVSQSDAGQYICKAivpQIGVGER 381
Cdd:cd05724     3 EPSDTQVAVGEMAVLECSppR-GHPEPTVSWRKDGQPLNLDNErvrivddGNLLIAEARKSDEGTYKCVA---TNMVGER 78

                  .
gi 1658177163 382 E 382
Cdd:cd05724    79 E 79

Seventh 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 seventh 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 1658177163 306 PLLLAEPTPVTADIETDVTLRCKWSGNPPPTLTWTK-------------KGSSVVLSNNAQLYLRSVSQSDAGQYICKAI 372
Cdd:cd20954     2 PRWIVEPVDANVAAGQDVMLHCQADGFPTPTVTWKKatgstpgeykdllYDPNVRILPNGTLVFGHVQKENEGHYLCEAK 81

Fourth immunoglobulin (Ig)-like domain in neogenin, and similar domains; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the fourth immunoglobulin (Ig)-like domain in neogenin and related proteins. Neogenin is a cell surface protein which is expressed in the developing nervous system of vertebrate embryos in the growing nerve cells. It is also expressed in other embryonic tissues, and may play a general role in developmental processes such as cell migration, cell-cell recognition, and tissue growth regulation. Included in this group is the tumor suppressor protein DCC which is deleted in colorectal carcinoma. DCC and neogenin each have four Ig-like domains followed by six fibronectin type III domains, a transmembrane domain, and an intracellular domain. 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
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....
gi 1658177163 309 LAEPTPVTADIETDVTLRCKWSGNPPPTLTWTKKGSSVVLSN------NAQLYLRSVSQSDAGQYICKA 371
Cdd:cd05723     1 LKKPSNIYAHESMDIVFECEVTGKPTPTVKWVKNGDVVIPSDyfkivkEHNLQVLGLVKSDEGFYQCIA 69

Second immunoglobulin (Ig) domain of nectin-3 and nectin-4 (poliovirus receptor related protein 4), and similar domains; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the second immunoglobulin (Ig) domain of nectin-3 (also known as poliovirus receptor related protein 3 or cluster of differentiation (CD) 113) and nectin-4 (poliovirus receptor related protein 4). Nectin-3 and nectin-4 belong to the nectin family comprised of four transmembrane glycoproteins (nectin-1 through -4). Nectins are synaptic cell adhesion molecules (CAMs) which facilitate adhesion and signaling at various intracellular junctions. Nectins form homophilic cis-dimers, followed by homophilic and heterophilic trans-dimers involved in cell-cell adhesion. Nectin-2 and nectin-3 localize at Sertoli-spermatid junctions where they form heterophilic trans-interactions between the cells that are essential for the formation and maintenance of the junctions and for spermatid development. Nectin-3 has also been shown to form a heterophilic trans-interaction with nectin-1 in ciliary epithelia, establishing the apex-apex adhesion between the pigment and non-pigment cell layers. Nectin-4 has recently been identified in several types of breast carcinoma and can be used as a histological and serological marker for breast cancer.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 113 TVLVPPGDPVIYEGpqvllrAGEAYNLSCVSRGAKPPARIHWEKDGmpldGGFSVTEVLADRKRTTTRSFLPILPVESDT 192
Cdd:cd07704     2 LVSLNPGPALLIDG------GNETLAASCTAETGKPAASVTWETDL----GGMESSRTFEHNRTATVTSEYHLVPTRFAN 71

                          90
                  ....*....|..
gi 1658177163 193 GRNITCVSSNAA 204
Cdd:cd07704    72 GRPLTCVVSHPA 83

First immunoglobulin (Ig)-like constant domain in Robo (roundabout) receptors, and similar domains; The members here are composed of the first immunoglobulin (Ig)-like domain in Roundabout (Robo) 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, and Robo3), and three mammalian Slit homologs (Slit1, Slit2, Slit3), 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 Slit1, Slit2,and Slit3 are expressed at the ventral midline. Robo3 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 is the Ig1 domain, while the Ig2 domain has been proposed to harbor a weak secondary binding site.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 306 PLLLAEPTPVTADIETDVTLRCKWSGNPPPTLTWTKKGSSV------------VLSNNAQLYLRSV----SQSDAGQYIC 369
Cdd:cd07693     1 PRIVEHPSDLIVSKGDPATLNCKAEGRPTPTIQWLKNGQPLetdkddprshriVLPSGSLFFLRVVhgrkGRSDEGVYVC 80

                  ..
gi 1658177163 370 KA 371
Cdd:cd07693    81 VA 82

agrin-responsive second immunoglobulin-like domains (Ig2) of the Muscle-specific kinase (MuSK) ectodomain; a member of the I-set of Ig superfamily domains; The members here are composed of the second immunoglobulin-like (Ig) domains of the Muscle-specific kinase (MuSK) ectodomain. 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
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*..
gi 1658177163 312 PTPVTADIETDVTLRCKWSGNPPPTLTWTkKGSSVVLSNN-------AQLYLRSVSQSDAGQYICKA 371
Cdd:cd20968     6 PTNVTIIEGLKAVLPCTTMGNPKPSVSWI-KGDDLIKENNriavlesGSLRIHNVQKEDAGQYRCVA 71

Immunoglobulin domain; Members of the immunoglobulin superfamily are found in hundreds of proteins of different functions. Examples include antibodies, the giant muscle kinase titin and receptor tyrosine kinases. Immunoglobulin-like domains may be involved in protein-protein and protein-ligand interactions.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|..
gi 1658177163 311 EPTPVTADIETDVTLRCKWS-GNPPPTLTWTKKGSSVVLSN----------NAQLYLRSVSQSDAGQYICKA 371
Cdd:pfam00047   2 APPTVTVLEGDSATLTCSAStGSPGPDVTWSKEGGTLIESLkvkhdngrttQSSLLISNVTKEDAGTYTCVV 73

Fourth immunoglobulin (Ig)-like domain of peroxidasin; The members here are composed of the fourth immunoglobulin (Ig)-like domain in peroxidasin. Peroxidasin has a peroxidase domain and interacting extracellular motifs containing four Ig-like domains. It has been suggested that peroxidasin is secreted, and has functions related to the stabilization of the extracellular matrix. It may play a part in various other important processes such as removal and destruction of cells which have undergone programmed cell death and protection of the organism against non-self.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163 323 VTLRCKWSGNPPPTLTWTKKGSSVV------LSNNAQLYLRSVSQSDAGQYICKAIVPqIGVGEREVTLT 386
Cdd:cd05746     1 VQIPCSAQGDPEPTITWNKDGVQVTesgkfhISPEGYLAIRDVGVADQGRYECVARNT-IGYASVSMVLS 69

Immunoglobulin domain; This family contains immunoglobulin-like domains.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1658177163  20 RFSQEPADQSVILGQRAVLSCVVFNY-SGIVQWTKDGLALGIGDDLRawpryrvlRGMEMGQYDLEILSAELSDDSLYEC 98
Cdd:pfam13927   3 VITVSPSSVTVREGETVTLTCEATGSpPPTITWYKNGEPISSGSTRS--------RSLSGSNSTLTISNVTRSDAGTYTC 74

                  ...
gi 1658177163  99 QAT 101
Cdd:pfam13927  75 VAS 77

Sixth 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 sixth 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 1658177163 134 GEAYNLSCVSRGAKPPARIHWEKDGMPL--DGGFSVTEVlaDRKrtttRSFLPILPVESDTGRNITCVSSNAAApTGKYa 211
Cdd:cd20959    17 GMRAQLHCGVPGGDLPLNIRWTLDGQPIsdDLGITVSRL--GRR----SSILSIDSLEASHAGNYTCHARNSAG-SASY- 88

                  ....*.
gi 1658177163 212 TVSLNV 217
Cdd:cd20959    89 TAPLTV 94

Third immunoglobulin domain of the Drosophila melanogaster Dscam protein, and similar domains; a member of the Constant 2 (C2)-set of IgSF domains; The members here are composed of the third 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. 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. This group belongs to the C2-set of IgSF domains, having A, B, and E strands in one beta-sheet and A', G, F, C, and C' in the other. Unlike other Ig domain sets, the C2-set lacks the D strand.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|....
gi 1658177163 312 PTPVTADIETDVTLRCKWSGNPPPTLTWTKKG------SSVVLSNNAQLYLRSVSQSDAGQYIC 369
Cdd:cd20957     8 PPVQTVDFGRTAVFNCSVTGNPIHTVLWMKDGkplghsSRVQILSEDVLVIPSVKREDKGMYQC 71