C-terminal domain of adaptor protein (AP) complexes medium mu subunits and its homologs (MHD); This family corresponds to the C-terminal domain of heterotetrameric AP complexes medium mu subunits and its homologs existing in monomeric stonins, delta-subunit of the heteroheptameric coat protein I (delta-COPI), a protein encoded by a pro-death gene referred as MuD (also known as MUDENG, mu-2 related death-inducing gene), an endocytic adaptor syp1, the mammalian FCH domain only proteins (FCHo1/2), SH3-containing GRB2-like protein 3-interacting protein 1 (SGIP1), and related proteins. AP complexes participate in the formation of intracellular coated transport vesicles and select cargo molecules for incorporation into the coated vesicles in the late secretory and endocytic pathways. Stonins have been characterized as clathrin-dependent AP-2 mu chain related factors and may act as cargo-specific sorting adaptors in endocytosis. Coat protein complex I (COPI)-coated vesicles function in the early secretory pathway. They mediate the retrograde transport from the Golgi to the ER, and intra-Golgi transport. MuD is distantly related to the C-terminal domain of mu2 subunit of AP-2. It is able to induce cell death by itself and plays an important role in cell death in various tissues. Syp1 represents a novel type of endocytic adaptor protein that participates in endocytosis, promotes vesicle tabulation, and contributes to cell polarity and stress responses. It shares the same domain architecture with its two ubiquitously expressed mammalian counterparts, FCHo1/2, which represent key initial proteins ultimately controlling cellular nutrient uptake, receptor regulation, and synaptic vesicle retrieval. They bind specifically to the plasma membrane and recruit the scaffold proteins eps15 and intersectin, which subsequently engage the adaptor complex AP2 and clathrin, leading to coated vesicle formation. Another mammalian neuronal-specific protein SGIP1 does have a C-terminal MHD and has been classified into this family as well. It is an endophilin-interacting protein that plays an obligatory role in the regulation of energy homeostasis. It is also involved in clathrin-mediated endocytosis by interacting with phospholipids and eps15.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 119625812  32 NAVSWCSERIKYKKNEVFIDVIESANLLVNAKGSILLSEIFGTIKLKVFLSGMPELRLGLNDSMLFELTGCHKNKLVKLE 111
Cdd:cd09259    2 NAVSWRSEGIKYKKNEVFIDVIESVNVLVNANGSVLSSEIVGCIKLKVFLSGMPELRLGLNDRVLFELTGRDKNKTVELE 81

                 ....*.
gi 119625812 112 DVKFQH 117
Cdd:cd09259   82 DVKFHQ 87

C-terminal domain of medium Mu1B subunit in epithelial cell-specific clathrin-associated adaptor protein (AP) complex AP-1; AP complexes participate in the formation of intracellular coated transport vesicles and select cargo molecules for incorporation into the coated vesicles in the late secretory and endocytic pathways. There are four AP complexes, AP-1, AP-2, AP-3, and AP-4, described in various eukaryotic organisms. Each AP complex consists of four subunits: two large chains (one each of gamma/alpha/delta/epsilon and beta1-4, respectively), a medium mu chain (mu1-4), and a small sigma chain (sigma1-4). Each of the four subunits from different AP complexes exhibits similarity with each other. This subfamily corresponds to the C-terminal domain of heterotetrameric clathrin-associated adaptor protein complex 1 (AP-1) medium mu1B subunit encoded by ap1m2 gene exclusively expressed in polarized epithelial cells. Epithelial cell-specific AP-1 is used to sort proteins to the basolateral plasma membrane, which involves the formation of clathrin-coated vesicles (CCVs) from the trans-Golgi network (TGN). Recruitment of AP-1 to the TGN membrane is regulated by a small GTPase, ADP-ribosylation factor 1 (ARF1). The phosphorylation/dephosphorylation events can also regulate the function of AP-1. The membrane-anchored cargo molecules can be linked to the outer lattice of CCVs by AP-1. Those cargo molecules interact with adaptors through short sorting signals in their cytosolic segments. Tyrosine-based endocytotic signals are one of the most important sorting signals. They are of the form Y-X-X-Phi, where Y is tyrosine, X is any amino acid and Phi is a bulky hydrophobic residue that can be Leu, Ile, Met, Phe, or Val. These kinds of sorting signals can be recognized by the C-terminal domain of AP-1 mu1B subunit, also known as Y-X-X-Phi signal-binding domain that contains two hydrophobic pockets, one for the tyrosine-binding and one for the bulky hydrophobic reside-binding. Besides, AP-1 mu1B subunit mediates the basolateral recycling of low-density lipoprotein receptor (LDLR) and transferrin receptor (TfR) from the sorting endosomes, where the basolateral sorting signal does not belong to the tyrosine-based signals. Thus, the binding site in mu1B subunit of AP-1 for the signals of LDLR and TfR might be distinct from that for YXXPhi signals.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 119625812  32 NAVSWCSERIKYKKNEVFIDVIESANLLVNAKGSILLSEIFGTIKLKVFLSGMPELRLGLNDSMLFELTGCHKNKLVKLE 111
Cdd:cd09259    2 NAVSWRSEGIKYKKNEVFIDVIESVNVLVNANGSVLSSEIVGCIKLKVFLSGMPELRLGLNDRVLFELTGRDKNKTVELE 81

                 ....*.
gi 119625812 112 DVKFQH 117
Cdd:cd09259   82 DVKFHQ 87

AP-1 complex subunit mu N-terminal domain; AP-1 complex mu subunit is part of the heterotetrameric adaptor protein (AP)-1 complex which consists of one large gamma-, one beta-, one mu-, and one sigma-subunit. AP complexes link the cytosolic domains of the cargo proteins to the protein coat that induces vesicle budding in the donor compartment during vesicle transport. In the case of AP-1 the coat protein is clathrin. AP-1 binds the phospholipid PI(4)P which plays a role in its localisation to the trans-Golgi network (TGN)/endosome. The mu subunit is comprised of an N-terminal longin domain followed by a C-terminal domain which is involved in the binding of the Y-X-X-Phi sorting signal.

                         10
                 ....*....|....*....
gi 119625812   1 QTTNSKILQEYTTQQSNKL 19
Cdd:cd14835  121 QTTESKILQEYITQESHKL 139

C-terminal domain of medium Mu1B subunit in epithelial cell-specific clathrin-associated adaptor protein (AP) complex AP-1; AP complexes participate in the formation of intracellular coated transport vesicles and select cargo molecules for incorporation into the coated vesicles in the late secretory and endocytic pathways. There are four AP complexes, AP-1, AP-2, AP-3, and AP-4, described in various eukaryotic organisms. Each AP complex consists of four subunits: two large chains (one each of gamma/alpha/delta/epsilon and beta1-4, respectively), a medium mu chain (mu1-4), and a small sigma chain (sigma1-4). Each of the four subunits from different AP complexes exhibits similarity with each other. This subfamily corresponds to the C-terminal domain of heterotetrameric clathrin-associated adaptor protein complex 1 (AP-1) medium mu1B subunit encoded by ap1m2 gene exclusively expressed in polarized epithelial cells. Epithelial cell-specific AP-1 is used to sort proteins to the basolateral plasma membrane, which involves the formation of clathrin-coated vesicles (CCVs) from the trans-Golgi network (TGN). Recruitment of AP-1 to the TGN membrane is regulated by a small GTPase, ADP-ribosylation factor 1 (ARF1). The phosphorylation/dephosphorylation events can also regulate the function of AP-1. The membrane-anchored cargo molecules can be linked to the outer lattice of CCVs by AP-1. Those cargo molecules interact with adaptors through short sorting signals in their cytosolic segments. Tyrosine-based endocytotic signals are one of the most important sorting signals. They are of the form Y-X-X-Phi, where Y is tyrosine, X is any amino acid and Phi is a bulky hydrophobic residue that can be Leu, Ile, Met, Phe, or Val. These kinds of sorting signals can be recognized by the C-terminal domain of AP-1 mu1B subunit, also known as Y-X-X-Phi signal-binding domain that contains two hydrophobic pockets, one for the tyrosine-binding and one for the bulky hydrophobic reside-binding. Besides, AP-1 mu1B subunit mediates the basolateral recycling of low-density lipoprotein receptor (LDLR) and transferrin receptor (TfR) from the sorting endosomes, where the basolateral sorting signal does not belong to the tyrosine-based signals. Thus, the binding site in mu1B subunit of AP-1 for the signals of LDLR and TfR might be distinct from that for YXXPhi signals.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 119625812  32 NAVSWCSERIKYKKNEVFIDVIESANLLVNAKGSILLSEIFGTIKLKVFLSGMPELRLGLNDSMLFELTGCHKNKLVKLE 111
Cdd:cd09259    2 NAVSWRSEGIKYKKNEVFIDVIESVNVLVNANGSVLSSEIVGCIKLKVFLSGMPELRLGLNDRVLFELTGRDKNKTVELE 81

                 ....*.
gi 119625812 112 DVKFQH 117
Cdd:cd09259   82 DVKFHQ 87

C-terminal domain of medium Mu1 subunit in clathrin-associated adaptor protein (AP) complex AP-1; AP complexes participate in the formation of intracellular coated transport vesicles and select cargo molecules for incorporation into the coated vesicles in the late secretory and endocytic pathways. There are four AP complexes, AP-1, AP-2, AP-3, and AP-4, described in various eukaryotic organisms. Each AP complex consists of four subunits: two large chains (one each of gamma/alpha/delta/epsilon and beta1-4, respectively), a medium mu chain (mu1-4), and a small sigma chain (sigma1-4). Each of the four subunits from the different AP complexes exhibits similarity with each other. This family corresponds to the C-terminal domain of heterotetrameric clathrin-associated adaptor protein complex 1 (AP-1) medium mu1 subunit, which includes two closely related homologs, mu1A (encoded by ap1m1) and mu1B (encoded by ap1m2). Mu1A is ubiquitously expressed, but mu1B is expressed exclusively in polarized epithelial cells. AP-1 has been implicated in bi-directional transport between the trans-Golgi network (TGN) and endosomes. It plays an essential role in the formation of clathrin-coated vesicles (CCVs) from the trans-Golgi network (TGN). Epithelial cell-specific AP-1 is also involved in sorting to the basolateral surface of polarized epithelial cells. Recruitment of AP-1 to the TGN membrane is regulated by a small GTPase, ADP-ribosylation factor 1 (ARF1). Phosphorylation/dephosphorylation events can also regulate the function of AP-1. The membrane-anchored cargo molecules can be linked to the outer lattice of CCVs by AP-1. Those cargo molecules interact with adaptors through short sorting signals in their cytosolic segments. Tyrosine-based endocytotic signals are one of the most important sorting signals. They are of the form Y-X-X-Phi, where Y is tyrosine, X is any amino acid and Phi is a bulky hydrophobic residue that can be Leu, Ile, Met, Phe, or Val. These kinds of sorting signals can be recognized by the C-terminal domain of AP-1 mu1 subunit, also known as Y-X-X-Phi signal-binding domain that contains two hydrophobic pockets, one for the tyrosine-binding and one for the bulky hydrophobic residue-binding.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 119625812  32 NAVSWCSERIKYKKNEVFIDVIESANLLVNAKGSILLSEIFGTIKLKVFLSGMPELRLGLNDSMLFELTGCH-KNKLVKL 110
Cdd:cd09250    2 NAVSWRPEGIKYKKNEVFLDVIESVNLLVDLNGQVLRSEIVGAIKMRSYLSGMPELKLGLNDKVLFEATGRSsKGKAVEL 81

                 ....*
gi 119625812 111 EDVKF 115
Cdd:cd09250   82 EDVKF 86

Adaptor complexes medium subunit family; This family also contains members which are coatomer subunits.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 119625812   34 VSWCSERIKYKKNEVFIDVIESANLLVNAKGSILLSEIFGTIKLKVFLSGMPELRLGLNDsmlfeltgchKNKLVKLEDV 113
Cdd:pfam00928   1 VPWRPPGIKYKKNEVFLDVIERVSVIVDKDGGLLNSEVQGTIDLKCFLSGMPELRLGLND----------KLLLIELDDV 70

                  ....
gi 119625812  114 KFQH 117
Cdd:pfam00928  71 SFHQ 74

C-terminal domain of medium Mu4 subunit in adaptor protein (AP) complex AP-4; AP complexes participate in the formation of intracellular coated transport vesicles and select cargo molecules for incorporation into the coated vesicles in the late secretory and endocytic pathways. There are four AP complexes, AP-1, AP-2, AP-3, and AP-4, described in various eukaryotic organisms. Each AP complex consists of four subunits: two large chains (one each of gamma/alpha/delta/epsilon and beta1-4, respectively), a medium mu chain (mu1-4), and a small sigma chain (sigma1-4). Each of the four subunits from the different AP complexes exhibits similarity with each other. This family corresponds to the C-terminal domain of heterotetrameric adaptor protein complex 4 (AP-4) medium mu4 subunit. AP-4 plays a role in signal-mediated trafficking of integral membrane proteins in mammalian cells. Unlike other AP complexes, AP-4 is found only in mammals and plants. It is believed to be part of a nonclathrin coat, since it might function independently of clathrin, a scaffolding protein participating in the formation of coated vesicles. Recruitment of AP-4 to the trans-Golgi network (TGN) membrane is regulated by a small GTPase, ADP-ribosylation factor 1 (ARF1) or a related protein. Membrane-anchored cargo molecules interact with adaptors through short sorting signals in their cytosolic segments. One of the most important sorting signals binding to mu subunits of AP complexes are tyrosine-based endocytotic signals, which are of the form Y-X-X-Phi, where Y is tyrosine, X is any amino acid and Phi is a bulky hydrophobic residue that can be Leu, Ile, Met, Phe, or Val. However, AP-4 does not bind most canonical tyrosine-based signals except for two naturally occurring ones from the lysosomal membrane proteins CD63 and LAMP-2a. It binds YX [FYL][FL]E motif, where X can be any residue, from the cytosolic tails of amyloid precursor protein (APP) family members in a distinct way.

                         10        20        30        40        50
                 ....*....|....*....|....*....|....*....|....*....|
gi 119625812  44 KKNEVFIDVIESANLLVNAKGSILLSEIFGTIKLKVFLSGMPELRLGLND 93
Cdd:cd09253    9 KRNEIFVDVLERLSVVFNANGQVLNSEIDGSIQMKSYLPGNPELRLALNE 58

C-terminal domain of medium Mu3A subunit in ubiquitously expressed adaptor protein (AP) complex AP-3; AP complexes participate in the formation of intracellular coated transport vesicles and select cargo molecules for incorporation into the coated vesicles in the late secretory and endocytic pathways. There are four AP complexes, AP-1, AP-2, AP-3, and AP-4, described in various eukaryotic organisms. Each AP complex consists of four subunits: two large chains (one each of gamma/alpha/delta/epsilon and beta1-4, respectively), a medium mu chain (mu1-4), and a small sigma chain (sigma1-4). Each of the four subunits from the different AP complexes exhibits similarity with each other. This subfamily corresponds to the C-terminal domain of heterotetrameric adaptor protein complex 3 (AP-3) medium mu3A subunit encoded by ap3m1gene. Mu3A is ubiquitously expressed in all mammalian tissues and cells. It appears to be localized to the trans-Golgi network (TGN) and/or endosomes and participates in trafficking to the vacuole/lysosome in yeast, flies, and mammals. Unlike AP-1 and AP-2, which function in conjunction with clathrin which is a scaffolding protein participating in the formation of coated vesicles, the nature of the outer shell of ubiquitous AP-3 containing coats remains to be elucidated. Membrane-anchored cargo molecules interact with adaptors through short sorting signals in their cytosolic segments. Tyrosine-based endocytotic signals are one of the most important sorting signals. They are of the form Y-X-X-Phi, where Y is tyrosine, X is any amino acid and Phi is a bulky hydrophobic residue that can be Leu, Ile, Met, Phe, or Val. These kinds of sorting signals can be recognized by the C-terminal domain of AP-3 mu3A subunit, also known as Y-X-X-Phi signal-binding domain that contains two hydrophobic pockets, one for the tyrosine-binding and one for the bulky hydrophobic residue-binding.

                         10        20        30        40        50        60        70
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 119625812  34 VSWCSERIKYKKNEVFIDVIESANLLVNAKGSILLSEIFGTIKLKVFLSGMPELRLGLNDSMLFELTGCH 103
Cdd:cd09260    1 IPWRRAGVKYTNNEAYFDVVEEIDAIIDKSGSTVFAEIQGVIDACIKLSGMPDLSLSFMNPRLLDDVSFH 70

Mu homology domain (MHD) of adaptor-like proteins (AP-like), stonins; A small family of proteins named stonins has been characterized as clathrin-dependent AP-2 mu2 chain related factors, which may act as cargo-specific sorting adaptors in endocytosis. Stonins include stonin 1 and stonin 2, which are only mammalian homologs of Drosophila stoned B, a presynaptic protein implicated in neurotransmission and synaptic vesicle (SV) recycling. They are conserved from C. elegans to humans, but are not found in prokaryotes or yeasts. This family corresponds to the mu homology domain of stonins, which is distantly related to the C-terminal domain of mu chains among AP complexes. Due to the low degree of sequence conservation of the corresponding binding site, the mu homology domain of stonins is unable to recognize tyrosine-based endocytic sorting signals. To data, little is known about the localization and function of stonin 1. Stonin 2, also known as stoned B, acts as an AP-2-dependent synaptotagmin-specific sorting adaptors for SV endocytosis. Stoned A is not a stonin. It is structurally unrelated to the adaptins and does not appear to have mammalian homologs. It is not included in this family.

                         10        20        30        40        50        60        70
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....
gi 119625812  41 IKYKKNEVFIDVIESANLLVNAKGSILLSEIFGTIKLKVFLSGMPELRLGLNDSMLfeltgcHKNKLVKLEDVK 114
Cdd:cd09255    6 ITYREDEITVDVTDEFHGKVTKTGEIKKLGVTVQIHILSFVTGDPECVLGLNDLEV------EGREVVRRQDIM 73

AP-1 complex subunit mu N-terminal domain; AP-1 complex mu subunit is part of the heterotetrameric adaptor protein (AP)-1 complex which consists of one large gamma-, one beta-, one mu-, and one sigma-subunit. AP complexes link the cytosolic domains of the cargo proteins to the protein coat that induces vesicle budding in the donor compartment during vesicle transport. In the case of AP-1 the coat protein is clathrin. AP-1 binds the phospholipid PI(4)P which plays a role in its localisation to the trans-Golgi network (TGN)/endosome. The mu subunit is comprised of an N-terminal longin domain followed by a C-terminal domain which is involved in the binding of the Y-X-X-Phi sorting signal.

                         10
                 ....*....|....*....
gi 119625812   1 QTTNSKILQEYTTQQSNKL 19
Cdd:cd14835  121 QTTESKILQEYITQESHKL 139