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.
Feature 1:signal peptide binding site [polypeptide binding site]
Evidence:
Structure:3L81; Homo sapiens AP-4 complex with bound sorting paptide from amyloid precursor protein, contacts 4A
Comment: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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",