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.
Feature 1:signal peptide binding site [polypeptide binding site]
Evidence:
Structure:4IKN; Rattus norvegicus mu3A subunit of the AP-3 adaptor complex with bound sorting peptide, contacts 4A
Comment:C-terminal domain of heterotetrameric adaptor protein (AP) complexes medium mu subunits binds Y-X-X-Phi-type tyrosine-based signals, 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.
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