cd09252

C-terminal domain of medium Mu3 subunit in 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 family corresponds to the C-terminal domain of heterotetrameric adaptor protein complex 3 (AP-3) medium mu3 subunit, which includes two closely related homologs, mu3A (P47A, encoded by ap3m1) and mu1B (P47B, encoded by ap3m2). Mu3A is ubiquitously expressed, but mu3B is specifically expressed in neurons and neuroendocrine cells. AP-3 is particularly important for targeting integral membrane proteins to lysosomes and lysome-related organelles at trans-Golgi network (TGN) and/or endosomes, such as the yeast vacuole, fly pigment granules and mammalian melanosomes, platelet dense bodies and the secretory lysosomes of cytotoxic T lymphocytes. 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 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 mu3 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.