C-terminal domain of medium Mu1A subunit in ubiquitously expressed 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 subfamily corresponds to the C-terminal domain of heterotetrameric clathrin-associated adaptor protein complex 1 (AP-1) medium mu1A subunit encoded by ap1m1 gene, which is ubiquitously expressed in all mammalian tissues and cells. AP-1 has been implicated in bidirectional transport between the trans-Golgi network (TGN) and endosomes. It is involved in the formation of clathrin-coated vesicles (CCVs) from the trans-Golgi network (TGN). The ubiquitous AP-1 is recruited to the TGN membrane, as well as to immature secretory granules. 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 mu1A 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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",