C-terminal domain of medium Mu2 subunit in ubiquitously expressed clathrin-associated adaptor protein (AP) complex AP-2
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, -2, -3, and -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 2 (AP-2) medium mu2 subunit. Mu2 is ubiquitously expressed in mammals. In higher eukaryotes, AP-2 plays a critical role in clathrin-mediated endocytosis from the plasma membrane in different cells. The membrane-anchored cargo molecules can be linked to the outer lattice of CCVs by AP-2. 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-2 mu2 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. Since the Y-X-X-Phi binding site is buried in the core structure of AP-2, a phosphorylation induced conformational change is required when the cargo molecules binds to AP-2. In addition, the C-terminal domain of mu2 subunit has been shown to bind other molecules. For instance, it can bind phosphoinositides, in particular PI[4,5]P2, which might be involved in the recognition process of the tyrosine-based signals. It can also interact with synaptotagmins, a family of important modulators of calcium-dependent neurosecretion within the synaptic vesicle (SV) membrane. Since many of the other endocytic adaptors responsible for biogenesis of synaptic vesicles exist, in the absence of AP-2, clathrin-mediated endocytosis can still occur. However, the cells may not survive in the complete absence of clathrin as well as AP-2.
Feature 1:signal peptide binding site [polypeptide binding site]
Evidence:
Structure:1H6E; Human mu2 adaptin subunit of AP-2 adaptor with bound signal peptides
Comment:Based on structure that shows that human mu2 adaptin subunit of AP-2 adaptor binds signal peptides.
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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",