Mu-homology domain (MHD) of endocytic adaptor protein (AP), Syp1
This family corresponds to the MHD found in the metazoan counterparts of yeast Syp1, which includes two ubiquitously expressed membrane-sculpting F-BAR domain-containing Fer/Cip4 homology domain-only proteins 1 and 2 (FCH domain only 1 and 2, or FCHo1/FCHo2), neuronal-specific SH3-containing GRB2-like protein 3-interacting protein 1 (SGIP1), and related uncharacterized proteins. FCHo1/FCHo2 represent key initial proteins ultimately controlling cellular nutrient uptake, receptor regulation, and synaptic vesicle retrieval. They are required for plasma membrane clathrin-coated vesicle (CCV) budding and marked sites of CCV formation. They bind specifically to the plasma membrane and recruit the scaffold proteins eps15 and intersectin, which subsequently engage the adaptor complex AP2 and clathrin, leading to coated vesicle formation. Both FCHo1/FCHo2 contain an N-terminal EFC/F-BAR domain that induces membrane tabulation, a proline-rich domain (PRD) in the middle region, and a C-terminal MHD responsible for the binding of eps15 and intersectin. Another mammalian neuronal-specific protein, neuronal-specific transcript Scr homology 3 (SH3)-domain growth factor receptor-bound 2 (GRB2)-like (endophilin) interacting protein 1 [SGIP1] does not contain EFC/F-BAR domain, but does have a PRD and a C-terminal MHD and has been classified into this family as well. SGIP1 is an endophilin-interacting protein that plays an obligatory role in the regulation of energy homeostasis. It is also involved in clathrin-mediated endocytosis by interacting with phospholipids and eps15.
Jiyao W et al.(2023). "The conserved domain database in 2023.",