Mu-homology domain (MHD) of muniscins adaptor proteins (AP) and similar proteins
This family corresponds to the MHD found in muniscins, a novel family of endocytic adaptor proteins. The term, muniscins, has been assigned to name the MHD of proteins with both EFC/F-BAR domain and MHD. These two domains are responsible for the membrane-tubulation activity associated with transmembrane cargo proteins. Members in this family include an endocytic adaptor Syp1, the mammalian FCH domain only proteins (FCHo1/2), SH3-containing GRB2-like protein 3-interacting protein 1 (SGIP1), and related uncharacterized proteins. Syp1 is a poorly characterized yeast protein with multiple biological functions. Syp1 contains 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 that can directly binds to the endocytic adaptor/scaffold protein Ede1 or a transmembrane stress sensor cargo protein Mid2. Thus, Syp1 represents a novel type of endocytic adaptor protein that participates in endocytosis, promotes vesicle tabulation, and contributes to cell polarity and stress response. Syp1 shares the same domain architecture with its two ubiquitously expressed mammalian counterparts, the membrane-sculpting F-BAR domain-containing Fer/Cip4 homology domain-only proteins 1 and 2 (FCHo1/2). FCHo1/2 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. 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.",