C-terminal domain of adaptor protein (AP) complexes medium mu subunits and its homologs (MHD)
This family corresponds to the C-terminal domain of heterotetrameric AP complexes medium mu subunits and its homologs existing in monomeric stonins, delta-subunit of the heteroheptameric coat protein I (delta-COPI), a protein encoded by a pro-death gene referred as MuD (also known as MUDENG, mu-2 related death-inducing gene), an endocytic adaptor syp1, the mammalian FCH domain only proteins (FCHo1/2), SH3-containing GRB2-like protein 3-interacting protein 1 (SGIP1), and related proteins. 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. Stonins have been characterized as clathrin-dependent AP-2 mu chain related factors and may act as cargo-specific sorting adaptors in endocytosis. Coat protein complex I (COPI)-coated vesicles function in the early secretory pathway. They mediate the retrograde transport from the Golgi to the ER, and intra-Golgi transport. MuD is distantly related to the C-terminal domain of mu2 subunit of AP-2. It is able to induce cell death by itself and plays an important role in cell death in various tissues. Syp1 represents a novel type of endocytic adaptor protein that participates in endocytosis, promotes vesicle tabulation, and contributes to cell polarity and stress responses. It shares the same domain architecture with its two ubiquitously expressed mammalian counterparts, FCHo1/2, which represent key initial proteins ultimately controlling cellular nutrient uptake, receptor regulation, and synaptic vesicle retrieval. 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 SGIP1 does have a C-terminal MHD and has been classified into this family as well. It 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.",