Mu homology domain (MHD) of adaptor protein (AP) coat protein I (COPI) delta subunit
COPI complex-coated vesicles function in the early secretory pathway. They mediate the retrograde transport from the Golgi to the ER, and intra-Golgi transport. COPI complex-coated vesicles consist of a small GTPase, ADP-ribosylation factor 1 (ARF1) and a heteroheptameric coatomer composed of two subcomplexes, F-COPI and B-COPI. ARF1 regulates COPI vesicle formation by recruiting the coatomer onto Golgi membranes to initiate its coat function. Coatomer complexes then bind cargo molecules and self-assemble to form spherical cages that yield COPI-coated vesicles. The heterotetrameric F-COPI subcomplex contains beta-, gamma-, delta-, and zeta-COP subunits, where beta- and gamma-COP subunits are related to the large AP subunits, and delta- and zeta-COP subunits are related to the medium and small AP subunits, respectively. Due to the sequence similarity to the AP complexes, the F-COPI subcomplex might play a role in the cargo-binding. The heterotrimeric B-COPI contains alpha-, beta-, and epsilon-COP subunits, which are not related to the adaptins. This subcomplex is thought to participate in the cage-forming and might serve a function similar to that of clathrin. This family corresponds to the mu homology domain of delta-subunit of COPI complex (delta-COP), which is distantly related to the C-terminal domain of mu chains among AP complexes. The delta-COP subunit appears tightly associated with the beta-COP subunit to confer its interaction with ARF1. In addition, both delta- and beta-COP subunits contribute to a common binding site for arginine (R)-based signals, which are sorting motifs conferring transient endoplasmic reticulum (ER) localization to unassembled subunits of multimeric membrane proteins.
Jiyao W et al.(2023). "The conserved domain database in 2023.",