COPII is a vesicle-coat system associated with export from the endoplasmic reticulum. Sar1 recruits an inner Sec23/Sec24 layer, and Sec13/Sec31 forms an outer coat. Cargo selection and membrane deformation are coupled but experimentally separable.
The Sec13-Sec31 complex forms the outer layer of a COPII coat. Its assemblies provide a scaffold around a budding carrier, while the inner Sec23-Sec24 complex couples coat formation to Sar1 GTPase and cargo selection. Reconstitution using purified coat components and defined liposomes demonstrated that this machinery can generate coated carriers without the full complexity of a cell. [ref]
COPII-coated vesicle formation reconstituted with purified coat proteins and chemically defined liposomes by Ken Matsuoka and others. 1998, purified Sar1p, Sec23-24p and Sec13-31p reconstitution
The Sec23-Sec24 complex forms the inner COPII coat layer. Sec24 provides cargo-recognition sites, while Sec23 binds Sar1 GTPase and contributes to regulation of its GTPase cycle. The outer Sec13-Sec31 complex then organizes the coat scaffold. Distinct cargo-binding sites on Sec24 explain how one coat can capture different cargo classes rather than relying on a single universal recognition sequence. [ref]
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