COPII and selective export from the endoplasmic reticulum

Forward transport of proteins from the endoplasmic reticulum (ER) to the Golgi complex depends on COPII, a membrane coat that forms ER-derived vesicles. Based on experimental observations, a series of integrated events must be accomplished during the formation of COPII coated vesicles. First, the subunits of the COPII coat must be recruited to the correct site on the surface of the ER. Second, soluble and integral membrane cargo proteins destined for the Golgi complex are concentrated into nascent buds. Third, a set of molecules that must cycle between the ER and Golgi compartments (such as SNARE proteins) are incorporated into vesicles. And fourth, the COPII coat is disassembled after release of ER-derived vesicles thus allowing vesicle fusion and recycling of COPII components. Incorporation of soluble cargo infers the existence of membrane spanning receptor molecules that link lumenal cargo to the vesicle coat. Some candidate proteins have been identified (including the p24 family) that appear to participate in the selection of soluble cargo; however, the mechanistic details of this selection procedure remain obscure. This review will focus on the molecular constituents of the COPII coat and emerging interactions of the coat subunits with proteins involved in selective export from the ER.