COPII AND SELECTIVE EXPORT FROM THE ENDOPLASMIC-RETICULUM

Forward transport of proteins from the endoplasmic reticulum (ER) to t he Golgi complex depends on COPII, a membrane coat that forms ER-deriv ed vesicles. Based on experimental observations, a series of integrate d events must be accomplished during the formation of COPII coated ves icles. First, the subunits of the COPII coat must be recruited to the correct site on the surface of the ER. Second, soluble and integral me mbrane cargo proteins destined for the Golgi complex are concentrated into nascent buds. Third, a set of molecules that must cycle between t he ER and Golgi compartments (such as SNARE proteins) are incorporated into vesicles. And fourth, the COPII coat is disassembled after relea se of ER-derived vesicles thus allowing vesicle fusion and recycling o f COPII components. Incorporation of soluble cargo infers the existenc e of membrane spanning receptor molecules that link lumenal cargo to t he vesicle coat. Some candidate proteins have been identified (includi ng the p24 family) that appear to participate in the selection of solu ble cargo; however, the mechanistic details of this selection procedur e 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. (C) 1998 Elsevier Science B.V. All rights reserved.