COPII-coated endoplasmic reticulum (ER)-derived transport vesicles contain a distinct set of membranebound polypeptides. We have obtained the NH 2 -terminal amino acid sequence of polypeptide constituents found on purified vesicles and in this report investigate the 24and 25-kDa species. The 24-kDa protein is identical to Emp24p, a type I transmembrane protein that is required for transport of a subset of secretory proteins from the ER to the Golgi complex (Schimmö ller, F., Singer-Krü ger, B.,

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... Schrö der, S., Krü ger, U., Barlowe, C., and Riezman, H. (1995) EMBO J. 14, 1329 -1339). The 25-kDa protein, termed Erv25p (ER vesicle protein of 25 kDa), corresponds to an open reading frame found on chromosome XIII of Saccharomyces cerevisiae. Erv25p shares overall sequence identity with Emp24p, but the two proteins are not functionally interchangeable. Antibodies directed against Erv25p reveal that Emp24p and Erv25p depend on each other for stability and form a protein complex that can be isolated after chemical cross-linking. Yeast strains lacking Erv25p (erv25⌬) are viable and display the same selective defect in transport of secretory proteins from the ER to Golgi complex as an emp24⌬ strain. A cell-free assay that measures vesicle formation from ER membranes demonstrates that Erv25p and Emp24p are incorporated equally into ERderived vesicles when COPII-coated budding is reconstituted. Vesicle formation from an erv25⌬ strain, an emp24⌬ strain and a double erv25⌬ emp24⌬ strain proceed at wild-type levels; however, incorporation of the Erv25p or the Emp24p protein into COPII-coated vesicles requires expression of both subunits. A potential model for transport of the Erv25p-Emp24p complex between the ER and Golgi compartments is discussed.