Role of Erv29p in collecting soluble secretory proteins into ER-derived transport vesicles.

Proteins are transported from the endoplasmic reticulum (ER) in vesicles formed by coat protein complex II (COPII). Soluble secretory proteins are thought to leave the ER in these vesicles by "bulk flow" or through recognition by hypothetical shuttling receptors. We found that Erv29p, a conserved transmembrane protein, was directly required for packaging glycosylated pro-alpha-factor (gpalphaf) into COPII vesicles in Saccharomyces cerevisiae. Further, an Erv29p-gpalphaf complex was isolated from ER-derived transport vesicles. In vivo, export of gpalphaf from the ER was saturable and depended on the expression level of Erv29p. These results indicate that membrane receptors can link soluble cargo proteins to the COPII coat.