BOS1P, AN INTEGRAL MEMBRANE-PROTEIN OF THE ENDOPLASMIC-RETICULUM TO GOLGI TRANSPORT VESICLES, IS REQUIRED FOR THEIR FUSION COMPETENCE

BOS1 encodes an integral endoplasmic reticulum (ER) membrane protein a nd genetically interacts with three other yeast genes (BET1, SEC22, an d YPT1) whose products are required for membrane traffic between the E R and the Golgi apparatus. Using an assay that reconstitutes transport at this stage of the pathway, we find that anti-Bos1p antibody blocks protein export after vesicles bud from the ER but prior to fusion wit h the Golgi. Additionally, the depletion of Bos1p from the ER leads to the formation of transport-incompetent vesicles. Carrier vesicles, im munoisolated with anti-Bos1p antibody, are approximately 50 nm in size . These vesicles contain Bos1p, Sec22p, and Ypt1p, but not Bet1p. The functional interactions of Bos1p with Ypt1p and Sec22p may be necessar y for the fusion competence of the ER to Golgi transport vesicles.