SED4 ENCODES A YEAST ENDOPLASMIC-RETICULUM PROTEIN THAT BINDS SEC16P AND PARTICIPATES IN VESICLE FORMATION

SEC16 is required for transport vesicle budding from the ER in Sacchar omyces cerevisiae, and encodes a large hydrophilic protein found on th e ER membrane and as part of the coat of transport vesicles. In a scre en to find functionally related genes, we isolated SED4 as a dosage-de pendent suppressor of temperature-sensitive SEC16 mutations. Sed4p is an integral ER membrane protein whose cytosolic domain binds to the CO OH-tenninal domain of Sec16p as shown by two-hybrid assay and coprecip itation. The interaction between Sed4p and Sec16p probably occurs befo re budding is complete, because Sed4p is not found in budded vesicles. Deletion of SED4 decreases the rate of ER to Golgi transport, and exa cerbates mutations defective in vesicle formation, but not those that affect later steps in the secretory pathway. Thus, Sed4p is important, but not necessary, for vesicle formation at the ER. Sec12p, a close h omologue of Sed4p, also acts early in the assembly of transport vesicl es. However, SEC12 performs a different function than SED4 since Sec12 p does not bind Sec16p, and genetic tests show that SEC12 and SED4 are not functionally interchangeable. The importance of Sed4p for vesicle formation is underlined by the isolation of a phenotypically silent m utation, sar1-5, that produces a strong ER to Golgi transport defect w hen combined with sed4 mutations. Extensive genetic interactions betwe en SAR1, SED4, and SEC16 show close functional links between these pro teins and imply that they might function together as a multisubunit co mplex on the ER membrane.