SEC12 Encodes a Guanine-nucleotide-exchange Factor Essential for Transport Vesicle Budding from the ER

In yeast a type II integral membrane glycoprotein that is essential for transport vesicle budding from the endoplasmic reticulum (ER) is encoded by SEC12 (refs 1-3). SAR1 was discovered as a multicopy suppressor of the sec12-1ts strain and encodes a GTPase of M(r) 21,000 (21K) also essential for vesicle budding from the ER. Sar1 is a peripherally associated membrane protein which shows enhanced membrane binding in cells containing elevated levels of Sec12 protein (refs 6, 7). We show here that a purified fragment of Sec12 promotes guanine-nucleotide dissociation from Sar1 whereas the purified mutant Sec12-1 has only 15% of the wild-type activity. GTP hydrolysis by Sar1 is not enhanced by Sec12, but is stimulated more than 50-fold by a mixture of Sec12 and Sec23, a GTPase-activating protein specific for Sar1 (ref. 8). We propose that Sec12 catalyses Sar1 guanine-nucleotide exchange in a process that recruits Sar1 to a vesicle formation site on the ER membrane.