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.
Barlowe
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Department of Molecular and Cell Biology, Howard Hughes Medical Institute, University of California, Berkeley 94720.
Schekman
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D020727:ADP-Ribosylation Factors; D001692:Biological Transport; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism*; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D005800:Genes, Fungal; D020662:Guanine Nucleotide Exchange Factors; D006150:Guanine Nucleotides / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D020559:Monomeric GTP-Binding Proteins; D010446:Peptide Fragments / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins
D005656:Fungal Proteins; D020662:Guanine Nucleotide Exchange Factors; D006150:Guanine Nucleotides; D008562:Membrane Glycoproteins; D010446:Peptide Fragments; C071239:SEC12 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D020727:ADP-Ribosylation Factors; D020559:Monomeric GTP-Binding Proteins; C077395:SAR1 protein, S cerevisiae