Abstract
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.
Publication types
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Research Support, Non-U.S. Gov't
MeSH terms
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ADP-Ribosylation Factors
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Biological Transport
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Endoplasmic Reticulum / metabolism*
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Fungal Proteins / genetics
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Fungal Proteins / metabolism*
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GTP Phosphohydrolases / metabolism
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GTP-Binding Proteins / metabolism*
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Genes, Fungal
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Guanine Nucleotide Exchange Factors
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Guanine Nucleotides / metabolism*
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Guanosine Diphosphate / metabolism
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Guanosine Triphosphate / metabolism
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Intracellular Membranes / metabolism
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Membrane Glycoproteins / genetics
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Membrane Glycoproteins / metabolism*
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Monomeric GTP-Binding Proteins*
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Peptide Fragments / metabolism
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Saccharomyces cerevisiae / genetics
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Saccharomyces cerevisiae / metabolism
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Saccharomyces cerevisiae Proteins*
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Vesicular Transport Proteins
Substances
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Fungal Proteins
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Guanine Nucleotide Exchange Factors
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Guanine Nucleotides
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Membrane Glycoproteins
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Peptide Fragments
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SEC12 protein, S cerevisiae
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Saccharomyces cerevisiae Proteins
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Vesicular Transport Proteins
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Guanosine Diphosphate
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Guanosine Triphosphate
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GTP Phosphohydrolases
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GTP-Binding Proteins
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ADP-Ribosylation Factors
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Monomeric GTP-Binding Proteins
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SAR1 protein, S cerevisiae