Journal of Biological Chemistry
Volume 271, Issue 6, 9 February 1996, Pages 3132-3140
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Cell Biology and Metabolism
The OST4 Gene of Saccharomyces cerevisiae Encodes an Unusually Small Protein Required for Normal Levels of Oligosaccharyltransferase Activity (∗)

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Sodium vanadate is an effective drug for the enrichment of yeast mutants defective in glycosylation reactions that are carried out in the Golgi complex(1). We have isolated vanadate-resistant, hygromycin B-sensitive mutants that act at very early steps of N-linked glycosylation, occurring in the endoplasmic reticulum. Here we describe the phenotypic characterization of ost4, a vanadate-resistant mutant that is defective in oligosaccharyltransferase (OTase) activity both in vivo and in vitro. The OST4 open reading frame is unusual in that it predicts a protein of only 36 amino acids. We demonstrate that the OST4 gene product is, in fact, an unusually small protein of approximately 3.6 kDa, predicted to lie almost entirely in the hydrophobic environment of the membrane. Strains carrying a disruption of the OST4 gene are viable but grow poorly at 25°C. The null mutant is inviable at 37°C, demonstrating that the OST4 gene product is essential for growth at high temperatures. Deletion of the OST4 gene greatly diminishes OTase activity but does not abolish it. These results suggest that the OST4 gene encodes a subunit or accessory component of OTase that is essential at high temperature.

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This work was supported in part by Grant JFRA457 from the American Cancer Society (to N. D.) and Grant GM33185 from the National Institutes of Health (to W. J. L.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank(™)/EMBL Data Bank with accession number(s) U43175 [GenBank] and U43176.

§

Present address: Hormone Research Inst., and Dept. of Biochemistry and Biophysics, University of California, San Francisco, CA 94143-0534.