THE STT3 PROTEIN IS A COMPONENT OF THE YEAST OLIGOSACCHARYLTRANSFERASE COMPLEX

N-Linked protein glycosylation is an essential process in eukaryotic c ells. In the central reaction, the oligosaccharyltransferase (OTase) c atalyzes the transfer of the oligosaccharide Glc(3)Man(9)GlcNAc(2) fro m dolicholpyrophosphate onto asparagine residues of nascent polypeptid e chains in the lumen of the endoplasmic reticulum. The product of the essential gene STT3 is required for OTase activity in vivo, but is no t present in highly purified OTase preparations. Using affinity purifi cation of a tagged Stt3 protein, we now demonstrate that other compone nts of the OTase complex, namely Ost1p, Wbp1p and Swp1p, specifically co-purify with the Stt3 protein. Tn addition, different conditional st t3 alleles can be suppressed by overexpression of either OST3 and OST4 , which encode small components of the OTase complex, These genetic an d biochemical data show that the highly conserved Stt3p is a component of the oligosaccharyltransferase complex.