The STT3 protein is a component of the yeast oligosaccharyltransferase complex

N-linked protein glycosylation is an essential process in eukaryotic cells. In the central reaction, the oligosaccharyltransferase (OTase) catalyzes the transfer of the oligosaccharide Glc3Man9GlcNac2 from dolicholpyrophosphate onto asparagine residues of nascent polypeptide chains in the lumen of the endoplasmic reticulum. The product of the essential gene STT3 is required for OTase activity in vivo, but is not present in highly purified OTase preparations. Using affinity purification of a tagged Stt3 protein, we now demonstrate that other components of the OTase complex, namely Ost1p, Wbp1p and Swp1p, specifically co-purify with the Stt3 protein. In addition, different conditional stt3 alleles can be suppressed by overexpression of either OST3 and OST4, which encode small components of the OTase complex. These genetic and biochemical data show that the highly conserved Stt3p is a component of the oligosaccharyltransferase complex.