The Saccharomyces cerevisiae oligosaccharyltransferase is a protein complex composed of Wbp1p, Swp1p, and four additional polypeptides.

Abstract

Asparagine-linked glycosylation of proteins in the lumen of the endoplasmic reticulum is catalyzed by the oligosaccharyltransferase. Previously, the mammalian oligosaccharyltransferase was shown to co-purify with a protein complex consisting of three integral membrane proteins: ribophorin I and ribophorin II and a nonglycosylated 48-kDa polypeptide designated OST48. Here, we describe the purification of the oligosaccharyltransferase from Saccharomyces cerevisiae. The yeast oligosaccharyltransferase complex is composed of six subunits (alpha, beta, gamma, delta, epsilon, and zeta). The alpha subunit of the yeast oligosaccharyltransferase complex is a heterogeneously glycosylated protein with three glycoforms of 64, 62, and 60 kDa that contain, respectively, four, three, and two asparagine-linked oligosaccharide chains. The beta and delta subunits were shown to correspond to the 45-kDa Wbp1 glycoprotein and the 30-kDa Swp1 protein, respectively. The Wbp1 and Swp1 proteins were previously shown to be essential for asparagine-linked glycosylation in vivo. The nonglycosylated gamma, epsilon, and zeta subunits have apparent molecular masses of 34, 16, and 9 kDa. Homology between the yeast and mammalian oligosaccharyltransferase complexes first became evident when the 48-kDa subunit of the mammalian enzyme was found to be 25% identical in sequence with the Wbp1 protein. Here we present an alignment between the Swp1 protein and the carboxyl-terminal half of human ribophorin II that reveals that these two proteins are related gene products.