Human oligosaccharyltransferase: isolation, characterization, and the complete amino acid sequence of 50-kDa subunit

Oligosaccharyltransferase (OT) catalyzes the glycosylation of asparagine residues in nascent polypeptides in the endoplasmic reticulum. In a previous communication we reported the purification and characterization of this enzyme from chicken oviduct. Here we describe the purification and sequence analysis of OT from human liver microsomes. Oligosaccharyltransferase copurified with three proteins designated 50-kDa, 65-I and 65-II based on their molecular weights by gel electrophoresis. The N-terminal sequence of the 50-kDa component was homologous to the 50-kDa subunit of avian OT. The N-terminal sequences of 65-I and 65-II were identical to the primary structures of human ribophorins I and II, respectively, predicted by cDNA sequencing. The complete amino acid sequence of the 50-kDa subunit of human OT was determined by chemical sequencing of peptides isolated from chemical and enzymatic digests. The 50-kDa subunit of human OT is 98% identical to its canine homolog, 93% identical to its avian homolog, and 25% identical to the beta subunit of yeast OT. These data indicate that structural features of oligosaccharyltransferase are conserved in all eukaryotes.