The oligosaccharyltransferase complex from yeast

N-Glycosylation of eukaryotic secretory and membrane-bound proteins is an e ssential and highly conserved protein modification. The key step of this pa thway is the en bloc transfer of the high mannose core oligosaccharide Glc( 3)Man(9)GlcNAc(2) from the lipid carrier dolichyl phosphate to selected Asn -X-Ser/Thr sequences of nascent polypeptide chains during their translocati on across the endoplasmic reticulum membrane. The reaction is catalysed by the enzyme oligosaccharyltransferase (OST). Recent biochemical and molecula r genetic studies in yeast have yielded novel insights into this enzyme wit h multiple tasks. Nine proteins have been shown to be OST components. These are assembled into a heterooligomeric membrane-bound complex and are requi red for optimal expression of OST activity in vivo in wild type cells. In a ccord with the evolutionary conservation of core N-glycosylation, there are significant homologies between the protein sequences of OST subunits from yeast and higher eukaryotes, and OST complexes from different sources show a similar organisation as well. (C) 1999 Elsevier Science B.V. All rights r eserved.