Ost4 is a very short, approximately 30 residues, enzyme found from fungi to vertebrates. It is a member of the ER oligosaccaryltansferase complex, EC 2.4.1.119, that catalyses the asparagine-linked glycosylation of proteins. It appears to be an integral membrane protein that mediates the en bloc transfer of a preassembled high-mannose oligosaccharide onto asparagine residues of nascent polypeptides as they enter the lumen of the rough endoplasmic reticulum (RER).

1: An evolving view of the eukaryotic oligosaccharyltransferase. Kelleher DJ, Gilmore R; Glycobiology. 2006;16:47. PMID:16317064