During N-linked glycosylation of proteins, oligosaccharide chains are assembled on the carrier molecule dolichyl pyrophosphate in the following order: 2 molecules of N-acetylglucosamine (GlcNAc), 9 molecules of mannose, and 3 molecules of glucose. These 14-residue oligosaccharide cores are then transferred to asparagine residues on nascent polypeptide chains in the endoplasmic reticulum (ER). As proteins progress through the Golgi apparatus, the oligosaccharide cores are modified by trimming and extension to generate a diverse array of glycosylated proteins (reviewed in 3, 4).

The oligosaccharyl transferase complex (OST complex) (EC 2.4.1.119) transfers 14-sugar branched oligosaccharides from dolichyl pyrophosphate to asparagine residues. The complex contains nine protein subunits: Ost1p, Ost2p, Ost3p, Ost4p, Ost5p, Ost6p, Stt3p, Swp1p, and Wbp1p, all of which are integral membrane proteins of the ER. The OST complex interacts with the Sec61p pore complex (5) involved in protein import into the ER.

The ninth component of the OST complex isolated, Ost6p was identified as a previously uncharacterized open reading frame (ORF) homologous to Ost3p (1). Ost6p was probably not identified in earlier purifications of the OST complex because it is present at substoichiometric levels and migrates very close to Swp1p (1).

Ost3p is homologous to Ost6p (1), and several lines of evidence indicate that they are alternative members of the OST complex. Both proteins are present in the complex at substoichiometric levels. They interact with the same subset of OST complex proteins, and are not co-immunoprecipitated (6). Disruption of either OST3 or OST6 causes only a minor defect in N-glycosylation (7), but deletion of both causes severe underglycosylation of soluble and membrane-bound glycoproteins and a defect in OST complex formation, although this does not result in a growth defect (1).

Although Ost3p and Ost6p share low sequence identity, they have very similar hydropathy profiles (1). They are homologous to the human tumor suppressor N33 (OMIM) (1). One difference between them: ost6 mutants are sensitive to caffeine, SDS, and Calcofluor White--indicating a possible defect in cell wall biogenesis--while ost3 mutants are not (1).

Last updated: 2005-06-17