InterPro domain: IPR021149

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 [ 1 , 2 ].

The oligosaccharyl transferase complex (OST complex) 2.4.1.119 transfers 14-sugar branched oligosaccharides from dolichyl pyrophosphate to asparagine residues [ 3 ]. 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 [ 4 ] involved in protein import into the ER.

This entry represents subunits OST3 and OST6. OST3 is homologous to OST6 [ 5 ], and several lines of evidence indicate that they are alternative members of the OST complex. Disruption of both OST3 and OST6 causes severe underglycosylation of soluble and membrane-bound glycoproteins and a defect in the assembly of the complex. Hence, the function of these genes seems to be essential for recruiting a fully active complex necessary for efficient N-glycosylation [ 6 ]. This entry also includes the magnesium transporter protein 1, also known as OST3 homologue B, which might be involved in N-glycosylation through its association with the oligosaccharyl transferase (OST) complex.

1. Glycoprotein biosynthesis in yeast. FASEB J. 7, 540-50
2. The dolichol pathway of N-linked glycosylation. Biochim. Biophys. Acta 1426, 239-57
3. Functional characterization of Ost3p. Loss of the 34-kD subunit of the Saccharomyces cerevisiae oligosaccharyltransferase results in biased underglycosylation of acceptor substrates. J. Cell Biol. 130, 567-77
4. Subunits of the translocon interact with components of the oligosaccharyl transferase complex. J. Biol. Chem. 280, 22917-24
5. The oligosaccharyltransferase complex from Saccharomyces cerevisiae. Isolation of the OST6 gene, its synthetic interaction with OST3, and analysis of the native complex. J. Biol. Chem. 274, 17249-56