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 5, 6).

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 (7) involved in protein import into the ER.

Ost4p and Stt3p were co-purified as the seventh and eighth components of the OST complex (8). Originally isolated as a vanadate-resistant mutant defective in oligosaccharyltransferase activity, OST4 encodes a protein of only 36 amino acids (1). Deletion of OST4 greatly reduces, but does not abolish, oligosaccharyltransferase activity, is lethal at 37° C, and causes cells to grow slowly at 25° C (1).

Last updated: 2005-06-17