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.

Ost1p is the alpha subunit of the OST complex, one of the original six subunits purified (1, 6). Also known as Nlt1p, for N-linked oligosaccharyl transferase (7), Ost1p was once thought to be responsible for recognition of the glycosylation target site in proteins (8), but this function is now assigned to Stt3p (9, 10, 11).

Deletion of OST1 is lethal in haploid cells (1, 7), and temperature-sensitive (ts) mutants arrest within four generations (1). In addition to glycosylation defects, some ts mutants grow slowly and form clumps in liquid culture at the permissive temperature, possibly due to defective separation of mother and daughter cells (1).

Ost1p is homologous to mammalian ribophorin I (OMIM) (1, 7), a component of the mammalian OST complex, along with ribophorin II (OMIM) and OST48 (OMIM).

Last updated: 2005-06-27