SUMMARY PARAGRAPH for STT3
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 6, 7).
The oligosaccharyl transferase complex (OST complex) (EC 220.127.116.11) 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 (8) involved in protein import into the ER.
Stt3p and Ost4p were co-purified as the seventh and eighth components of the OST complex (3). Stt3p is involved in recognition of the target peptide and/or catalysis (4, 9, 10). STT3 is an essential gene, but conditional mutants have defective glycosylation (1, 4) and oligosaccharide transfer (11). Like ost6 cells, stt3 mutants are defective in cell wall biosynthesis (12, 13).
The name STT3 comes from the staurosporine and temperature sensitive phenotype (5), which is unique among components of the OST complex (14). The "stt" phenotype is only observed in alleles mutated in the hydrophobic N-terminal domain, in particular the first cytosolic loop. Mutations in the highly conserved (4) C-terminal lumenal domain lead to general glycosylation defects and/or death (14). Staurosporine inhibits Pkc1p, so Stt3p may be a target of the protein kinase C pathway (PKC1 is STT1) (14).
STT3 is homologous to the human genes SIMP (OMIM) (15) and ITM1 (OMIM) (16). Mouse Itm1 and S. pombe stt3+ complement stt3 on rich medium (YPD) but not on minimal medium (SD) (17). Arabidopsis thaliana has two STT3 homologs, STT3a and STT3b, with overlapping but nonidentical functions, but only stt3a mutants are defective in glycosylation and neither gene complements S. cerevisiae stt3.
Last updated: 2005-06-27