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).

Och1p adds a mannose moiety to core N-linked oligosaccharides upon their arrival in the Golgi apparatus; this is the last modification before the N-linked glycosylation pathway forks to produce either large mannan outer chains or small core-type oligosaccharides (reviewed in 7). Och1p is a membrane-bound (1, 8, 9) alpha-1,6-mannosyltransferase (1, 8, 9, 10) of the cis-Golgi cisternae (2, 10), but it is recycled between the trans-Golgi network (2, 11) and a late compartment of the ER (12). Despite its homology to and partial complementation of S. pombe och1(+) (13), S. cerevisiae Och1p is not a component of the alpha-1,6-mannosyltransferase complex. OCH1 expression is regulated by SKN7 (14, 15), SLN1 (14, 15), and CDC4 (16).

Deletion or depletion of OCH1 causes lethality or slow-growth (1, 4, 9). och1 mutants cannot form high mannose oligosaccharides (~50+ mannose residues) (1, 4, 8, 9) so they have decreased levels of cell wall mannoproteins (4, 17), causing weakness and defects in bud formation (4, 8, 17) and hypersensitivity to agents that attack the cell wall (calcofluor white, hygromycin B, and SDS) (17). Weakening of cell walls in hypotonic solution can be partially suppressed by the addition of osmotic stabilizers such as salt or sorbitol (17, 18).

Last updated: 2005-07-12