Class I alpha-1,2-mannosidases are conserved throughout eukaryotic evolution and are members of the glycoside hydrolase family 47. They regulate the maturation of N-glycans during glycoprotein biosynthesis. N-glycan formation begins with the transfer of a preformed oligosaccharide precursor, usually Glc3Man9ClcNac2, to nascent polypeptide chains. The oligosaccharide precursor is then trimmed immediately by alpha-glucosidases and alpha-mannosidases in the endoplasmic reticulum or the golgi apparatus. Besides their importance in N-glycan maturation, endoplasmic reticulum and golgi apparatus processing glycosidases and mannosidases also play a role in protein folding \"quality control\". Trimming of mannose residues in the endoplasmic reticulum acts as a signal to target misfolded glycoproteins for degradation by the proteasome, which ensures only correctly folded proteins are transported to their final destination.
Mannosyl-oligosaccharide 1,2-alpha-mannosidase is the only alpha-mannosidase in Saccharomyces cerevisae and it removes a single mannose residue from Man(9)(GlcNAc)(2) to form Man(8)(GlcNAc)(2) in the endoplasmic reticulum as do equivalent enzymes in higher organisms. Class I enzymes found in the golgi apparatus remove all four linked alpha-mannose residues.
Defined by 4 residues: GLU:A-99 [auth A-132]ARG:A-103 [auth A-136]ASP:A-242 [auth A-275]GLU:A-397 [auth A-435]