Filamentous fungus Aspergillus oryzae has two types of alpha-1,2-mannosidases, one of which is a microsomal enzyme that removes a single mannose residue from Man9GlcNAc2.

alpha-Mannosidase activities towards high-mannose oligosaccharides were examined with a detergent-solubilized microsomal preparation from a filamentous fungus, Aspergillus oryzae. In the enzymatic reaction, the pyridylaminated substrate Man(9)GlcNAc(2)-PA was trimmed to Man(8)GlcNAc(2)-PA which lacked one alpha-1,2-mannose residue at the nonreducing terminus of the middle branch (Man8B isomer), and this mannooligosaccharide remained predominant through the overall reaction. Trimming was optimal at pH 7.0 in PIPES buffer in the presence of calcium ion and kifunensine was inhibitory with IC(50) below 0.1 microM. These results suggest that the activity is the same type as was previously observed with human and yeast endoplasmic reticulum (ER) alpha-mannosidases. Considering these results together with previous data on a fungal alpha-1,2-mannosidase that trimmed Man(9)GlcNAc(2) to Man(5)GlcNAc(2) (Ichishima, E., et al. (1999) Biochem J, 339: 589-597), the filamentous fungi appear to have two types of alpha-1,2-mannosidases, each of which acts differently on N-linked mannooligosaccharides.