alpha-Mannosidase from Trichoderma reesei participates in the postsecretory deglycosylation of glycoproteins

The 160 kDa alpha-mannosidase (E.C. 3.2.1.24) isolated from culture filtrate of Trichoderma reesei has wide aglycon specificity but cleaves the alpha1 --> 2 and alpha1 --> 3 mannosidic bonds with higher rate than alpha1 --> 6 bond and slowly hydrolyses yeast mannan and 1,6-alpha-mannan. The specific activity of the enzyme and rate constant in the reaction with p-nitrophenyl-alpha-D-mannopyranoside were 0.15 U/mg and 1.62 x 10(-4) microM/min/microg, respectively, at optimal pH 6.5. We have found that in vitro enzyme is able to cleave off 30% of total alpha-mannopyranosyl residues from N- and O-linked glycans of secreted glycoproteins. The activity of the alpha-mannosidase toward glycoproteins in vivo was studied comparing the structures of O- and N-linked glycans of glycoproteins isolated from the cultures growing with and without 1-deoxymannojirimycin, an inhibitor of alpha-mannosidases. Difference in structures of these glycans may be explained by postsecretory deglycosylation catalysed by the alpha-mannosidase.