1,2-alpha-Mannosidase catalyzes the specific cleavage of 1,2-alpha-mannose residues from protein-linked N-glycan. In this study, a novel DNA sequence homologous to the authentic 1,2-alpha-mannosidase was cloned from a cDNA library prepared from solid-state cultured Aspergillus oryzae. The fmanIB cDNA consisted of 1530 nucleotides and encoded a protein of 510 amino acids in which all consensus motifs of the class I alpha-mannosidase were conserved. Expression of the full length of 1,2-alpha-mannosidase cDNA by the Aspergillus host, though it has rarely been done with other filamentous-fungal mannosidase, was successful with fmanIB and caused an increase in both intracellular and extracellular mannosidase activity. The expressed protein (FmanIBp) specifically hydrolyzed 1,2-alpha-mannobiose with maximal activity at a pH of 5.5 and a temperature of 45 degrees C. With Man(9)GlcNAc(2) as the substrate, Man(5)GlcNAc(2) finally accumulated while hydrolysis of the 1,2-alpha-mannose residue of the middle branch was rate-limiting. To examine the intracellular localization of the enzyme, a chimeric protein of FmanIBp with green fluorescent protein was constructed. It showed a dotted fluorescence pattern in the mycelia of Aspergillus, indicative of the localization in intracellular vesicles. Based on these enzymatic and microscopic results, we estimated that FmanIBp is a fungal substitute for the mammalian Golgi 1,2-alpha-mannosidase isozyme IB. This and our previous report on the presence of another ER-type mannosidase in A. oryzae (Yoshida et al., 2000) support the notion that the filamentous fungus has similar steps of N-linked glycochain trimming to those in mammalian cells.

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The hydrolysis of tri-, di-, and monooleoylglycerols by highly purified lysosomal acid lipase from rabbit liver (Imanaka, T., et al. (1984) J. Biochem. 96, 1089-1101) was examined. The degradative products were separated by thin layer chromatography on silicic acid impregnated with boric acid. Trioleoylglycerol was hydrolyzed with intermediate accumulation of 1,2(2,3)-dioleoylglycerol and 2-monooleoylglycerol, but no detectable production of the 1,3-isomer. 1,2(2,3)-Dioleoylglycerol was also hydrolyzed with the intermediate accumulation of 2-monooleoylglycerol. The 1(3)-isomer of monooleoylglycerol was hydrolyzed exclusively, with no hydrolysis of the 2-isomer. These results indicate that the enzyme shows preference for 1(3)-ester bonds and that the main lipolytic reaction sequence catalyzed is triacylglycerol---1,2(2,3)-diacylglycerol---2-monoacylglycerol.