Characterization of a Human Core-specific Lysosomal α1,6-Mannosidase Involved in N-Glycan Catabolism

• Published in: The Journal of biological chemistry 2005-11, Vol.280 (44), p.37204
• Main Authors: Chaeho Park, Lu Meng, Leslie H. Stanton, Robert E. Collins, Steven W. Mast, Xiaobing Yi, Heather Strachan, Kelley W. Moremen
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M508930200

In humans and rodents, the lysosomal catabolism of core Man 3 GlcNAc 2 N -glycan structures is catalyzed by the concerted action of several exoglycosidases, including a broad specificity lysosomal α-mannosidase (LysMan), core-specific α1,6-mannosidase, β-mannosidase, and cleavage at the reducing terminus by a di- N -acetylchitobiase. We describe here the first cloning, expression, purification, and characterization of a novel human glycosylhydrolase family 38 α-mannosidase with catalytic characteristics similar to those established previously for the core-specific α1,6-mannosidase (acidic pH optimum, inhibition by swainsonine and 1,4-dideoxy-1,4-imino- d -mannitol, and stimulation by Co 2+ and Zn 2+ ). Substrate specificity studies comparing the novel human α-mannosidase with human LysMan revealed that the former enzyme efficiently cleaved only the α1-6mannose residue from Man 3 GlcNAc but not Man 3 GlcNAc 2 or other larger high mannose oligosaccharides, indicating a requirement for chitobiase action before α1,6-mannosidase activity. In contrast, LysMan cleaved all of the α-linked mannose residues from high mannose oligosaccharides except the core α1-6mannose residue. α1,6-Mannosidase transcripts were ubiquitously expressed in human tissues, and expressed sequence tag searches identified homologous sequences in murine, porcine, and canine databases. No expressed sequence tags were identified for bovine α1,6-mannosidase, despite the identification of two sequence homologs in the bovine genome. The lack of conservation in 5′-flanking sequences for the bovine α1,6-mannosidase genes may lead to defective transcription similar to transcription defects in the bovine chitobiase gene. These results suggest that the chitobiase and α1,6-mannosidase function in tandem for mammalian lysosomal N -glycan catabolism.