Lepidoptera insects, for example, silkworm (Bombyx mori) and cabbage looper (Trichoplusia ni), are known to produce proteins modified with mannose-rich non-sialylated sugar moieties, the so-called high-mannose-type N-glycans. However, we reported that lepidopteran insect cells synthesized sialylated moieties under an acetylglucosaminidase (GlcNAcase)-inhibited condition, suggesting the requirement of the enzyme activity for assembling Lepidoptera-specific N-glycans. In this study, we cloned the gene encoding for a novel silkworm GlcNAcase, GlcNAcase II, and characterized its biological properties in comparison with those of the authentic GlcNAcase, GlcNAcase I. GlcNAcase II shows a significant but limited homology (27.3% amino acid identity) with GlcNAcase I, and reveals similar catalytic activity on both acetylglucosamine and acetylgalactosamine residues at the non-reduced ends of the sugar moiety. Interestingly, GlcNAcase II, but not GlcNAcase II, is strongly inhibited by a sugar-mimicking alkaloid, 2-deoxynojirimycin, suggesting distinct roles of these enzymes in the carbohydrate metabolism of silkworm. Another striking difference between these isozymes is concerning intracellular localization. GlcNAcase I resides in lysosome-like compartments in insect cells. In contrast, GlcNAcase II localizes in the cytoplasm, exhibiting a punctate pattern. Although the site of action and biological role of the enzyme remain unclear, the GlcNAcase may be a key to assembling Lepidoptera-specific N-glycans.
(Research Team for Advanced Biologicals)
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