A digestive beta-glucosidase from the silkworm, Bombyx mori: cDNA cloning, expression and enzymatic characterization

A digestive beta-glucosidase cDNA was cloned from the silkworm, Bombyx mori. The B. mori beta-glucosidase cDNA contains an open reading frame of 1473 bp encoding 491 amino acid residues. The B. mori beta-glucosidase possesses the amino acid residues involved in catalysis and substrate binding conserved in glycosyl hydrolase family 1. Southern blot analysis of genomic DNA suggested the B. mori beta-glucosidase to be a single gene. Northern blot analysis of B. mori beta-glucosidase gene confirmed larval midgut-specific expression. The B. mori beta-glucosidase mRNA expression in larval midgut was detectable only during feeding period, whereas its expression was downregulated during starvation. The B. mori beta-glucosidase cDNA was expressed as a 57-kDa polypeptide in baculovirus-infected insect Sf9 cells, and the recombinant beta-glucosidase was active on cellobiose and lactose, but not active on salicin, indicating that the B. mori beta-glucosidase possesses the characteristics of the Class 2 enzyme. The enzyme activity of the purified recombinant beta-glucosidase expressed in baculovirus-infected insect cells was approximately 665 U per microg of recombinant B. mori beta-glucosidase. The purified recombinant B. mori beta-glucosidase showed the highest activity at 35 degrees C and pH 6.0, and were stable at 50 degrees C at least for 10 min. Treatment of recombinant virus-infected Sf9 cells with tunicamycin, a specific inhibitor of N-glycosylation, revealed that the recombinant B. mori beta-glucosidase is N-glycosylated, but the carbohydrate moieties are not essential for enzyme activity.