β-Fructofuranosidase Genes of the Silkworm, Bombyx mori

Mulberry latex contains extremely high concentrations of alkaloidal sugar mimic glycosidase inhibitors, such as 1,4-dideoxy-1,4-imino- d -arabinitol ( d -AB1) and 1-deoxynojirimycin (DNJ). Although these compounds do not harm the silkworm, Bombyx mori , a mulberry specialist, they are highly toxic to insects that do not normally feed on mulberry leaves. d -AB1 and DNJ are strong inhibitors of α-glucosidases (EC 3.2.1.20); however, they do not affect the activity ofβ-fructofuranosidases (EC 3.2.1.26). Althoughα-glucosidase genes are found in a wide range of organisms, β-fructofuranosidase genes have not been identified in any animals so far. In this study, we report the identification and characterization of β-fructofuranosidase genes ( BmSuc1 and BmSuc2 ) from B. mori . The BmSuc1 gene was highly expressed in the midgut and silk gland, whereas the expression of BmSuc2 gene was not detected. BmSuc1 encodes a functional β-fructofuranosidase, whose enzymatic activity was not inhibited by DNJ or d -AB1. We also showed that BmSUC1 protein localized within the midgut goblet cell cavities. Collectively, our data clearly demonstrated that BmSuc1 serves as a sugar-digesting enzyme in the silkworm physiology. This anomalous presence of the β-fructofuranosidase gene in the B. mori genome may partly explain why the silkworm can circumvent the mulberry's defense system.