β-fructofuranosidase genes of the silkworm, Bombyx mori -: Insights into enzymatic adaptation of B. mori to toxic alkaloids in mulberry latex

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 alpha-glucosidases (EC 3.2.1.20); however, they do not affect the activity of beta-fructofuranosidases (EC 3.2.1.26). Although alpha-glucosidase genes are found in a wide range of organisms, beta-fructofuranosidase genes have not been identified in any animals so far. In this study, we report the identification and characterization of beta-fructofuranosidase genes (BmSuc1 and BmSuc2) from B. mod. 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 beta-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 beta-fructofuranosidase gene in the B. mori genome may partly explain why the silkworm can circumvent the mulberry's defense system.