dsRNA Binding Activity of Silkworm Larval Hemolymph is Mediated by Lipophorin Complex

RNAi induction by the injection of long dsRNA into hemocoel has been tried to archive efficient gene knockdown in silkworm larvae or pupae, and severas reports successfully applying RNAi to silkworm have been brought out. However, effect of RNAi in silkworm is often insufficient and improvement of efficiency is desirable. On the other hand, the metabolic turnover or organ uptake of dsRNA injected into insects has not been sufficiently clarified so far. In this study, we report that silkworm larval hemolymph has dsRNA binding activity. We also show that the molecular entity of dsRNA binding activity of hemolymph is lipophorin, a major lipoprotein complex in insects. dsRNA binding activity of silkworm larval hemolymph was observed by electrophoretic mobility shift assay. dsRNA binding activity–associated protein was purified by affinity chromatography with immobilized dsRNA as a ligand. Amino acid sequence of the purified protein was determined by Edman degradation and the protein was revealed to be a silkworm homolog of Apolipophorin I, an unexchangeable apolipoprotein subunit of lipophorin. Lipophorin fraction prepared from hemolymph by density gradient ultracentrifugation also exhibited the similar dsRNA binding activity. We determined full– length sequence of Apolipophorin II/I precursor homolog of silkworm, whose structure was highly conserved among insects. Although physiological role of dsRNA binding activity is still unknown, the present study indicates potential effect by injection of dsRNA into silkworm individuals.