ATP-dependent RNA helicase that is essential and required for cellular function, larval growth, and for male and female fertility. Also required for RNA interference (RNAi), double-stranded RNA induces potent and specific gene silencing, by acting downstream of dsRNA internalization. RNAi is mediated by the RNA-induced silencing complex (RISC), a sequence-specific, multicomponent nuclease that destroys or silences messenger RNAs homologous to the silencing trigger. ATP + H2O = ADP + H(+) + phosphate In nurse cells, follicle cells, and within the oocyte. Vas and bel colocalize in the perinuclear region of nurse cells. Vas and bel colocalize in nuage (perinuclear, electron-dense granules in germline cells) and at the oocyte posterior during oogenesis. Expressed throughout development, highest expression in second larval instar and adult females. Flies are recessive lethal with a larval growth defect phenotype. Hypomorphic bel mutants are male-sterile due to defects in spermatogenesis and female sterile as oogenesis usually arrests around stages 8-9 and egg chambers completely degenerate. Belongs to the DEAD box helicase family. DDX3/DED1 subfamily.