Mosquito cathepsin B-like protease involved in embryonic degradation of vitellin is produced as a latent extraovarian precursor

Here we report identification of a novel member of the thiol protease super family in the yellow fever mosquito, Aedes aegypti. It is synthesized and s ecreted as a latent proenzyme in a sex-, stage-, and tissue-specific manner by the fat body, an insect metabolic tissue, of female mosquitoes during v itellogenesis in response to blood feeding. The secreted, hemolymph form of the enzyme is a large molecule, likely a hexamer, consisting of 44-kDa sub units. The deduced amino acid sequence of this 44-kDa precursor shares high similarity with cathepsin B but not with other mammalian cathepsins. We ha ve named this mosquito enzyme vitellogenic cathepsin B (VCB). VCB decreases to 42 kDa after internalization by oocytes, In mature yolk bodies, VCB is located in the matrix surrounding the crystalline yolk protein, vitellin. A t the onset of embryogenesis, VCB is further processed to 33 kDa. The embry o extract containing the 33-kDa VCB is active toward benzoyloxycarbonyl-Arg -Arg-para-nitroanilide, a cathepsin B-specific substrate, and degrades vite llogenin, the vitellin precursor. Both of these enzymatic activities are pr evented by trans-epoxysuccinyl-L-leucylamido- (4-guanidino)butane (E-64), a thiol protease inhibitor. Furthermore, addition of the anti-VCB antibody t o the embryonic extract prevented cleavage of vitellogenin, strongly indica ting that the activated VCB is involved in embryonic degradation of vitelli n.