Hemolymph protease-5 links the melanization and Toll immune pathways in the tobacco hornworm, Manduca sexta

Significance Phenoloxidase-catalyzed melanization and Spätzle-triggered Toll signaling are critical innate immune responses of insects that are triggered by specific proteolysis. It has been unclear whether separate protease cascades or an integrated network of serine proteases coordinates these responses. Here we present evidence that hemolymph protease HP5 acts in pathways eliciting activation of both phenoloxidase and Spätzle-1 in Manduca sexta . HP5 has a unique specificity by cleaving proHP6 at His 112 , resulting in HP6 activation. CLIP proteases related to HP6 from other insect species, including Drosophila Persephone, share this activation site feature with proHP6, and therefore, HP5 orthologs in other species are candidates for key roles in regulating and integrating protease cascade pathways in innate immune responses.