A silkworm hemolymph protein is a prophenoloxidase activation blocker

Melanization in insect hemolymph is triggered by the recognition of pathogen-associated molecular patterns via pattern recognition receptors. The signal transduction leads to the activation of the prophenoloxidase and hence the generation of melanin. The proPO activation process must be tightly controlled to minimize the host damage caused by reactive intermediates during melanin synthesis. The full-length cDNA sequence of a 20 kDa hemolymph protein from Bombyx (Bmhp20) was determined. Bmhp20 gene was expressed in larval fat body, integument, trachea, and ovary and was induced by the challenge of B. bombyseptieus. Binding of recombinant Bmhp20 to microbial cell wall components as well as gram-positive bacteria and fungi was confirmed. Phenoloxidase activity assay indicated that recombinant Bmhp20 blocked the proPO activation in hemolymph that was triggered by peptidoglycan or beta-1, 3-glucan. Our data suggest that Bmhp20 plays bifunctional roles in silkworm humoral responses: to participate in pattern recognition and to block the activation of proPO.