A novel 43-kDa protein as a negative regulatory component of phenoloxidase-induced melanin synthesis

The melanization reaction induced by activated phenoloxidase in arthropods is important in the multiple host defense innate immune reactions, leading to the sequestration and killing of invading microorganisms. This reaction ought to be tightly controlled because excessive formation of quinones and systemic hypermelanization are deleterious to the hosts, suggesting that a negative regulator(s) of melanin synthesis may exist in hemolymph. Here, we report the purification and cloning of a cDNA of a novel 43-kDa protein, from the mealworm Tenebrio molitor, which functions as a melanization-inhibiting protein (MIP). The deduced amino acid sequence of 352 residues has no homology to known sequences in protein data bases. When the concentration of the 43-kDa protein was examined by Western blot analysis in a melanin-induced hemolymph prepared by injection of Candida albicans into T. molitor larvae, the 43-kDa protein specifically decreased in the melanin-induced hemolymph compared with control hemolymph. Recombinant MIP expressed in a baculovirus system had an inhibitory effect on melanin synthesis in vitro. RNA interference using a synthetic 445-mer double-stranded RNA of MIP injected into Tenebrio larvae showed that melanin synthesis was markedly induced. These results suggest that this 43-kDa MIP inhibits the formation of melanin and thus is a modulator of the melanization reaction to prevent the insect from excessive melanin synthesis in places where it should be inappropriate.