A Novel Protein Acts as a Negative Regulator of Prophenoloxidase Activation and Melanization in the Freshwater Crayfish Pacifastacus leniusculus

Melanization is an important immune component of the innate immune system of invertebrates and is vital for defense as well as for wound healing. In most invertebrates melanin synthesis is achieved by the prophenoloxidase-activating system, a proteolytic cascade similar to vertebrate complement. Even though melanin formation is necessary for host defense in crustaceans and insects, the process needs to be tightly regulated because of the hazard to the animal of unwanted production of quinone intermediates and melanization in places where it is not suitable. In the present study we have identified a new melanization inhibition protein (MIP) from the hemolymph of the crayfish, Pacifastacus leniusculus. Crayfish MIP has a similar function as the insect MIP molecule we recently discovered in the beetle Tenebrio molitor but interestingly has a completely different sequence. Crayfish MIP as well as Tenebrio MIP do not affect phenoloxidase activity in itself but instead interfere with the melanization reaction from quinone compounds to melanin. Importantly, crayfish MIP in contrast to Tenebrio MIP contains a fibrinogen-like domain, most similar to the substrate recognition domain of vertebrate l-ficolins. Surprisingly, an Asp-rich region similar to that found in ficolins that is likely to be involved in Ca2+ binding is present in crayfish MIP. However, crayfish MIP did not show any hemagglutinating activity as is common for the vertebrate ficolins. A mutant form of MIP with a deletion lacking four Asp amino acids from the Asp-rich region lost most of its activity, implicating that this part of the protein is involved in regulating the prophenoloxidase activating cascade. Overall, a new negative regulator of melanization was identified in freshwater crayfish that shows interesting parallels with proteins (i.e. ficolins) involved in vertebrate immune response.