The role of clip-domain serine protease homologue CLIPA14 and two carboxypeptidases, CP1 and CP2 in A. gambiae immunity.

Abstract

Functional genetic and molecular studies in the malaria vector Anopheles gambiae showed that mosquitoes are not passive to Plasmodium infections, rather A. gambiae mounts potent immune responses against malaria parasites as well as bacterial and fungal infections. Gene discovery tools and systematic functional genetic screens by RNAi helped identify key immunity genes, among which are clip domain serine proteases (CLIPs) that play key roles in diverse aspects of insect immunity. CLIPs are unique to arthropods. They are composed of one or more clip domains at the N-terminus, and a serine protease domain at the C-terminus containing the catalytic His-Asp-Ser residues. A group of CLIPs is non-catalytic [also known as serine protease homologs (SPHs)], whereby the serine and sometimes the aspartic acid residues are substituted by another amino acid. CLIP cascades trigger several downstream effector responses including, antimicrobial peptide synthesis, coagulation and melanization. Melanization is a potent insect immune response characterized by the deposition of melanin on microbial surfaces leading to their elimination. Genetic screens by RNAi identified several catalytic CLIPs and SPHs with strong RNAi phenotypes. Among SPHs, CLIPA8 was initially shown to be essential for the melanization of ookinetes as well as bacteria and fungi. Later, SPCLIP1 and CLIPA2 were found to act as positive and negative regulators of the mosquito melanization response indirectly by amplification of complement. TEP1 is a hallmark effector molecule in mosquito immunity required for defense against Plasmodium, bacteria and fungi. These results altogether reveal that cSPHs play intricate roles in the regulation of the melanization and complement responses, and point to a cross talk between complement and melanization. To identify novel SPHs with immune functions, a proteomic approach was adopted whereby key proteins involved in the melanization response including CLIPA2 and CTL4-CTLMA2 complex were immunoprecipitated and interacting prot