Molecular cloning of the complement C1r/C1s/MASP2-like serine proteases from the common carp (Cyprinus carpio)

The classical pathway of complement composed of C1, C4, and C2 is an antibo dy-dependent activation cascade that is present in jawed vertebrates. C1 is a Ca2+-dependent complex of C1q, C1r, and C1s, and analogous to an initiat ion complex of the lectin pathway of complement, which consists of the man- nose-binding lectin (MBL) homologous to C1q and the MEL-associated serine p roteases (MASPs) homologous to C1r and C1s. Thus divergence of C1q and MBL and that of C1r, C1s and the MASPs are considered to be crucial events in t he establishment and evolution of the classical complement pathway. However , molecular information on the C1 subcomponents is very limited in lower ve rtebrates. Here we describe two distinct C1r/C1s/MASP2-like cDNA clones (C1 r/s-A, C1r/s-B) isolated from the common carp (Cyprinus carpio). They share 83% identity at the amino acid level and have a domain structure similar t o that of C1r/C1s/MASPs from other species. The serine protease domain of t he carp homologues lacks the histidine loop and is encoded by a single exon containing an AGY codon for the active serine residue, as in mammalian C1r , C1s, and MASP2. Southern blot and PCR analyses indicated that the carp ha s at least three copies of the C1r/s-A gene and a single C1r/s-B gene. Alth ough phylogenetic tree analysis does not definitively assign carp C1r/s-A a nd C1r/s-B, they might represent ancestral molecules which later diverged i nto Clr, Cls, and MASP2 of higher vertebrates.