A 100-kDa protein in the C4-activating component of Ra-reactive factor is a new serine protease having module organization similar to C1r and C1s.

Ra-reactive factor (RaRF), a C-dependent bactericidal factor in mice, is composed of one polysaccharide-binding component and one C4/C2-activating component. The former is an oligomer of 28-kDa protein corresponding to the mannose-binding protein of mice. The 100-kDa protein, P100, has been shown to be present in the C4/C2-activating component. This protein generates 29- and 70-kDa polypeptide chains when reduced. In this study, we determined the nucleotide sequence of cDNA coding for P100. cDNAs were prepared by reverse transcription PCR and cassette-ligation-mediated PCR on mRNA from BALB/c mouse liver, using primers synthesized by reference to the sequence determined in a previous study. The results of cDNA sequencing indicate that the precursor protein of P100 containing a 24-residue signal peptide consists of 704 amino acid residues. Taking the results of the previous electrophoretic study into consideration, it is thought that the cleavage of mature P100 protein generates a 29-kDa chain of 251 residues and a 70-kDa chain of 429 residues. Although homology in the amino acid sequence of P100 with that of human C1r and C1s subcomponents of C was less than 40%, a striking similarity in domain organization was found among these proteins, indicating that P100 is a new C4-activating serine protease structurally similar to C1r and C1s. Northern hybridization showed that the liver was the primary site of the expression of the P100 gene.