The mannose-binding lectin (MBL) and MBL-associated serine proteases (MASPs) play crucial roles in activation of the lectin pathway of the complement system. Mammals and Xenopus possess two distinct MASPs, MASP1 and MASP2, with different substrate specificity. Recently, a truncated form named MAp19 or sMAP, composed of N-terminal C1r/C1s/Uegf/bone morphogenetic protein (CUB)-1 and epidermal growth factor domains of MASP2, has been shown to be generated by alternative polyadenylation and splicing from the MASP2 gene. In the present study, we isolated cDNA encoding a novel MASP-related protein, designated MRP, from carp. MRP is distinct from MAp19/sMAP in containing two additional domains, CUB-2 and short concensus repeat (SCR)-1, followed by a unique C-terminal 21 amino acids, but resembles it by also lacking the serine protease domain, suggesting that carp MRP is a functional homologue of human MAp19/sMAP. Analyses of polymerase chain reaction (PCR)-amplified carp genomic DNA, from CUB-2 to SCR-2 of MASP, indicated that carp possess duplicated MASP genes, designated MASP-A and MASP-B, both of which contain an exon encoding the MRP-specific C-terminal stretch between the exons coding for SCR-1 and SCR-2 domains. Reverse transcription-PCR analysis showed that both MASP genes of carp produce the two MASP isoforms, MASP and MRP, through alternative polyadenylation and splicing. The conservation of MASP isoforms that lack the catalytic domain in both carp and human implies that they meet an essential requirement in the MBL-MASP complex of the lectin pathway.
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