Two constituents of the initiation complex of the mannan-binding lectin activation pathway of complement are encoded by a single structural gene

Mannan-binding lectin (MBL) forms a multimolecular complex with at least two MEL-associated serine proteases, MASP-1 and MASP-2, This complex initiates the MBL pathway of complement activation by binding to carbohydrate structures present on bacteria, yeast, and viruses, MASP-1 and MASP-2 are composed of modular structural motifs similar to those of the C1q-associated serine proteases C1r and C1s, Another protein of 19 kDa with the same N-terminal sequence as the 76-kDa MASP-2, protein is consistently detected as part of the MBL/MASP complex, In this study, we present the primary structure of this novel MEL-associated plasma protein of 19 kDa, MAp19, and demonstrate that MAp19 and MASP-2 are encoded by two different mRNA species generated by alternative splicing/polyadenylation from one structural gene.