ANCIENT ORIGIN OF THE COMPLEMENT LECTIN PATHWAY REVEALED BY MOLECULAR-CLONING OF MANNAN-BINDING - PROTEIN-ASSOCIATED SERINE-PROTEASE FROM AUROCHORDATE, THE JAPANESE ASCIDIAN, HALOCYNTHIA-RORETZI

Recent identification of a C3-like gene in sea urchins revealed the pr esence of a complement system in invertebrates, To elucidate further t he components and function of the pre-vertebrate complement system, me attempted to isolate an ascidian (urochordata) C3 convertase, After i dentification of C3 cDNA from Halocynthia roretzi, a Japanese ascidian , reverse transcriptase-PCR amplification of hepatopancreas RNA was pe rformed using primers encoding highly conserved amino acid sequences o f the vertebrate Bf and C2 serine protease domain, Two candidate seque nces were identified, and the corresponding cDNA clones were isolated from a hepatopancreas library, Surprisingly, neither clone is related to Bf/C2 but rather share the same domain structure of mammalian C1r/C 1s/MASP (mannan binding protein-associated serine protease), and are m ore related evolutionarily to mammalian MASP than to mammalian C1r or C1s, The identification of the tunitate MASP clones, amplified with pr imers designed to amplify Bf or C2, suggests that the lectin pathway a ntedated the classical and alternative pathways of complement activati on.