Eighth component of guinea pig complement. Purification and characterization

Abstract

To elucidate the molecular architecture and function of the possibly primitive complement system of the solitary ascidian, Halochynthia roretzi, cDNA clones for the third component (C3) and mannose-binding lectin (MBL)-associated serine protease (MASP) were isolated from the hepatopancreas cDNA library. The deduced primary structure of ascidian C3 (AsC3) shows overall similarity to mammalian C3 including a typical thioester site. Two distinct ascidian MASPs, termed AsMASPa and AsMASPb, have the same domain structure as mammalian C1r/C1s/MASP-1/MASP-2. Both of them show a closer similarity to mammalian MASP-1 than to mammalian C1r/C1s/MASP-2. Ascidian body fluid contains an opsonic activity which enhances phagocytosis of yeast by ascidian blood cells, and an antibody against AsC3 inhibits this opsonic activity. These results indicate that the lectin-dependent, opsonic complement system was present prior to the emergence of the vertebrates and well ahead of the establishment of adaptive immunity.