Opsonic complement component C3 in the solitary ascidian, Halocynthia roretzi

The recent identification of two mannose-binding lectin-associated serine p rotease clones from Halocynthia roretzi, an ascidian, suggested the presenc e of a complement system in urochordates. To elucidate the structure and fu nction of this possibly primitive complement system, we have isolated cDNA clones for ascidian C3 (AsC3) and purified AsC3 protein from body fluid. Th e deduced primary structure of AsC3 shows overall similarity to mammalian C 3, including a typical thioester site with the His residue required far nuc leophilic activation of the thioester. AsC3 has a two-subunit chain structu re, and the cu-chain is cleaved at a specific site near to the N terminus u pon activation. Ascidian body fluid contains an opsonic activity which enha nces phagocytosis of yeast by ascidian blood cells, and Ab against AsC3 inh ibits this opsonic activity. These results indicate that the complement sys tem played a pivotal role in innate immunity by enhancing phagocytosis befo re the emergence of the vertebrates and well ahead of the establishment of adaptive immunity, which is believed to have occurred at about the time of the appearance of cartilaginous fish.