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*
Department of Biochemistry, Nagoya City University Medical School, Mizuho-ku, Japan;
Department of Biochemistry, Graduate School of Pharmaceutical Sciences, Hokkaido University, Hokkaido, Japan;
Department of Biological Sciences, Graduate School of Science, Tokyo Metropolitan University, Tokyo, Japan;
§
Medical Research Council Immunochemistry Unit, University of Oxford, Oxford, United Kingdom; and
¶
Department of Biochemistry, Fukushima Medical University School of Medicine, Fukushima, Japan
The recent identification of two mannose-binding lectin-associated
serine protease clones from Halocynthia roretzi, an
ascidian, suggested the presence of a complement system in
urochordates. To elucidate the structure and function of this possibly
primitive complement system, we have isolated cDNA clones for ascidian
C3 (AsC3) and purified AsC3 protein from body fluid. The deduced
primary structure of AsC3 shows overall similarity to mammalian C3,
including a typical thioester site with the His residue required for
nucleophilic activation of the thioester. AsC3 has a two-subunit chain
structure, and the 
-chain is cleaved at a specific site near to the
N terminus upon activation. Ascidian body fluid contains an opsonic
activity which enhances phagocytosis of yeast by ascidian blood cells,
and Ab against AsC3 inhibits this opsonic activity. These results
indicate that the complement system played a pivotal role in innate
immunity by enhancing phagocytosis before 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.
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