Sea urchin coelomocytes specifically express a homologue of the complement component C3.
A homologue of complement component C3 (SpC3) has been cloned and sequenced from the purple sea urchin, Strongylocentrotus purpuratus. The preprocessed, deduced protein size is estimated to be 186 kDa with a short leader and two chains, alpha and beta. There are cysteines in conserved positions for interchain disulfide bonding, and there is a conserved thioester site in the alpha-chain with an associated histidine. There are five consensus N-linked glycosylation sites, and putative cleavage sites for factor I and C3 convertase. Partially purified SpC3 on protein gels shows a nonreduced size of 210 kDa and, under reducing conditions, reveals an alpha-chain of 130 kDa and a beta-chain of 80 kDa. These sizes are larger than the deduced sizes, suggesting that the protein has carbohydrates added to most of the consensus N-linked glycosylation sites. Phylogenetic analysis of SpC3 compared with other members of the thioester protein family, which includes C3, C4, C5, and alpha2-macroglobulin, shows that SpC3 is the first divergent complement protein, falling at the base of the complement protein clade. Transcripts from the SpC3 gene (Sp064) are 9 kb, and the gene is expressed specifically in coelomocytes, which are the immunocytes in the sea urchin. Genome blots suggest that SpC3 is encoded by a single copy gene per haploid genome. This is the first identification of a complement component in an invertebrate, and suggests homology of the innate immune system within the deuterostome lineage of animals.
Al-Sharif
W Z
WZ
Department of Biological Sciences, George Washington University, Washington, DC 20052, USA.
Sunyer
J O
JO
Lambris
J D
JD
Smith
L C
LC
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003176:Complement C3 / Q000032:analysis / Q000737:chemistry / Q000235:genetics*; D018076:DNA, Complementary / Q000737:chemistry / Q000302:isolation & purification; D018628:Gene Dosage; D006801:Humans; D008969:Molecular Sequence Data; D010802:Phylogeny; D012617:Sea Urchins / Q000276:immunology*
D003176:Complement C3; D018076:DNA, Complementary