Sea urchin coelomocytes specifically express a homologue of the complement component C3

J Immunol. 1998 Mar 15;160(6):2983-97.


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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Base Sequence
  • Complement C3 / analysis
  • Complement C3 / chemistry
  • Complement C3 / genetics*
  • DNA, Complementary / chemistry
  • DNA, Complementary / isolation & purification
  • Gene Dosage
  • Humans
  • Molecular Sequence Data
  • Phylogeny
  • Sea Urchins / immunology*


  • Complement C3
  • DNA, Complementary

Associated data

  • GENBANK/AF025526