Thioester function is conserved in SpC3, the sea urchin homologue of the complement component C3.
The amino acid sequence of the thioester site in the alpha chain of SpC3, the sea urchin homologue of C3, is conserved. This implies a conserved function of covalent bond formation with amine or hydroxyl groups on target molecules. When coelomic fluid (CF) was incubated with 14C-methylamine, a classic assay for thioester binding function, the alpha chain became labeled. When CF was treated to induce autolysis, peptide bond cleavage occurred at the thioester site. Autolysis could be blocked or reduced by pre-treating CF with either methylamine or yeast, both of which are known to bind to thioester sites C3 proteins from other organisms. The data suggest that SpC3 can bind to target cell surfaces, constituting indirect evidence that it can covalently bind to pathogen surfaces and function as an opsonin in vivo. This activity may be an important aspect of host defense in the sea urchin.
Smith
L Courtney
LC
Department of Biological Sciences and the Institute of Biomedical Sciences, George Washington University, 2023 George Washington Street, DC 20052, USA. csmith@gwu.edu
D000595:Amino Acid Sequence; D000818:Animals; D001329:Autolysis; D001665:Binding Sites; D002247:Carbon Isotopes; D003176:Complement C3 / Q000378:metabolism*; D017124:Conserved Sequence; D006801:Humans; D008744:Methylamines / Q000378:metabolism; D008969:Molecular Sequence Data; D011956:Receptors, Cell Surface / Q000378:metabolism*; D012617:Sea Urchins; D017386:Sequence Homology, Amino Acid
D002247:Carbon Isotopes; D003176:Complement C3; D008744:Methylamines; D011956:Receptors, Cell Surface; C027451:methylamine