THE 3RD COMPONENT OF XENOPUS COMPLEMENT - CDNA CLONING, STRUCTURAL AND FUNCTIONAL-ANALYSIS, AND EVIDENCE FOR AN ALTERNATE C3 TRANSCRIPT

Although the third component of complement has been purified from two amphibian species, Xenopus laevis and the axolotl, only limited inform ation is available about its primary structure in these species. We no w present (a) 95% of the cDNA sequence encoding C3 from a Xenopus laev is/Xenopus gilli (Xenopus LG) hybrid (b) an analysis of the C3 convert ase and factor I cleavage sites in Xenopus C3, and (c) evidence for an alternative form of C3. The Xenopus LG sequence has a 57% nucleotide and 52% amino acid sequence identity to human C3 and contains one pote ntial N-glycosylation site in the P-chain. The deduced amino acid sequ ence showed that the C3 convertase and factor I cleavage sites (Arg-Se r) are conserved in Xenopus C3 and protein sequencing of Xenopus C3 fr agments fixed on zymosan during complement activation demonstrated tha t Xenopus C3 is indeed cleaved by C3 convertase and factor I at these sites. Our screening of a liver cDNA library identified an unusual C3 clone with a deletion of 2502 bp, suggesting the presence of a novel C 3 transcript in Xenopus LG liver. The presence of this C3 transcript w as confirmed by reverse transcription polymerase chain reaction using Xenopus LG liver mRNA and specific oligonucleotide probes. This transc ript encoded a putative 102-kDa protein comprising the beta-chain of C 3, together with the first 59 residues and the last 103 residues of th e alpha-chain; it would therefore lack many of the ligand binding site s found in the intact alpha-chain. However, the molecule may be an ana log of a truncated C3 molecule that is found in the serum of allergic dermatitis patients and acts as an inhibitor of eosinophil cytotoxicit y and neutrophil adherence.