The nucleotide sequence coding for 726 amino acid residues of the αchain of rabbit C3 was determined from a cDNA clone. Subfragments of the cDNA produced by restriction endonucleasas were inserted into the bacteriophage M13 and sequenced using the dideoxynucleotide technique. The derived amino acid sequence was compared with those of human and mouse C3, which have been previously reported [by De Brujn, M.H.L. and Fey, G.H. (1985) Proc.Natl.Acad.sci.USA 82, 708, and Westel, R.A. et al.(1984) J.Biol.chem. 259, 13857,respectively]. There was 79% or 78% homology in amino acid sequence between rabbit and human or mouse C3, respectively. All of the cysteinyl residues were conserved among the three molecules, and the sequence around the thioestar site was also highly conserved. Several regions having low homology were found: one of them was the small fragment released by factor I cleavage.
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