cDNA sequence coding for the alpha '-chain of the third complement component in the African lungfish

cDNA clones coding for almost the entire C3 alpha-chain of the African lung fish (Protopterus aethiopicus), a representative of the Sarcopterygii (lobe -finned fishes), were sequenced and characterized. From the sequence it is deduced that the lungfish C3 molecule is probably a disulphide-bonded alpha :beta dimer similar to that of the C3 components of other jawed vertebrates . The deduced sequence contains conserved sites presumably recognized by pr oteolytic enzymes (e.g. factor I) involved in the activation and inactivati on of the component. It also contains the conserved thioester region and th e putative site for binding properdin. However, the site for the interactio n with complement receptor 2 and factor H are poorly conserved. Either comp lement receptor 2 and factor H are not present in the lungfish or they bind to different residues at the same or a different site than mammalian compl ement receptor 2 and factor H. The C3 alpha-chain sequences faithfully refl ect the phylogenetic relationships among vertebrate classes and can therefo re be used to help to resolve the long-standing controversy concerning the origin of the tetrapods.