Isolation of a hagfish gene that encodes a complement component

It has been widely accepted that cyclostomes are the most primitive vertebrates extant with the ability to produce antibodies. We isolated cDNA clones that encode a putative 'antibody' from one of the cyclostomes, Eptatretus burgeri. The amino acid sequence predicted from the nucleotide sequences of the cDNA clones indicated that this gene does actually encode the proteins isolated as hagfish 'antibodies' by various investigators. However, these proteins are not similar to mammalian immunoglobulins but have some characteristics common to complements C3, C4 and C5 in higher vertebrates. We discuss the relationships of the isolated gene for hagfish complement with the mammalian genes for complements C3, C4 and C5. We also discuss the possibility of the presence of antibodies in cyclostomes.