Hagfish humoral defense protein exhibits structural and functional homology with mammalian complement components

A genomic clone and cDNA fragment encoding a portion of a humoral recognition molecule from the hagfish were isolated and sequenced. The serum protein has previously been described as having structural features that are immunoglobulin-like. Amino acid sequence obtained from the 77-kDa H1 heavy chain facilitated the isolation of a genomic clone containing at least two coding regions. Through use of primers derived from the genomic sequences, a 231-base-pair cDNA fragment was obtained by PCR from liver RNA. Comparison of the deduced 120-amino acid sequence from the N terminus of H1 with known protein sequences revealed substantial sequence similarity with the beta chain of the murine fourth complement component C4 and with the related third and fifth complement molecules C5 and C3 and the major histocompatibility complex-encoded sex-limited protein. Observation of structural and functional similarities associated with the sequence similarity indicate that these molecules share an evolutionary relationship: the polypeptide chain structure of hagfish complement-like protein (CLP) resembles that of C4; CLP contains a hidden thioester group on the 70-kDa chain; CLP binds to streptococcal cells and enhances the phagocytosis of yeast by hagfish leukocytes. These data suggest that CLP forms part of a non-clonally-derived complement-related humoral defense system in the hagfish.