Identification of a homologue of CD59 in a cyclostome: implications for the evolutionary development of the complement system.

We have employed a COS cell expression cloning procedure to isolate a full length cDNA clone encoding a hagfish leukocyte-associated membrane protein (HLMP1). The protein, which is identified by a monoclonal antibody (JB3) generated in our laboratory, is present on the majority of hagfish leukocytes and is also expressed on erythrocytes. The cDNA clone contained an open reading frame encoding a 120 residue polypeptide which exhibits 33% amino acid sequence identity with the precursor protein of human CD59, a leukocyte-associated membrane protein which regulates the action of the complement membrane attack complex on homologous cells. CD59 belongs to a family of structurally related glycoproteins which includes the Ly-6 proteins expressed on mouse lymphocytes. In addition to significant overall sequence homology HLMP1 shows conservation of 8 key cysteine residues with members of the CD59/Ly-6 family. Comparison of the hagfish sequence with that of the mature human CD59 protein suggested a processed protein consisting of 74 amino acids associated with the cell membrane via a GPI anchor. The latter was confirmed by immuno-flow cytometry following treatment of transfected COS cells with phospholipase. Phylogenetic analysis and tissue distribution of this protein in the hagfish are consistent with HLMP1 being a homologue of CD59. A three-dimensional model of HLMP1, constructed using the NMR-determined structure for human CD59 as a template, indicated conservation of a core structure of five strands of beta-sheet and a short helix stabilised by four disulfide bonds. These findings, when taken together with our previous identification of C5a-like chemotactic activity in LPS-activated serum, provide indirect evidence for the existence of the terminal lytic complement pathway (C5 to C9) in these primitive vertebrates.