Evolutionary relationships among proteins encoded by the regulator of complement activation gene cluster.

Evolutionary relationships among members of the regulator of complement activation (RCA) gene cluster were analyzed using neighbor-joining and parsimony methods of phylogenetic tree inference. We investigated the structural and functional similarities among short consensus repeats (SCRs) of the following human proteins: the alpha chain of the C4b-binding protein (C4bpalpha), factor H (FH), factor H-related proteins (FHR-1 through FHR-4), complement receptors type 1 (CR1) and type 2 (CR2), the CR1-like protein (CR1L), membrane cofactor protein (MCP), decay accelerating factor (DAF), and the sand bass proteins, the cofactor protein (SBP1) and its homolog, the cofactor-related protein (SBCRP-1). Also included are the beta chain of the human C4b-binding protein (C4bpbeta) and the b subunit of human blood-clotting factor XIII (FXIIIb). Our results indicate that the human plasma complement regulators, FH and C4bpalpha, fall into two distinct groups on the basis of their sequence divergence. Homology among RCA proteins is in agreement with their chromosomal location, with the exception of C4bpbeta. The evolutionary relationships among individual short consensus repeats are confirmed by the exon/intron structure of the RCA members. Structural similarities among repeats of the RCA proteins correlate with their functional activities and demonstrate the importance of the N-terminal SCRs.