Molecular engineering of an efficient four-domain DAF-MCP chimera reveals the presence of functional modularity in RCA proteins

Significance Complement regulation is critical for protecting the body’s own cells from homologous complement. This function is primarily mediated by the regulators of complement activation (RCA) proteins through two functional mechanisms termed decay-accelerating activity (DAA) and cofactor activity (CFA). Here, we show the presence of functional modularity in the RCA proteins. We demonstrate that the basic functional blocks for DAA and CFA in these proteins are formed by only two domains and that these are arranged in a specific order. Furthermore, we show that linking functional blocks of the decay-accelerating factor and membrane cofactor protein—which possess DAA and CFA, respectively—and adding C-terminal domains for enhancing avidity toward C3b/C4b results in a regulator molecule with dual activity.