Electrostatic Exploration of Membrane Cofactor Protein (MCP) using Computational Alanine Scans, Poisson-Boltzmann Electrostatics and Experimental Mutagenesis Data

The complement system is part of innate immunity and a link between innate and adaptive immunities. One of the functions of the complement system is tagging foreign pathogens by coating their surfaces with the protein C3b. The complement system discriminates “self” from “nonself” through tight regulation, which involves cleavage of C3b when it coats self-tissues. The membrane cofactor protein (MCP) contributes to the regulation of the complement system by cleaving the protein C3b, with the aid of Factor I, a process called cofactor activity. The interaction between MCP and C3b is driven by charge hotspots, which are localized in specific MCP modules and C3b domains. To delineate the role of each charged amino acid in the MCPC3b interaction, we performed computational alanine scans, Poisson-Boltzmann electrostatic potential calculations, and hierarchical clustering of spatial distributions of electrostatic potentials. Based on our physicochemical analysis and existing experimental data, we present a model for the MCP-C3b interaction, by focusing on the whole MCP and individual contributions of the four MCP modules. Introduction Membrane Cofactor Protein (MCP; CD46) is a transmembrane glycoprotein ubiquitously expressed by nucleated cells except for human erythrocytes. Initially identified as a C3b and C4b binding protein of human red blood cells, MCP has now proven to have more roles in the human system. MCP is a member of the regulators of the complement activation (RCA) proteins and is composed of four N-terminal complement control proteins (CCP) called CCP modules, followed by a Serine, Threonine, Proline (STP) transmembrane domain and one of two spliced cytoplasmic tails. MCP protects the host cells from autoimmunity by regulating the complement system. It effectively accomplishes regulation through the process of cofactor activity, which involves MCP as a cofactor for the Factor I-mediated cleavage of C3b molecules that are deposited on host cells (Fig. 1). The convertase complex C3bBb, which is the cleaving