Dissecting sites important for complement regulatory activity in membrane cofactor protein (MCP; CD46)

Membrane cofactor protein (MCP; CD46), a widely distributed regulator of co mplement activation, is a cofactor for the factor I-mediated degradation of C3b and C4b deposited on host cells. MCP possesses four extracellular, con tiguous complement control protein modules (CCPs) important for this inhibi tory activity. The goal of the present study was to delineate functional si tes within these modules. We employed multiple approaches including mutagen esis, epitope mapping, and comparisons to primate MCP to make the following observations. First, functional sites were located to each of the four CCP s. Second, some residues were important for both C3b and C4b interactions w hile others were specific for one or the other. Third, while a reduction in ligand binding was invariably accompanied by a parallel reduction in cofac tor activity (CA), other mutants lost or had reduced CA but retained ligand binding. Fourth, two C4b-regulatory domains overlapped measles virus inter active regions, indicating that the hemagglutinin docks to a site important for complement inhibition. Fifth, several MCP regulatory areas corresponde d to functionally critical, homologous positions in other CCP-bearing C3b/C 4b-binding proteins. Based on these data and the recently derived crystal s tructure of repeats one and two, computer modeling was employed to predict MCP structure and examine active sites.