Membrane Cofactor Protein
Membrane cofactor protein (MCP) is a widely expressed transmembrane complement regulator built from four short consensus repeat modules, a serine/threonine-rich stalk, a transmembrane segment, and a cytoplasmic tail subject to alternative splicing. On the cell surface it serves as an essential cofactor for factor I, which cleaves C3b to iC3b and C4b to C4b2, dismantling convertases assembled on host membranes while leaving neighboring cells untouched. Unlike DAF, MCP is not GPI-anchored, and its expression on virtually all nucleated cells makes them resistant to complement lysis. MCP also acts as a receptor for measles virus and several adenovirus species.
