A polymorphism of the complement regulatory protein MCP (membrane cofactor protein or gp45-70)

Membrane cofactor protein (MCP or gp45-70) is a recently described regulatory glycoprotein of the complement system which binds iC3 or C3b and is present on human platelets, T cells, B cells, monocytes, and mononuclear-derived cell lines. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, MCP migrates as a doublet with an M(r) of the upper band of 63,000 and the lower band of 58,000. The same pattern was found on all cell populations in a given individual and was stable over time. In order to further characterize the two band pattern on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, MCP was isolated by affinity chromatography or immunoprecipitation from 72 healthy unrelated donors. All individuals expressed both bands and, based on the densitometric scanning of gels, three patterns were noted: 1) upper band predominant in 65%, 2) approximately equal distribution of upper and lower bands in 29%, and 3) lower band predominant in 6%. These observed phenotypic frequencies fit with expectations based on Hardy-Weinberg equilibrium for a two-allele system. Family studies also support this model as none of the 26 offspring had a phenotype that deviated from the expected, assuming an autosomal codominant model of inheritance. These results are consistent with a simple two-allele system that controls the expression of the two bands of MCP.