Glycosylation of membrane cofactor protein (CD46) in human trophoblast, kidney and platelets

Many cell surface glycoconjugates are differentiation markers and are involved in cell-cell and intermolecular interactions in development, immunity and cancer. Membrane cofactor protein (MCP) comprises structurally related 65 and 55 kDa glycoproteins that bear O- and N-linked glycans. MCP prevents amplification of autologous complement action on human cells. We used immunoblotting with MCP-specific monoclonal antibody TRA-2-10 to determine lectin-binding properties and glycosidase sensitivities of MCP in a study of cell-specific variation in glycosylation of this protein. The results showed that N-linked glycans on placental syncytiotrophoblast and cytotrophoblast, kidney and platelet MCP are similar in binding to concanavalin A and Lens culinaris lectins, but are not bound by leucophytohemagglutinin. Lectin binding prior to and after neuraminidase digestion indicates that MCP from these sources is highly sialylated. 65 kDa MCP was confirmed to contain more O-linked glycans than 55 kDa MCP. A fraction of platelet 65 kDa MCP is distinct, however, in bearing peripheral fucose residues. Syncytiotrophoblast is unique in containing a 110 kDa form of MCP in non-reducing SDS-PAGE that resembles 65 kDa MCP in glycosylation. Chorion laeve MCP in 4 of 8 preparations was unusually heterogeneous and differed from syncytiotrophoblast MCP after neuraminidase digestion in the forms bound to peanut agglutinin and WGA. The results indicated for the first time, differences in O-linked glycosylation of MCP in chorion laeve cytotrophoblast relative to syncytiotrophoblast, platelet and kidney MCP. We conclude that structures of MCP glycans can differ between trophoblasts and other cell types.