Membrane cofactor protein: importance of N- and O-glycosylation for complement regulatory function.
Membrane cofactor protein (MCP; CD46) is a type 1 membrane glycoprotein that inhibits complement activation on host cells. It also is a measles virus (MV) receptor, an adherence factor for group A Streptococcus pyogenes, and a cellular pilus receptor for pathogenic Neisseria. The amino terminus of MCP consists of four complement control protein (CCP) repeats, three of which (CCP-1, -2, and -4) possess N-glycans. Immediately following the CCP modules is an alternatively spliced region for extensive O-glycosylation (termed the STP domain). Previous studies established that the N-glycan of CCP-2 is essential for MV binding and infection and that the splicing variants of the STP domain not only affect MV binding and fusion, but also differentially protect against complement-mediated cytolysis. In this report, we dissect the role of these carbohydrates on complement regulatory function. We constructed, expressed, and characterized proteins deleting these carbohydrates. For MCP-mediated protection against cytolysis, the N-glycans of CCP-2 and -4 were necessary, the STP segment influenced but was not essential, and the N-glycan of CCP-1 was not required. In addition, the rate and magnitude of cell surface cleavage of C4b to C4c and C4d by MCP and factor I correlated with cytoprotection. These studies expand the structure-function understanding of the active sites of MCP and elucidate an important role for carbohydrates in its function, a finding consistent with their conservation in the MCP of other species.
Liszewski
M K
MK
Division of Rheumatology, Washington University School of Medicine, St. Louis, MO 63110, USA.
Leung
M K
MK
Atkinson
J P
JP
D000818:Animals; D015703:Antigens, CD / Q000235:genetics / Q000378:metabolism*; D001216:Asparagine / Q000235:genetics; D016466:CHO Cells; D002458:Cell Fractionation; D002999:Clone Cells; D003167:Complement Activation / Q000235:genetics; D003179:Complement C3b / Q000235:genetics / Q000378:metabolism; D015935:Complement C4b / Q000235:genetics / Q000378:metabolism; D006224:Cricetinae; D019610:Cytoprotection / Q000276:immunology; D005973:Glutamine / Q000235:genetics; D006031:Glycosylation; D006801:Humans; D006868:Hydrolysis; D007700:Kinetics; D051916:Membrane Cofactor Protein; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D011392:Proline / Q000235:genetics; D017434:Protein Structure, Tertiary; D017384:Sequence Deletion / Q000276:immunology; D012694:Serine / Q000235:genetics; D013912:Threonine / Q000235:genetics; D014162:Transfection / Q000276:immunology
D015703:Antigens, CD; C497857:CD46 protein, human; D051916:Membrane Cofactor Protein; D008562:Membrane Glycoproteins; D005973:Glutamine; D013912:Threonine; D012694:Serine; D001216:Asparagine; D003179:Complement C3b; D015935:Complement C4b; D011392:Proline