Membrane CofactorProteinof the Complement System:AlternativeSplicingof Serine/Threonine/Prohne-richExons and Cytoplasmic TailsProduces MultipleIsoformsthatCorrelatewith ProteinPhenotype

Summary Membrane cofactorprotein(MCP) isacomplementregulatory proteinthatisexpressedonhuman cellsandcelllinesastwo relatively broadspecieswithM, of58,000-68,000and48,000-56,000. The structure ofapreviously reportedcDNA cloneindicatedthatMCP was atype1membrane glycoproteinand amember of theregulatorsofcomplement activation gene/proteincluster . However,itdidnotprovideanexplanation fortheunusualphenotypicpatternofMCP .Therefore, inparallel with an analysis ofthegene,additional cDNAs were clonedand characterized .Six different MCP cDNA classes wereidentified .Allencodethesame5'untranslated signalpeptide, fourSCRs, transmembranedomain,andbasicamino acidanchor.However,theydifferinthe lengthandcompositionofan extracellular serine/threonine/proline (STP)-richarea,a siteof heavyO-glycosylation, andcytoplasmictail .AnalysisoftheMCP genedemonstratedthatthe variation incDNA structurewas aresultofalternative splicing .Peripheral bloodcellsandcell linespredominantlyexpressedfourofthesixisoforms.Thesevariedby thepresenceorabsence ofan STP-richsegmentof 15amino acids(STPB)and by theuseofone oftwo cytoplasmic domains.Analysisbypolymerasechainreaction, Northernblots,andtransfection indicatedthat thepredominanceof MCP cDNA isoformswith STPB correlatedwith thehighmolecular weightproteinphenotype,whilethepredominanceofisoformswithoutSTPB correlated with thelowermolecularweightphenotype.The expressionina singlecelloffourdistinct protein specieswith variableSTP-richregionsand cytoplasmictailsrepresents an interesting example oftheuseofalternative splicingtoprovidevariability ina mammalian protein.