Molecular cloning, expression and characterization of the rat analogue of human membrane cofactor protein (MCP/CD46)

In humans, host cells are protected from homologous complement by membrane proteins encoded in the regulators of complement activation (RCA) gene clus ter. These include complement receptor I (CRI), decay-accelerating factor ( DAF, CD55) and membrane cofactor protein (MCP, CD46). In mouse and rat a si ngle membrane inhibitor, Crry, appeared to perform the functions of both DA F and MCP and was proposed to be the functional analogue of both. Recently, however, murine homologues of DAF and MCP have been identified, prompting a search for the rat counterparts. We have described the identification of rat DAF and here describe the cloning of rat MCP from cDNA. and genomic lib raries, using a probe based on the mouse MCP cDNA sequence. The domain stru cture for rat MCP was identical to that of mouse MCP with four short consen sus repeats (SCRs) followed by a STP domain, transmembrane segment and cyto plasmic tail. Overall identity of rat and mouse MCP was 77% at the amino ac id level and 88% at the nucleotide level. Northern blot analysis from a ran ge of tissues indicated that high-level expression was limited to the testi s, although expression in other tissues was detected using reverse transcri ption-polymerase chain reaction. Rat MCP mRNA localized to Sertoli cells an d spermatogonia in seminiferous tubules by in situ hybridization, but was a bsent in mature sperm. In cofactor assays utilizing human factor I, a recom binant soluble form of rat MCP catalysed cleavage of human C3ma.