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 cluster. These include complement receptor I (CRI), decay-accelerating factor (DAF, CD55) and membrane cofactor protein (MCP, CD46). In mouse and rat a single membrane inhibitor, Crry, appeared to perform the functions of both DAF 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 libraries, using a probe based on the mouse MCP cDNA sequence. The domain structure for rat MCP was identical to that of mouse MCP with four short consensus repeats (SCRs) followed by a STP domain, transmembrane segment and cytoplasmic tail. Overall identity of rat and mouse MCP was 77% at the amino acid level and 88% at the nucleotide level. Northern blot analysis from a range of tissues indicated that high-level expression was limited to the testis, although expression in other tissues was detected using reverse transcription-polymerase chain reaction. Rat MCP mRNA localized to Sertoli cells and spermatogonia in seminiferous tubules by in situ hybridization, but was absent in mature sperm. In cofactor assays utilizing human factor I, a recombinant soluble form of rat MCP catalysed cleavage of human C3ma.