Molecular cloning and chromosomal localization of human membrane cofactor protein (MCP). Evidence for inclusion in the multigene family of complement-regulatory proteins.

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Abstract
Membrane cofactor protein (MCP), a regulatory molecule of the complement system with cofactor activity for the factor I-mediated inactivaition of C3b and C4b, is widely distriubted, being present on leukocytes, platelets, endothelial cells epithelial cells, and fibroblasts. MCP was purified from a human T cell line (HSB2) and the NH2-terminal 24-amino acid sequence obtained by Edman degradation. An oligonucleotide probe based on this sequence was used to identify a clone from a human monocytic (U937) cDNA library. Nucleotide squencing showed a 43-bp 5''-untranslated region, an open reading frame of 1,152 bp, and a 335-bp 3''-untranslated region followed by a 16-bp poly(A) track. The deduced full-length MCP protein consists of a 34-amino acid signal peptide and a 350-amino acid mature protein. The protein has, beginning at the NH2 terminus, four .apprx. 60-amino acid repeat units that match the consensus sequence found in a multigene family of complement regulatory proteins (C3b-receptor or CR1, C3d-receptor or CR2, decay-acclerating factor, C4-binding protein, and factor H), and well as several other complement and non-complement proteins. The remainder of the MCP protein consists of 25 amino acids that are rich in serine and threonine (probable site of heavy O-linked glycosylation Of MCP), 17 amino acids of unknown significance, and a 23-amino acid transmembrane hydrophobic region followed by a 33-amino acid cytoplasmic tail. The MCP gene was localized to human chromosome 1, bands 1q31-41, by analysis of human .times. rodent somatic cell hybrid clones and by in situ hybridization. This same genetic region contains the multigene family of complement-regulatory proteins, which is thereby enlarged to include the functionally and structurally related MCP.