Characterization of CR1- and membrane cofactor protein-like proteins of two primates. 
We have identified and characterized C3b binding proteins of two primates, orangutan (Pongo pygmaeus) and gorilla (Gorilla gorilla).  Detergent solubilized 125I surface-labeled E and PBMC were subjected to affinity chromatography with homologous or human iC3/C3b.  These ligands bound a 225,000 single chain protein from orangutan E and PBMC and a 220,000 protein from gorilla E. Proteins of the same Mr were immunoprecipitated by a rabbit polyclonal and two murine mAb to the human CR1 (CD35).  The C3b binding protein of gorilla E aligned with that of the common human CR1 polymorphic size variant.  Human or orangutan iC3 was also a ligand for a surface-labeled protein doublet of 59,000 and 65,000 from orangutan E. The doublet pattern and mol wts are similar to membrane cofactor protein (or CD46).  Further, this doublet was immunoprecipitated by a mAb to human MCP.  The MCP-like protein doublet was not isolated from gorilla or human E. Decay accelerating factor (DAF) of orangutan E was also identified and was structurally and antigenically distinct from the MCP-like protein.  Orangutan or gorilla E preparations were a cofactor for the cleavage of human iC3 by human factor I and produced the same cleavage fragments as human CR1.  Cofactor activity of orangutan E was partially inhibited by preclearance of CR1 and more completely inhibited by preclearance of MCP.  Cofactor activity of gorilla E was inhibited by coincubation with a monoclonal antibody to human CR1.  These data indicate that the orangutan and gorilla high m.w.  proteins are equivalent to human CR1.  The orangutan E membrane protein doublet with m.w.  of 59,000 and 65,000 possesses biochemical, antigenic, and functional properties of human membrane cofactor protein.