ANALYSIS OF C3B C3D BINDING-SITES AND FACTOR-I COFACTOR REGIONS WITHIN MOUSE COMPLEMENT RECEPTOR-1 AND RECEPTOR-2/

Human and murine CR1 and CR2 are defined as evolutionary homologues on the basis of their in vitro activities and a shared structural motif known as a short consensus repeat (SCR). To identify additional simila rities between the two species, we analyzed the functional domains wit hin the mouse receptors by constructing mouse-human chimeric cDNAs in which the C3 binding site of human CR2 has been replaced by different regions within the first eight SCRs of mouse CR1. Rosette analysis of cells expressing chimeric proteins, with erythrocytes bearing differen t mouse C3 fragments, coupled with rosette inhibition studies using sp ecific anti-mouse CR1/CR2 mAbs reveal a weak C3b binding site within S CRs 1 and 2 of mouse CR1. There is no independent C3b interacting doma in within SCRs 3 to 6, but their presence enhances C3 binding. A molec ule that contains only the first six SCRs of mouse CR1 also binds C3b, but with less efficiency. There is no C3d binding area within the fir st six SCRs, but our data confirms previous studies indicating an addi tional C3b/C3d binding region within SCRs 7 and 8 of mouse CR1 (SCRs 1 -2 of mouse CR2). The presence of SCRs 1 to 4 is required for C3 cofac tor activity. 8C12, a mAb which blocks C3b erythrocyte rosette binding and the C3 cofactor activity of mouse CR1, binds only to chimeras con taining SCRs 3 to 4. In summary, human and mouse CR1 contain two indep endent C3b binding sites and share a common framework consisting of SC Rs involved in ligand binding and nonbinding SCRs that optimize activi ty, further supporting the hypothesis that these two molecules act as functional homologues.