Binding of C3b and C4b by the CR1-like site in murine CR1.

We determined whether the six short consensus repeats (SCRs) that are appended to the amino terminus of murine CR2 to form murine CR1 contain a binding site for C4b in addition to that for C3b, and whether these sites overlap or are distinct. Human K562 transfectant cell lines were established that stably expressed constructs encoding variable combinations of these six murine SCRs attached to the amino terminus of a truncated form of human CR2 lacking its iC3b/C3dg binding site. These cell lines, and two others expressing full-length human CR1 and SCRs lacking its iC3b/C3dg binding site. These cell lines, and two others expressing full-length human CR1 and SCRs 8-11 of the C3b binding site of human CR1, respectively, were assessed for their capacity to form rosettes with sheep E bearing rat C4b or guinea pig C3b. K562 cells with full length human CR1 formed rosettes with both EC3b and EC4b, and the cells expressing the construct with human CR1 SCRs 8-11 bound only EC3b. The murine CR1/human CR2 chimera containing murine SCRs 1-6 resembled the full length human CR1 in binding both EC3b and EC4b. Deletion of SCRs 5-6 from the murine CR1/human CR2 chimera diminished in parallel, but did not abolish, binding of EC3b and EC4b. Constructs containing SCRs 2-5, SCRs 3-6, or SCRs 2-6 lacked activity, indicating an absolute requirement for SCR-1 for binding of both C3b and C4b. Therefore, murine CR1 binds both C3b and C4b, and the sites for these ligands have similar, if not identical, amino- and carboxyl-terminal boundaries.