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Binding of C3b and C4b by the CR1-like site in murine CR1
1Graduate Program in Immunology, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
Insights
Murine complement receptor 1 (CR1) binds both C3b and C4b complement proteins. The binding sites for these proteins are located within the N-terminal short consensus repeats (SCRs) and share similar boundaries.
Area of Science:
- Immunology
- Complement System
- Protein Interactions
Background:
- Complement receptors play crucial roles in immune responses.
- Complement receptor 1 (CR1) and complement receptor 2 (CR2) are key regulators of the complement cascade.
- Understanding the ligand-binding specificities of these receptors is vital for deciphering immune mechanisms.
Purpose of the Study:
- To investigate the binding site for C4b on murine CR1, specifically within the N-terminal six short consensus repeats (SCRs).
- To determine if the binding sites for C3b and C4b on murine CR1 overlap or are distinct.
- To elucidate the role of individual SCRs in the binding of C3b and C4b.
Main Methods:
- Generation of human K562 transfectant cell lines expressing variable combinations of murine SCRs fused to human CR2.
- Assessment of cell line binding capacity to sheep erythrocytes (E) bearing rat C4b or guinea pig C3b (rosette formation assay).
- Systematic deletion of SCRs to identify essential domains for ligand binding.
Main Results:
- Murine CR1, comprising SCRs 1-6, binds both C3b and C4b.
- Deletion of SCRs 5-6 partially reduced binding of both C3b and C4b.
- SCR-1 was found to be essential for the binding of both C3b and C4b, indicating overlapping binding site boundaries.
Conclusions:
- Murine CR1 possesses a binding site for C4b in addition to its known C3b binding site.
- The binding sites for C3b and C4b on murine CR1 are closely related, sharing similar amino- and carboxyl-terminal boundaries.
- SCR-1 is critically important for the interaction of murine CR1 with both C3b and C4b.
Abstract:
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.