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Diversity of sites for measles virus binding and for inactivation of complement C3b and C4b on membrane cofactor
Abstract:
The complement system membrane cofactor protein (MCP) CD46 serves as a C3b/C4b inactivating factor for the protection of host cells from autologous complement attack and as a receptor for measles virus (MV). MCP consists of four short consensus repeats (SCR) which are the predominant extracellular structural motif. In the present study, we determined which of the four SCR of MCP contribute to its function using Chinese hamster ovary cell clones expressing each SCR deletion mutants. The results were as follows: 1) SCR1 and SCR2 are mainly involved in MV binding and infection; 2) SCR2, SCR3, and SCR4 contribute to protect Chinese hamster ovary cells from human alternative complement pathway-mediated cytolysis; and 3) SCR2 and SCR3 are essential for protection of host cells from the classical complement pathway. These results on cell protective activity of the mutants against the human classical and the alternative complement pathways were compatible with factor I-mediated inactivation profiles of C4b and C3b, respectively, in the fluid-phase assay using solubilized mutants and factor I; the results were mostly consistent with those reported by Adams et al. (Adams, E. M., Brown, M. C., Nunge, M., Krych, M., and Atkinson, J. P. (1991) J. Immunol. 147, 3005-3011). SCR2 and SCR3 were required for C3b and C4b inactivation, and SCR4-deleted MCP showed weak cofactor activity for C4b cleavage but virtually no cofactor activity for C3b cleavage. The functional domains of MCP for the three natural ligands C3b, C4b, and MV, therefore, map to different, although partly overlapping, SCR domains.
Insights
Membrane cofactor protein (MCP) CD46 uses its short consensus repeats (SCR) to bind measles virus and inactivate complement proteins C3b and C4b, protecting host cells. Different SCR domains mediate distinct functions, including viral binding and complement regulation.
Area of Science:
- Immunology
- Virology
- Molecular Biology
Background:
- The complement system's membrane cofactor protein (MCP) CD46 is crucial for host defense, acting as a C3b/C4b inactivator and a measles virus receptor.
- MCP comprises four short consensus repeats (SCRs), which form its primary extracellular structure.
Purpose of the Study:
- To delineate the functional roles of each of the four SCR domains of MCP.
- To understand the specific contributions of SCR domains to measles virus binding and complement regulatory activities.
Main Methods:
- Generation and analysis of Chinese hamster ovary (CHO) cell clones expressing MCP with individual SCR deletions.
- Assessing measles virus binding and infection.
- Evaluating complement-mediated cytolysis via classical and alternative pathways.
- Conducting fluid-phase assays with solubilized MCP mutants and factor I to measure C3b and C4b inactivation.
Main Results:
- SCR1 and SCR2 are primarily involved in measles virus binding and infection.
- SCR2, SCR3, and SCR4 are essential for protecting CHO cells from alternative complement pathway-mediated lysis.
- SCR2 and SCR3 are critical for protecting host cells from classical complement pathway-mediated lysis.
- SCR2 and SCR3 are required for efficient C3b and C4b inactivation by factor I.
- SCR4-deleted MCP exhibits reduced cofactor activity for C4b and minimal activity for C3b.
Conclusions:
- The functional domains of MCP for its natural ligands (C3b, C4b, and measles virus) map to distinct, yet partially overlapping, SCR domains.
- SCR1 and SCR2 mediate viral interactions, while SCR2, SCR3, and SCR4 are involved in complement regulation through distinct pathways.