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Dissecting sites important for complement regulatory activity in membrane cofactor protein (MCP; CD46)
M K Liszewski1, M Leung, W Cui
1Division of Rheumatology, Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Abstract:
Membrane cofactor protein (MCP; CD46), a widely distributed regulator of complement activation, is a cofactor for the factor I-mediated degradation of C3b and C4b deposited on host cells. MCP possesses four extracellular, contiguous complement control protein modules (CCPs) important for this inhibitory activity. The goal of the present study was to delineate functional sites within these modules. We employed multiple approaches including mutagenesis, epitope mapping, and comparisons to primate MCP to make the following observations. First, functional sites were located to each of the four CCPs. Second, some residues were important for both C3b and C4b interactions while others were specific for one or the other. Third, while a reduction in ligand binding was invariably accompanied by a parallel reduction in cofactor activity (CA), other mutants lost or had reduced CA but retained ligand binding. Fourth, two C4b-regulatory domains overlapped measles virus interactive regions, indicating that the hemagglutinin docks to a site important for complement inhibition. Fifth, several MCP regulatory areas corresponded to functionally critical, homologous positions in other CCP-bearing C3b/C4b-binding proteins. Based on these data and the recently derived crystal structure of repeats one and two, computer modeling was employed to predict MCP structure and examine active sites.
Insights
Membrane cofactor protein (MCP; CD46) regulates complement activation. Functional sites across all four MCP modules were identified, revealing specific roles in C3b/C4b binding and cofactor activity.
Area of Science:
- Immunology
- Molecular Biology
- Structural Biology
Background:
- Membrane cofactor protein (MCP; CD46) is a key regulator of complement activation.
- MCP acts as a cofactor for factor I-mediated degradation of C3b and C4b on host cells.
- MCP contains four extracellular complement control protein (CCP) modules crucial for its inhibitory function.
Purpose of the Study:
- To delineate functional sites within the four CCP modules of MCP.
- To understand the specific roles of MCP residues in C3b and C4b interactions and cofactor activity.
Main Methods:
- Site-directed mutagenesis to alter specific residues within MCP CCP modules.
- Epitope mapping to identify regions involved in protein interactions.
- Comparative analysis with primate MCP sequences.
- Computational modeling based on existing crystal structures.
Main Results:
- Functional sites were identified in all four CCP modules of MCP.
- Specific residues were found to be critical for C3b and/or C4b binding and cofactor activity.
- Some MCP mutants exhibited reduced cofactor activity despite retained ligand binding.
- MCP regions involved in C4b regulation overlapped with measles virus binding sites.
- Identified MCP regulatory regions showed homology to functionally critical sites in other CCP-containing proteins.
Conclusions:
- MCP's four CCP modules contain distinct and overlapping functional sites for complement regulation.
- The interaction of measles virus hemagglutinin with MCP highlights a convergence of viral evasion and complement inhibition strategies.
- Structural and functional insights into MCP provide a basis for understanding its role in host defense and disease.