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Functional properties of membrane cofactor protein of complement
1Department of Immunology, Center for Adult Diseases, Osaka, Japan.
The Biochemical Journal
|December 1, 1989
Summary
Membrane cofactor protein (MCP) stabilizes complement system C3 convertases, enhancing C3b deposition. It shows potent cofactor activity for fluid-phase C3b but is inefficient on cell-bound C3b, suggesting a unique regulatory role.
Area of Science:
- Immunology
- Complement System Biology
- Protein Function Analysis
Background:
- Membrane cofactor protein (MCP) is known to regulate the complement system.
- MCP acts as a cofactor for factor I-mediated cleavage of C3b and C3.
- Its precise role in complement regulation, especially on cell surfaces, requires further elucidation.
Purpose of the Study:
- To further characterize the cofactor activity of Membrane cofactor protein (MCP).
- To investigate MCP's function in the absence and presence of factor I.
- To determine MCP's efficacy in cleaving cell-bound versus fluid-phase C3b.
Main Methods:
- Assessed MCP's effect on alternative and classical pathway cell-bound C3 convertases.
- Quantified cofactor activity of soluble MCP for factor I-mediated C3b cleavage on erythrocytes and zymosan.
- Investigated C3b cleavage by MCP and factor I on solubilized cell membranes.
Main Results:
- MCP unexpectedly stabilized cell-bound C3 convertases, enhancing C3b deposition without factor I.
- Soluble MCP exhibited inefficient cofactor activity for factor I in cleaving erythrocyte- or zymosan-bound C3b.
- MCP and factor I efficiently cleaved C3b bound to solubilized membrane molecules, but not on intact cells.
Conclusions:
- MCP possesses potent cofactor activity for fluid-phase C3b and C3b on solubilized molecules.
- MCP demonstrates inefficient extrinsic cofactor activity towards C3b on intact cells.
- This unique functional profile suggests a significant role for MCP in complement regulation across different cellular environments.