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Purification and functional analysis of the polymorphic variants of the C3b/C4b receptor (CR1) and comparison with H,
Insights
Complement receptor 1 (CR1) variants A, B, and C show similar functional activities in purified states. CR1 exhibits cofactor activity comparable to C4bp and H, highlighting its role in complement regulation.
Area of Science:
- Immunology
- Biochemistry
- Complement System
Background:
- Four complement receptor 1 (CR1) variants (A, B, C, D) exist in the normal population, differing in molecular weight.
- CR1 plays a crucial role in regulating the complement cascade, a key part of the innate immune system.
Purpose of the Study:
- To develop an improved purification method for CR1 that avoids C3b affinity chromatography.
- To assess the functional activities of isolated CR1 variants (A, B, and C).
- To compare the functional activities of CR1 with other complement regulatory proteins: C4bp, H, and decay accelerating factor (DAF).
Main Methods:
- Developed an improved chromatographic purification scheme for CR1.
- Isolated CR1 variants A, B, and C.
- Assessed functional activities using fluid phase assays for cofactor activity and inhibition of C3 convertases.
Main Results:
- Purified CR1 variants A, B, and C demonstrated similar cofactor activity for I-mediated C3(H2O) cleavage and inhibitory activity for fluid phase C3 convertases.
- On a molar basis, CR1 showed inhibitory activity similar to C4bp for the classical pathway convertase and similar to H for the alternative pathway convertase.
- DAF was a more potent inhibitor of the alternative pathway convertase but less potent for the classical pathway convertase compared to CR1, C4bp, and H.
- Fluid phase CR1 was less efficient than H as a cofactor for C3b cleavage, contrasting with its activity on cell membranes, suggesting environmental influence.
Conclusions:
- Despite molecular weight differences, CR1 variants A, B, and C possess similar functional activities in their purified state.
- CR1 integrates the functional capabilities of C4bp and H, acting as a versatile regulator of both classical and alternative complement pathways.
- The local environment significantly modulates CR1's cofactor activity, emphasizing the importance of its cellular context in complement regulation.
Abstract:
Four CR1 variants have been found in the normal population and are designated CR1-A (190,000 daltons), CR1-B (220,000 daltons), CR1-C (160,000 daltons), and CR1-D (250,000 daltons). In the present study, we first developed an improved chromatographic purification scheme for CR1 that does not employ a C3b affinity step. CR1 variants (A, B, and C) were then isolated, and their individual functional activity was assessed. Each possessed similar co-factor activity for I-mediated cleavage of C3(H2O), as well as for the inhibitory activity for fluid phase C3 convertases. These results indicate that, despite relatively large Mr differences, in the purified state these three CR1 variants have similar functional activities. The functional activity of CR1 was also compared with C4bp, H, and decay accelerating factor (DAF) in fluid phase assays designed to assess the inhibition of the C3 convertases and co-factor activity. On a molar basis, CR1 had approximately the same inhibitory activity as C4bp for the classical pathway convertase, and had the same as H for the alternative pathway convertase. These results indicate that CR1 encompasses the functional capabilities of both proteins. They also raise a number of interesting genetic and structural questions in regard to these complement regulatory proteins, because C4bp is thought to have multiple C4b binding domains, whereas H is reported to bind one C3b. DAF was an approximately fourfold better inhibitor of the alternative pathway convertase than CR1 or H, but was a fourfold less efficient inhibitor of the classical pathway convertase than CR1 or C4bp. The effective inhibitory capacity of DAF in these fluid phase assay systems suggests that the DAF substrate specificity is for the convertases. Fluid phase CR1 was twofold less efficient than H in serving as a co-factor for the first cleavage of fluid phase C3b, and hardly mediated the second cleavage. These data are in contrast to the co-factor activity of CR1 on a cell membrane, and provide additional evidence for the local environment being a critical modulator of the function of proteins that regulate the activation of C3.