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Function of the factor I modules (FIMS) of human complement component C6
R G DiScipio1, S M Linton, N K Rushmere
1La Jolla Institute for Experimental Medicine, La Jolla, California 92037, USA.
Insights
The factor I modules (FIMs) in complement component C6 are crucial for efficient C5b-6 complex formation. Deleting FIMs reduces C6 activity, particularly in the Classical Pathway, highlighting their role in C6-C5 interaction.
Area of Science:
- Immunology
- Complement System Biology
Background:
- The complement system is vital for innate and adaptive immunity.
- Complement component C6 plays a role in the terminal pathway of complement activation.
- Understanding C6 function is key to deciphering complement-mediated immune responses.
Purpose of the Study:
- To elucidate the functional role of specific modules within complement component C6.
- To investigate the contribution of Factor I Modules (FIMs) and Complement Control Protein (CCP) modules to C6 activity.
- To compare the functional impact of FIMs and CCPs on C6 in both Classical and Alternative Complement Pathways.
Main Methods:
- Recombinant expression of truncated C6 molecules, including C6 lacking FIMs (C6des-748-913) and C6 lacking both CCP and FIMs (C6des-611-913).
- Assays for hemolytic activity of full-length and truncated C6 in both Classical and Alternative Complement Pathways.
- Measurement of the stability and decay rates of metastable C5b* and (125)I-C5b in different complement pathways.
- Investigation of the binding kinetics of C5 to C6 and its truncated variants.
Main Results:
- C6 lacking FIMs (C6des-748-913) showed reduced hemolytic activity (60-70% in Alternative Pathway, 4-6% in Classical Pathway) compared to full-length C6.
- The stability of metastable C5b* and the half-life of (125)I-C5b were significantly longer in the Alternative Pathway than in the Classical Pathway.
- (125)I-C5 binding to C6 was ionic strength-dependent, weak to C6des-FIMs, and absent in C6des-CCP/FIMs.
Conclusions:
- Factor I Modules (FIMs) are important for efficient C6 interaction with C5, facilitating C5b-6 complex formation.
- While not absolutely essential, FIMs significantly enhance C6 activity, particularly in the Classical Pathway.
- Differences in C5b* stability between complement pathways contribute to the observed functional variations of truncated C6 molecules.
Abstract:
In order to elucidate the function of complement component C6, truncated C6 molecules were expressed recombinantly. These were either deleted of the factor I modules (FIMs) (C6des-748-913) or both complement control protein (CCP) modules and FIMs (C6des-611-913). C6des-748-913 exhibited approximately 60-70% of the hemolytic activity of full-length C6 when assayed for Alternative Pathway activity, but when measured for the Classical Pathway, C6des-748-914 was only 4-6% as effective as C6. The activity difference between C6 and C6des-748-913 for the two complement pathways can be explained by a greater stability of newly formed metastable C5b* when produced by the Alternative Pathway compared with that made by the Classical Pathway. The half-lives of metastable C5b* and the decay of (125)I-C5b measured from cells used to activate the Alternative Pathway were found to be about 5-12-fold longer than those same parameters derived from cells that had activated the Classical Pathway. (125)I-C5 binds reversibly to C6 in an ionic strength-dependent fashion, but (125)I-C5 binds only weakly to C6des-FIMs and not at all to C6des-CCP/FIMs. Therefore, although the FIMs are not required absolutely for C6 activity, these modules promote interaction of C6 with C5 enabling a more efficient bimolecular coupling ultimately leading to the formation of the C5b-6 complex.