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J Biol Chem, Vol. 274, Issue 45, 31811-31818, November 5, 1999

Function of the Factor I Modules (FIMS) of Human Complement Component C6

Richard G. DiScipio, Stuart M. Linton^¶, and Neil K. Rushmere^¶

From the  La Jolla Institute for Experimental Medicine, La Jolla, California 92037 and the ^¶ Department of Medical Biochemistry, University of Wales College of Medicine, Cardiff CF4 4XX, Wales, United Kingdom

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 ¹²⁵I-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. ¹²⁵I-C5 binds reversibly to C6 in an ionic strength-dependent fashion, but ¹²⁵I-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.

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