Co‐operativity between modules within a C3b‐binding site of complement receptor type 1

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Abstract
Complement receptor type 1 (CR1) has 30 modules in its extracellular portion. An understanding of structure‐function relationships within CR1 is being assembled gradually from studies of overlapping protein fragments. A CR1 fragment corresponding to modules 16 and 17 was expressed recombinantly as a non‐glycosylated protein and its stability and unfolding characteristics studied using biophysical techniques. The results were compared with data collected previously on a CR1 fragment encompassing modules 15, 16 and 17 which together constitute a C3b‐binding site (Kirkitadze, M.D., Krych, M., Uhrin, D., Dryden, D.T.F., Smith, B.O., Wang, X., Hauhart, R., Atkinson, J.P. and Barlow, P.N. (1999) Biochemistry 38, 7019–7031). Modules within CR1 were found to co‐operate during unfolding. The folding, stability and flexibility of this protein is therefore likely to be a complex function, and not just the sum, of contributions from individual modules.