Evidence for calmodulin inter-domain compaction in solution induced by W-7binding

Small-angle X-ray scattering and nuclear magnetic resonance were used to in vestigate the structural change of calcium-bound calmodulin (Ca2+/CaM) in s olution upon binding to its antagonist, N-(6-aminohexyl)-5-chloro-1-naphtha lenesulfonamide (W-7). The radius of gyration was 17.3+/-0.3 Angstrom for C a2+/CAM-W-7 with a molar ratio of 1:5 and 20.3 +/- 0.7 Angstrom for Ca2+/Ca M. Comparison of the radius of gyration and the pair distance distribution function of the Ca2+/CaM-W-7 complex with those of other complexes indicate s that binding of two W-7 molecules induces a globular shape for Ca2+/CaM, probably caused by an inter-domain compaction. The results suggest a tenden cy for Ca2+/CaM to form a globular structure in solution, which is inducibl e by a small compound like W-7. (C) 1999 Federation of European Biochemical Societies.