Frictional resistance to motions of bimane-labelled spinach calmodulin in response to ligand binding.

The single cysteinyl residue 26 of spinach calmodulin was labelled with the thiol-specific bimane fluorescence probe. Following application of stoichiometric quantities of Ca2+ or aluminum ions to the protein, temperature-dependent fluorescence changes (anisotropy, lifetime) could be monitored via the label. From these data the Y function could be constructed which, as a function of temperature, seems to consist of two linear regions which intersect at the critical temperature, Tc. From the Y function the thermal coefficient, b(T), of the frictional resistance to fluorophore rotation could be determined. b(T) was dependent on the type and stoichiometry of the ligand(s) bound to calmodulin. Changes of the thermal coefficient apparently resulted in part from ligand-triggered structural pertubations transmitted over a considerable distance to calmodulin region I, the site of the fluorophore.