Effects of cation binding on the thermal transitions in calmodulin

Abstract

The thermal transitions in different forms of bovine brain calmodulin (0, 1, 2, 3 and 4 bound Ca 2+ ions per molecule) have been studied by means of microcalorimetry, intrinsic tyrosine fluorescence, circular dichroism and infrared spectroscopy. The heating of the apoprotein from 5 to 110°C induces at least three unfolding transitions. The heating of Ca 2+-loaded calmodulin causes at least two structural transitions, one of which occurs at relatively low temperatures, from approx. 30 to approx 50°C. The binding of the biologically significant Ca 2+, Mg 2+, Na + and K + ions has been measured at 12, 20, 28, 37 and 50°C by means of the fluorescence method. The values of the binding parameters for these cations do not depend on temperature within the range 12 to 50°C. It has been proposed that the temperature independence of the metal-ion-binding properties of calmodulin is achieved due to the temperature-induced structural changes, which adjust the protein conformation in such a way that the protein-binding parameters remain constant.