Enthalpic and entropic contributions to actin stability: calorimetry, circular dichroism, and fluorescence study and effects of calcium

The delta H associated with the thermal unfolding of G-actin has been determined by differential scanning calorimetry (DSC) to be 142 +/- 5 kcal/mol, with the Tm (melting temperature) at 57.2 +/- 0.5 degrees C, at pH 8.0 (heating rate 0.5 K/min). The transition is broad and cannot be treated as a single transition that mimics a two-state process, suggesting the existence of domains. Deconvolution is done to fit it into two quasi-independent two-state transitions. For F-actin, the transition is more cooperative, with a cooperative ratio (the ratio of van't Hoff enthalpy and calorimetric enthalpy) of 1.4, indicating intermonomer interaction. The delta H of the thermal unfolding of F-actin is 162 +/- 10 kcal/mol with a Tm at 67.0 +/- 0.5 degrees C. A state of G-actin similar to that of the heat-denatured form, designated D-actin, is obtained by removing tightly bound Ca2+ with EGTA. The DSC-detectable cooperative transition is completely lost when the free calcium concentration of the medium is 1 x 10(-11) M or lower, using a Ca2+/EGTA buffer system. However, circular dichroism (CD) shows that the helix content of actin, 32% in the G-form, is only partially reduced to 19% in this apo form. The CD spectrum and the helix content of the calcium-depleted actin are almost identical with those of the heat-denatured D form. This loss of 40% of the native helical content is irreversible in both cases. The remaining 60% of the native helical content cannot be further eliminated by heating to 95 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)