The effect of BeFx and a natural toxin (jasplakinolide) was examined on the thermal stability of actin filaments by using differential scanning calorimetry. The phosphate analogue beryllium fluoride shifted the melting temperature of actin filaments (67.4 degrees C) to 83.7 degrees C indicating that the filaments were thermodynamically more stable in their complex with ADP.BeFx. A similar tendency was observed when the jasplakinolide was used in the absence of BeFx. When both the ADP.BeFx and the jasplakinolide bound to the actin filaments their collective effect was similar to that observed with ADP.BeFx or jasplakinolide alone. These results suggested that ADP.BeFx and jasplakinotide probably stabilize the actin filaments by similar molecular mechanisms. (C) 2007 Elsevier B.V. All rights reserved.