CHANGES IN MOLAR VOLUME AND HEAT-CAPACITY OF ACTIN UPON POLYMERIZATION

We have used densimetry and microcalorimetry to measure the changes in molar volume and heat capacity of the actin molecule during Mg2+-indu ced polymerization. Molar volume is decreased by 720 ml/mol. This resu lt is in contradiction with previous measurements by Ikkai and Ooi [(1 966) Science 152, 1756-1757], and by Swezey and Somero [(1985) Biochem istry 24, 852-860]: both of these groups reported increases in actin v olume during polymerization, of 391 ml/mol and 63 ml/mol respectively. We also observed a decrease in heat capacity of about 69.5 kJ . K-1 . mol-1 during polymerization. This is in agreement with the concept of conformational fluctuation of proteins proposed by Lumry and Gregory [(1989) J. Mol. Liq. 42, 113-144] whereby either ligand binding by a p rotein or monomer-monomer interaction decreases the protein's conforma tional flexibility.