Compressibility and specific volume of actin decrease upon G to F transformation

We measured the densities as well as the sound velocities in solutions of G-actin, F-actin and the reconstituted thin filament. Using the data obtained, we determined their partial specific volumes and partial specific adiabatic compressibilities. The objectives were to investigate the volume change of actin upon polymerization and to detect the conformational change associated with the Ca2+-binding to the reconstituted thin filament. The partial specific volume and the partial specific adiabatic compressibility of G-actin were 0.749 cm(3)/g and 9.3 . 10(-12) cm(2)/dyne, respectively. The results suggest that G-actin is a rather soft protein compared with other globular proteins. The partial specific volumes of F-actin were in a range of 0.63-0.66 cm(3)/g depending on the solvent conditions. The partial specific adiabatic compressibilities of F-actin were negative (-(7-13). 10(-12) cm(3)/dyne). These data indicate that the amount of hydration may increase by several times upon polymerization assuming that the size of the cavity remains constant. We detected little difference between the partial specific adiabatic compressibility of the reconstituted thin filament in a Ca2+-bound state and that in a Ca2+-unbound state. This suggests that the Ca2+ binding affected not the subunit itself but the inter-subunit junction.