Strong Dependence of Hydration State of F-Actin on the Bound Mg super(2+)/Ca super(2+) Ions

Understanding of the hydration state is an important issue in the chemomechanical energetics of versatile biological functions of polymerized actin (F-actin). In this study, hydration-state differences of F-actin by the bound divalent cations are revealed through precision microwave dielectric relaxation (DR) spectroscopy. G- and F-actin in Ca- and Mg-containing buffer solutions exhibit dual hydration components comprising restrained water with DR frequency f sub(2) (f sub(w)). The hydration state of F-actin is strongly dependent on the ionic composition. In every buffer tested, the HMW signal D sub(hyme) ( identical with (f sub(1) - f sub(w)) delta sub(1)/(f sub(w) delta sub(w))) of F-actin is stronger than that of G-actin, where delta sub(w) is DR-amplitude of bulk solvent and delta sub(1) is that of HMW in a fixed-volume ellipsoid containing an F-actin and surrounding water in solution. D sub(hyme) value of F-actin in Ca2.0-buffer (containing 2 mM Ca super(2+)) is markedly higher than in Mg2.0-buffer (containing 2 mM Mg super(2+)). Moreover, in the presence of 2 mM Mg super(2+), the hydration state of F-actin is changed by adding a small fraction of Ca super(2+) ( similar to 0.1 mM) and becomes closer to that of the Ca-bound form in Ca2.0-buffer. This is consistent with the results of the partial specific volume and the Cotton effect around 290 nm in the CD spectra, indicating a change in the tertiary structure and less apparent change in the secondary structure of actin. The number of restrained water molecules per actin (N sub(2)) is estimated to be 1600-2100 for Ca2.0- and F-buffer and similar to 2500 for Mg2.0-buffer at 10-15 degree C. These numbers are comparable to those estimated from the available F-actin atomic structures as in the first water layer. The number of HMW molecules is roughly explained by the volume between the equipotential surface of -kT/2e and the first water layer of the actin surface by solving the Poisson-Boltzmann equation using UCSF Chimera.