title: Strong Dependence of Hydration State of F-Actin on the Bound Mg(2+)/Ca(2+) Ions.
abstract: 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 f2 (<fw: DR frequency of bulk solvent, 17 GHz at 20 °C) and hypermobile water (HMW) with DR frequency f1 (>fw). The hydration state of F-actin is strongly dependent on the ionic composition. In every buffer tested, the HMW signal Dhyme (≡ (f1 - fw)δ1/(fwδw)) of F-actin is stronger than that of G-actin, where δw is DR-amplitude of bulk solvent and δ1 is that of HMW in a fixed-volume ellipsoid containing an F-actin and surrounding water in solution. Dhyme value of F-actin in Ca2.0-buffer (containing 2 mM Ca(2+)) is markedly higher than in Mg2.0-buffer (containing 2 mM Mg(2+)). Moreover, in the presence of 2 mM Mg(2+), the hydration state of F-actin is changed by adding a small fraction of Ca(2+) (∼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 (N2) is estimated to be 1600-2100 for Ca2.0- and F-buffer and ∼2500 for Mg2.0-buffer at 10-15 °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.
authors.affiliation: Department of Materials Processing, Graduate School of Engineering, Tohoku University , 6-6-02 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan.|Frontier Research Institute for Interdisciplinary Sciences, Tohoku University , Sendai, Miyagi 980-8578, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Suzuki
authors.affiliation: Department of Materials Processing, Graduate School of Engineering, Tohoku University , 6-6-02 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan.
authors.forename: Asato
authors.initials: A
authors.lastname: Imao
authors.affiliation: Department of Materials Processing, Graduate School of Engineering, Tohoku University , 6-6-02 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan.|Frontier Research Institute for Interdisciplinary Sciences, Tohoku University , Sendai, Miyagi 980-8578, Japan.
authors.forename: George
authors.initials: G
authors.lastname: Mogami
authors.affiliation: Department of Materials Processing, Graduate School of Engineering, Tohoku University , 6-6-02 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan.
authors.forename: Ryotaro
authors.initials: R
authors.lastname: Chishima
authors.affiliation: Department of Materials Processing, Graduate School of Engineering, Tohoku University , 6-6-02 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Watanabe
authors.affiliation: Department of Materials Processing, Graduate School of Engineering, Tohoku University , 6-6-02 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan.
authors.forename: Takaya
authors.initials: T
authors.lastname: Yamaguchi
authors.affiliation: Department of Materials Processing, Graduate School of Engineering, Tohoku University , 6-6-02 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan.
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Morimoto
authors.affiliation: Department of Biomolecular Science and Engineering, The Institute of Scientific and Industrial Research, Osaka University , Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan.
authors.forename: Tetsuichi
authors.initials: T
authors.lastname: Wazawa
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000737:chemistry; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry*; D002942:Circular Dichroism; D058266:Dielectric Spectroscopy; D007477:Ions / Q000737:chemistry; D008274:Magnesium / Q000737:chemistry*; D008872:Microwaves; D055672:Static Electricity; D014867:Water / Q000737:chemistry
chemical_list: D000199:Actins; D007477:Ions; D014867:Water; D000255:Adenosine Triphosphate; D008274:Magnesium; D002118:Calcium