Role of Water in Actin–Myosin Binding and Actin Polymerization: Rotational and Translational Mobility of Water Molecules

Abstract

This paper introduces recent studies on the rotational mobility, translational water diffusion coefficient, and the local viscosity of water around actin filaments (F-actin, a polymerized form of G-actin). Microwave dielectric relaxation spectroscopy (DRS), pulsed-field-gradient spin-echo (PFGSE) proton NMR, and time-resolved fluorescence spectroscopy were employed. The DRS measurements showed that F-actin exhibits dual hydration properties as a structure-maker and a structure-breaker. Thus, in addition to the water molecules with lowered rotational mobility that form a constrained hydration shell around the F-actin, there exist water molecules with rotational relaxation frequencies higher than those of bulk water, which are here referred to as hyper-mobile water. Upon binding of myosin to F-actin, which is an endothermic reaction, the amount of hyper-mobile water increased. A PFGSE NMR study indicated that the water molecules around F-actin have self-diffusion coefficients higher than those around G-actin and in bulk water. The local viscosity of water around actin was also investigated by fluorescence measurements of cyanine dye, Cy3, conjugated to Cys 374 of actin. The estimated rotational correlation time of Cy3 attached to F-actin was 60% lower than that to G-actin, supporting the existence of lower viscosity water around F-actin than that around G-actin. Thus, these three measurements revealed the existence of highly mobile water around F-actin. This could be a clue to understanding the endothermic reactions of myosin/F-actin binding and actin polymerization and constructing the energetics of adenosine-triphosphate-driven protein functions.