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Differences in Internal Dynamics of Actin under Different Structural States Detected by Neutron Scattering

Satoru Fujiwara ^*, Marie Plazanet   , Fumiko Matsumoto ^* and Toshiro Oda ^¶

^* Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan; European Laboratory for Non-Linear Spectroscopy (LENS), University di Firenze, I-50019 Sesto Fiorentino, Firenze, Italy; INFM-CRS-Soft Matter (CNR), University la Sapienza, Rome, Italy; Institut Laue Langevin, F-38042 Grenoble cedex 9, France; and ^¶ RIKEN SPring-8 center, RIKEN Harima Institute/ERATO Actin Filament Dynamics Project, Japan Science and Technology Agency, Sayo, Hyogo 679-5148, Japan

Correspondence: Address reprint requests to Satoru Fujiwara, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan. Tel.: 81-29-282-6737; Fax: 81-29-282-5822, E-mail: fujiwara.satoru{at}jaea.go.jp.

F-actin, a helical polymer formed by polymerization of the monomers (G-actin), plays crucial roles in various aspects of cell motility. Flexibility of F-actin has been suggested to be important for such a variety of functions. Understanding the flexibility of F-actin requires characterization of a hierarchy of dynamical properties, from internal dynamics of the actin monomers through domain motions within the monomers and relative motions between the monomers within F-actin to large-scale motions of F-actin as a whole. As a first step toward this ultimate purpose, we carried out elastic incoherent neutron scattering experiments on powders of F-actin and G-actin hydrated with D₂O and characterized the internal dynamics of F-actin and G-actin. Well established techniques and analysis enabled the extraction of mean-square displacements and their temperature dependence in F-actin and in G-actin. An effective force constant analysis with a model consisting of three energy states showed that two dynamical transitions occur at 150 K and 245 K, the former of which corresponds to the onset of anharmonic motions and the latter of which couples with the transition of hydration water. It is shown that behavior of the mean-square displacements is different between G-actin and F-actin, such that G-actin is "softer" than F-actin. The differences in the internal dynamics are detected for the first time between the different structural states (the monomeric state and the polymerized state). The different behavior observed is ascribed to the differences in dynamical heterogeneity between F-actin and G-actin. Based on structural data, the assignment of the differences observed in the two samples to dynamics of specific loop regions involved in the polymerization of G-actin into F-actin is proposed.