Flexural and torsional rigidities of actin filaments are important factors in cell motility and muscle contraction, where actin filaments serve as mechanical elements. The flexural rigidity has already been determined by directly observing the bending of individual filaments under a microscope, but measurement of the torsional rigidity has been relatively scarce and indirect, because torsion of an actin filament is difficult to visualize. This paper shows that the torsional rigidity can be measured directly by visualizing the torsional Brownian motion of a single actin filament with a novel methodology based on an optical trapping technique. Actin filaments (F-actin) were prepared by polymerizing actin monomers binding Ca2+ ion or Mg2+ ion at the high affinity site. The torsional rigidity of F-Ca2+-actin ((8.5 (± 1.3)) x 10-26 N m2) was about three times as large as that of F-Mg2+-actin ((2.8 (± 0.3)) x 10-26 N m2), whereas the flexural rigidity ((6.0 (± 0.2)) x 10-26 N m2) was almost independent of the kind of the bound cation. The dynamic structure of F-actin is regulated by the bound metal in an anisotropic manner. The torsional rigidities above, whether of F-Ca2+-actin or F-Mg2+-actin, are one to two orders of magnitude greater than previous experimental estimates.
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