ATP-DEPENDENT FLUCTUATIONS OF SINGLE ACTIN-FILAMENTS IN-VITRO

Single filaments of actin were observed to fluctuate in the direction perpendicular to their longitudinal axes when they hydrolyzed ATP in t he presence of myosin. The transversal fluctuations of actin filaments were identified by reading the transversal displacements of the filam ents under a fluorescence microscope. The transversal fluctuations in the absence of ATP decreased their intensity as the number of myosin m olecules contacting directly with actin filaments increased. In the pr esence of ATP, on the other hand, the amplitude of the transversal flu ctuations increased in proportion to the ATP concentration up to a cer tain level, while the sliding velocity of the filaments did not increa se significantly over the same range of ATP concentration. The present observation suggests that the chemical energy released from ATP along actin filament binding to myosin molecules is first and primarily con verted into kinetic energy of fluctuations in the form of the displace ment movements of the filaments in the direction perpendicular to thei r longitudinal axes.