Twirling of Actin by Myosins II and V
PIs: John F. Beausang, Harry W. Schroeder, III, James A. Gilmour, Yale E. Goldman
Several factors are expected to determine whether actin experiences a torque during active myosin-directed sliding: the direction of the force vector between actin and myosin, the distribution of myosin binding sites on actin, and cooperation between myosins translocating an individual actin filament. Consequently, studies in the literature have disagreed on whether myosin moves axially along actin or helically with a right-handed or left-handed pitch. We developed a new assay to monitor azimuthal rotation of actin in a gliding filament assay. The actin is sparsely labeled with tetramethylrhodamine at Cys374 and the three-dimensional orientation of the individual rhodamines is monitored with 40 ms time resolution.
Results and Conclusions
During translocation by myosin, approximately half of the observed actin filaments exhibit a ‘twirling’ helical path of rotation around the filament axis. Myosin II and V induce a left-handed twirling motion (opposite to the long-pitch helix of actin) with pitch 1.0 ± 0.2 mm for myosin II and 1.5 ± 0.1 mm for myosin V, whereas myosin VI causes left-handed twirling. The observed twirling motions may be the result of an applied torque between the two proteins that is required in some theories of actomyosin motility. None of the myosin isoforms follow the actin helix.
Reference: Sun, Y., Schroeder, H.W., Beausang, J.F., Homma, K., Ikebe, M. and Goldman, Y.E. Single Molecule Fluorescence Polarization of Calmodulin in Myosin VI. Biophys J., 90:431a. 2006.