Submitted on January 18, 2005
Accepted on February 25, 2005
Myosin lever disposition during length oscillations when power stroke tilting is reduced
Peter J Griffiths1, Maria Angela Bagni2, Barbara Colombini2, Heinz Amenitsch3, Sigrid Bernstorff4, Christopher C Ashley1, and Giovanni Cecchi2*
1 University Lab of Physiology, University of Oxford, Oxford, United Kingdom
2 Dipartimento di Scienze Fisiologiche, Universita' di Firenze, Firenze, Italy
3 Inst of Byiophis and X-Ray Structure Research, Austrian Academy of Science, Graz, Austria
4 Elettra, Sincrotrone Trieste, Trieste, Italy
* To whom correspondence should be addressed. E-mail: giovanni.cecchi{at}unifi.it.
M3 reflection intensity (IM3) from tetanised, intact skeletal muscle fiber bundles was measured during sinusoidal length oscillations at 2.8kHz, a frequency at which the myosin motor's power stroke is greatly reduced. IM3)signals were approximately sinusoidal, but showed a 'double peak' distortion previously observed only at lower oscillation frequencies. A tilting lever arm model simulated this distortion, where IM3)was calculated from the molecular structure of myosin S1. Simulations showed an isometric lever arm disposition close to normal to the filament axis at isometric tension, similar to that found using lower oscillation frequencies, where the power stroke contributes more to total S1 movement. Inclusion of a second, detached S1 in each actin-bound myosin dimer, increased simulated IM3 signal amplitude and improved agreement with the experimental data. Best agreement was obtained when detached heads have a fixed orientation, insensitive to length changes, and similar to that of attached heads at tetanus plateau. This configuration also accounts for the variations in relative intensity of the two main peaks of the M3 reflection substructure following a length change. This evidence of an IM3)signal distortion when power stroke tilting is suppressed, providing that a sufficiently large amplitude of length oscillation is used, is consistent with the tilting lever arm model of the power stroke.
