Myosin lever disposition during length oscillations when power stroke tilting is reduced

1 Sincrotrone Trieste, Basovizza, Italy; 2 University Laboratory of Physiology, Oxford University, Oxford, United Kingdom; 3 Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, Florence, Italy; and 4 Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Graz Messendorf, Austria Submitted 18 January 2005 ; accepted in final form 25 February 2005 M3 reflection intensity (I M3 ) from tetanized, intact skeletal muscle fiber bundles was measured during sinusoidal length oscillations at 2.8 kHz, a frequency at which the myosin motor’s power stroke is greatly reduced. I M3 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 I M3 was calculated from the molecular structure of myosin subfragment 1 (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 toward total S1 movement. Inclusion of a second detached S1 in each actin-bound myosin dimer increased simulated I M3 signal amplitude and improved agreement with the experimental data. The 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 after a length change. This evidence of an I M3 signal distortion when power stroke tilting is suppressed, provided that a large enough amplitude of length oscillation is used, is consistent with the tilting lever arm model of the power stroke. skeletal muscle; X-ray diffraction; muscle mechanics; molecular motors; subfragment 1 structure Address for reprint requests and other correspondence: G. Cecchi, Dipartimento di Scienze Fisiologiche, Viale G. B. Morgagni 63, I-50134 Florence, Italy (e-mail: giovanni.cecchi{at}unifi.it )