Reconciling the working strokes of a single head of skeletal muscle myosin estimated from laser-trap experiments and crystal structures

Models, Molecular Myosin Type II 0301 basic medicine Models, Statistical Protein Conformation Lasers Biophysics Normal Distribution 500 Dyneins Kinesins Actomyosin Models, Theoretical Myosins Crystallography, X-Ray 530 Actins Adenosine Diphosphate Kinetics 03 medical and health sciences Adenosine Triphosphate Models, Chemical Animals Muscle, Skeletal
DOI: 10.1073/pnas.0506272103 Publication Date: 2006-01-21T01:33:19Z
ABSTRACT
Myosin generates force by a rotation of its lever arm. Crystal structures of myosin II indicate an unloaded working stroke of 10–12 nm, a range confirmed by recent x-ray interference experiments. However, when an actin filament, held between two weakly, optically trapped beads is made to interact with a single head of skeletal myosin, the bead displacements have often been reported as having a mean value of 5–6 nm, a value that is commonly interpreted as the working stroke. In general, the observed displacement is not expected to be equal to the working stroke because the kinetics of the stroke is necessarily strain-dependent: this effect biases the frequency of binding events to different actin sites so that displacements smaller than the working stroke are preferentially selected. Our analysis is tailored to current trap experiments, in which the time resolution is insufficient to detect prerigor states. If the preceding transitions are in equilibrium, the mean displacement is zero, contrary to observations in the presence of ATP. However, under ATP-cycling conditions, we find that the mean displacement is deflated to 0.3–0.7 of the true working stroke, depending on the equilibrium constant of the stroke and the rate at which the first myosin product state can detach from actin. The primary working stroke of processive myosin motors as measured by optical trapping is similarly uncertain.
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