Evidence for Structural Changes in Crossbridges During Force Generation
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During muscle contraction, it is generally thought that myosin heads undergo large scale conformational changes, such as an oar-like rotation between 90° and 45° while attached to actin. However, evidence for conformational changes of the attached crossbridges associated with force generation has been ambiguous. In this study, we compared the conformations of attached crossbridges in (i) the pre-force generating state, (ii) force generating state, (ii) rigor state.
High resolution equatorial X-ray diffraction patterns have been obtained from single chemically skinned rabbit psoas fibers under relaxed, fully Ca2+-activated and rigor conditions. The experimental condition was chosen (ionic strength = 50 mM and temperature = 5°C) such that there are large fractions (80–100%) of crossbridges attached in all the three states, and the attached crossbridges in the relaxed muscle represent the pre-force generating state.
Upon activation, changes in the two innermost intensities I10 and I11 did not follow the familiar reciprocal changes. Instead, there was almost no change in I11 while I10 decreased by 30%. Similarly, greater changes were found in I10 as the fiber goes into rigor from the activated state. Changes were also found in the higher order reflections suggesting that the structure of the force generating crossbridges is not a mixture of those found in the weakly bound and rigor crossbridges. Therefore, our data provides evidence that the average conformation of the force generating crossbridges is different from the weakly attached and from rigor crossbridges.
KeywordsThin Filament Myosin Head Thick Filament Rigor Condition Cross Bridge
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