Modelling muscle motor conformations using low-angle X-ray diffraction
Squire, John M.
AL-Khayat, Hind A.
Harford, Jeffrey J.
Irving, Thomas C.
Reedy, Michael K.
- Publisher: Institution of Engineering and Technology
mesheuropmc: macromolecular substances
New results on myosin head organization using analysis of low-angle X-ray diffraction\ud patterns from relaxed insect flight muscle (IFM) from a giant waterbug,bu ilding on previous\ud studies of myosin filaments in bony fish skeletal muscle (BFM),show that the information content\ud of such low-angle diffraction patterns is very high despite the 'crystallographically low' resolution\ud limit (65A( ) of the spacings of the Bragg diffraction peaks being used. This high information\ud content and high structural sensitivity arises because: (i) the atomic structures of the domains of the\ud myosin head are known from protein crystallography; and (ii) myosin head action appears to\ud consist mainly of pivoting between domains which themselves stay rather constant in structure;\ud thus (iii) the intensity distribution among diffraction peaks in even the low resolution diffraction\ud pattern is highly determined by the high-resolution distribution of atomically modelled domain\ud mass. A single model was selected among 5000+ computer-generated variations as giving the best\ud fit for the 65 reflections recorded within the selected resolution limit of 65A( . Clear evidence for a\ud change in shape of the insect flight muscle myosin motor between the resting (probably like the prepowerstroke)\ud state and the rigor state (considered to mimic the end-of-powerstroke conformation)\ud has been obtained. This illustrates the power of the low-angle X-ray diffraction method. The\ud implications of these new results about myosin motor action during muscle contraction are\ud discussed.