No abstract

Low-angle x-ray diffraction patterns from relaxed insect flight muscle recorded on the BioCAT beamline at the Argonne APS have been modeled to 6.5 nm resolution (R-factor 9.7%, 65 reflections) using the known myosin head atomic coordinates, a hinge between the motor (catalytic) domain and the light chain-binding (neck) region (lever arm), together with a simulated annealing procedure. The best head conformation angles around the hinge gave a head shape that was close to that typical of relaxed M*ADP*Pi heads, a head shape never before demonstrated in intact muscle. The best packing constrained the eight heads per crown within a compact crown shelf projecting at approximately 90 degrees to the filament axis. The two heads of each myosin molecule assume nonequivalent positions, one head projecting outward while the other curves round the thick filament surface to nose against the proximal neck of the projecting head of the neighboring molecule. The projecting heads immediately suggest a possible cross-bridge cycle. The relaxed projecting head, oriented almost as needed for actin attachment, will attach, then release Pi followed by ADP, as the lever arm with a purely axial change in tilt drives approximately 10 nm of actin filament sliding on the way to the nucleotide-free limit of its working stroke. The overall arrangement appears well designed to support precision cycling for the myogenic oscillatory mode of contraction with its enhanced stretch-activation response used in flight by insects equipped with asynchronous fibrillar flight muscles.

No abstract

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