Modelling muscle motor conformations using low-angle X-ray diffraction

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.

Invertebrate muscles: Thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

“…Troponin is visible in unaveraged electron micrographs as a small bead just below (Z-ward) the rear cross-bridge (Taylor et al, 1993;Reedy et al, 1994a). X-ray diffraction work shows that the myosin head shape in these rigor cross-bridges differs from its shape in resting muscle (Squire et al, 2003a).…”

“…Techniques for preparing the muscles for electron microscopy and X-ray diffraction in relaxed, rigor, and nucleotide (e.g., ATP) attached states have been fully developed (Reedy et al, 1983b;McDowall et al, 1984;Ménétret et al, 1988). Techniques for identifying, averaging, and extracting three-dimensional information from the repeating motifs present in electron micrographs {Al Khayat, 2004 37678 /id; 23274 /id} and of X-ray diffraction patterns (Squire et al, 2003a) of asynchronous muscle are well-developed. Structural modeling began early in asynchronous muscle studies (Holmes et al, 1982), and protocols for fitting atomic models of actin and myosin head and neck regions to cross-bridge images are now also well-developed (Chen et al, 2001).…”

The Myosin Filament Superlattice in the Flight Muscles of Flies: A-band Lattice Optimisation for Stretch-activation?

“…Such a sensor could be a myosin head interaction with troponin, or with the actin part of the thin filament close to troponin. 41,43,44 Such an interaction may not be as readily made as in the case of the myosin heads making interactions with actin. It is known that head freedom is sufficient to enable about 80% of the heads to make rigor attachments in Lethocerus muscle.…”

Axial Dispositions and Conformations of Myosin Crossbridges Along Thick Filaments in Relaxed and Contracting States of Vertebrate Striated Muscles by X-ray Fiber Diffraction

“…11,12 Various investigators have attempted to analyze the intensity distributions of these myosin reflections in order to elucidate the structures of the two-headed crossbridges in relaxed and isometrically contracting states of live skeletal muscles. 6,[13][14][15][16][17] There is a series of reflections indexing on the basic period of 42.9 nm observed on the meridian in the resting Xray diffraction pattern, 11,12 indicating that there are significant departures from the regular symmetry; the so-called forbidden meridional reflections on the layer-lines such as 1, 2, 4, 5, 7 etc. of the basic period that are introduced by the presence of a systematic perturbation of the three successive crown repeats within the basic period, 18,19 not by variations in the sizes or shapes of myosin crowns themselves.…”