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Hind A Al-Khayat

Publications and source records attributed to Hind A Al-Khayat.

6 recordsLinked to original sources
PubMed2006

Refined structure of bony fish muscle myosin filaments from low-angle X-ray diffraction data.

Application of X-ray diffraction methods to the elucidation of the arrangement of the myosin heads on myosin filaments in resting muscles is made simpler when the muscles themselves are well ordered in 3D. Bony fish muscle for the vertebrates and insect flight muscle for the invertebrates are the muscles of choice for this analysis. The rich, well-sampled, low-angle X-ray diffraction pattern from bony fish muscle has previously been modelled with an R-factor of 3.4% between observed and calculated transforms on the assumption that the two heads in one myosin molecule have the same shape. However, recent evidence from other kinds of analysis of other muscles has shown that this assumption may not be valid. There is evidence that the motor domain of one head in each pair may interact with the neck region of the second head. This possibility has been tested directly in the present analysis which extends the X-ray modelling of fish muscle myosin filaments by permitting independent shape changes of the two heads in one molecule. The new model, with a computed R-factor of 1.19% against 56 independent reflections, shows that in fish muscle also there is a marked asymmetry in the organisation of each head pair.

Animals↗
PubMed2006

3D structure of relaxed fish muscle myosin filaments by single particle analysis.

To understand the structural changes involved in the force-producing myosin cross-bridge cycle in vertebrate muscle it is necessary to know the arrangement and conformation of the myosin heads at the start of the cycle (i.e. the relaxed state). Myosin filaments isolated from goldfish muscle under relaxing conditions and viewed in negative stain by electron microscopy (EM) were divided into segments and subjected to three-dimensional (3D) single particle analysis without imposing helical symmetry. This allowed the known systematic departure from helicity characteristic of vertebrate striated muscle myosin filaments to be preserved and visualised. The resulting 3D reconstruction reveals details to about 55 A resolution of the myosin head density distribution in the three non-equivalent head 'crowns' in the 429 A myosin filament repeat. The analysis maintained the well-documented axial perturbations of the myosin head crowns and revealed substantial azimuthal perturbations between crowns with relatively little radial perturbation. Azimuthal rotations between crowns were approximately 60 degrees , 60 degrees and 0 degrees , rather than the regular 40 degrees characteristic of an unperturbed helix. The new density map correlates quite well with the head conformations analysed in other EM studies and in the relaxed fish muscle myosin filament structure modelled from X-ray fibre diffraction data. The reconstruction provides information on the polarity of the myosin head array in the A-band, important in understanding the geometry of the myosin head interaction with actin during the cross-bridge cycle, and supports a number of conclusions previously inferred by other methods. The observed azimuthal head perturbations are consistent with the X-ray modelling results from intact muscle, indicating that the observed perturbations are an intrinsic property of the myosin filaments and are not induced by the proximity of actin filaments in the muscle A-band lattice. Comparison of the axial density profile derived in this study with the axial density profile of the X-ray model of the fish myosin filaments which was restricted to contributions from the myosin heads allows the identification of a non-myosin density peak associated with the azimuthally perturbed head crown which can be interpreted as a possible location for C-protein or X-protein (MyBP-C or -X). This position for C-protein is also consistent with the C-zone interference function deduced from previous analysis of the meridional X-ray pattern from frog muscle. It appears that, along with other functions, C-(X-) protein may have the role of slewing the heads of one crown so that they do not clash with the neighbouring actin filaments, but are readily available to interact with actin when the muscle is activated.

Animals↗
PubMed2005

Single particle analysis: a new approach to solving the 3D structure of myosin filaments.

Knowledge of the structure of muscle myosin filaments is essential for a proper understanding of sarcomere structure and how myosin heads interact with the actin filaments to produce force and movement. Two principal methods have been used to define the myosin head arrays in filaments in the relaxed state, namely modelling from low-angle X-ray diffraction data and image processing of electron micrographs of isolated filaments. Analysis of filament images by 3D helical reconstruction, which imposes total helical symmetry on the structure, is very effective in some cases, but it relies on the existence of very highly ordered preparations of straight filaments. Resolutions achieved to date are about 70 angstroms. Modelling of X-ray diffraction data recorded from whole relaxed fish or insect muscles has also been used as an independent method. Although the resolution of the diffraction data is often also about 70 angstroms, the effective resolution of the modelling is very much higher than this because additional very high resolution data (e.g. from protein crystallography) is included in the analysis. However, the X-ray diffraction method has to date provided only limited data on non-myosin thick filament proteins such as C-protein and titin and it cannot provide the polarity of the myosin head arrangement. Both the helical reconstruction and X-ray diffraction techniques have advantages and disadvantages, but their disadvantages are avoided in the new approach of single particle analysis of electron micrograph data. Even using the same micrographs as for helical reconstruction, the resolution can be extended by this method to about 50 angstroms or better. In addition, it is not necessary to assume that the myosin filaments are helical; a significant advantage in the case of vertebrate myosin filaments where there is a known crossbridge perturbation. Here we describe the principles of all these approaches, but particularly that of single particle analysis. We outline the application of single particle analysis to myosin filaments from vertebrate skeletal and insect flight (IFM) muscle myosin filaments.

Animals↗
PubMed2004

A measure for the angle between projections based on the extent of correlation between corresponding central sections.

A pre-condition for the ab initio assignment of Euler angles to a set of projections from an asymmetric object is that at least three of the available projections correspond to rotations about different axes. For symmetric objects this condition may be relaxed. There are some applications of single-particle electron microscopy, such as the reconstruction of filamentous macromolecular assemblies, where all available projections more-or-less correspond to rotations about a common rotation axis making it difficult to satisfy this condition. Here, a method has been developed to overcome this problem, based on the fact that the correlation between two central sections of the Fourier transform of a compact object will not be limited to an infinitesimal central line but will have a finite extent, which is related to the angle between the corresponding projections. Projections from model filaments, with different degrees of rotational symmetry about the long axis, have been used to test the methodology. The results show that angle determination is robust down to signal-to-noise ratios as low as 2 and that, in general, the error decreases as the degree of symmetry increases. The method has been used to assign angles to a set of negatively stained muscle thick filament projections to obtain an initial 3D reconstruction. The main features of the projections are seen to be faithfully reproduced in the reprojections from the reconstruction. A real-space adaptation of this method is also discussed.

Fourier Analysis↗