Localization and Classification of Repetitive Structures in Electron Tomograms of Paracrystalline Assemblies
Electron tomography offers opportunities to study structures that are not amenable to 3D imaging by any of the classical methods, such as singleparticle reconstruction (Frank, 1996), helical reconstruction (Egelman, 2000; DeRosier and Moore, 1970) or electron crystallography (Glaeser, 1999) that require either a repetitive structure, or multiple copies of identical structures. Since electron tomography can produce a 3D image of a single copy of a structure, it is finding wide application in cell biology and material science. Paracrystalline specimens constitute another class of structure for which electron tomography can be particularly useful for obtaining detailed 3D images (Taylor et al., 1997). Paracrystals (para—Greek prefix meaning faulty) are arrays with various kinds of intrinsic disorder. Spatial averaging of such specimens usually blurs or even erases the disordered component, which may eliminate the functionally interesting feature. For this chapter, we define a paracrystalline specimen as one with partial ordering such that one component of the specimen may be highly regular while another may be irregular due to either low occupancy, lattice irregularity or both.
KeywordsElectron Tomography Myosin Head Class Average Actin Monomer Tilt Axis
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