Many important biological macromolecules exist as helical polymers. Examples are actin, tubulin, myosin, RecA, Rad51, flagellin, pill, and bacteriophage tails. The first application of three-dimensional reconstruction from electron microscopic images was to a helical polymer, and several laboratories today are using helical tubes of integral membrane proteins as specimens for solving the near-atomic structure of these proteins in the electron microscope. The existing methods for image analysis and three-dimensional reconstruction of helices require that the structure be very regular. Unfortunately, many of these helical polymers are quite disordered. This may mean that only the most ordered polymers will be studied with existing methods. But the disorder that is present in the other specimens, those that cannot be studied, may have a large biological role in the function of these assemblies, so many interesting systems may not lend themselves to methodologies that require near-crystalline order. Further, perhaps the most interesting aspects of the structure and dynamics of more regular polymers may be obscured when helical averaging is imposed upon a specimen that may have significant deviations from such a regular symmetry. Over the past year an iterative method has been developed in my lab for the application of single-particle image processing techniques to disordered helical polymers, and applied very successfully to actin and nucleoprotein filaments involved in homologous genetic recombination. Intensive development work needs to be done to allow this methodology to be widely used in other laboratories. Further, there are preliminary indications of how the methodology can be extended to make it more powerful in separating out different structural states within a population of polymers. This project is devoted to improving this methodology, and producing a set of tools that can be easily used by other investigators. It is expected that the availability of such tools will have a large impact in fields such as cell biology, molecular biology and structural biology.
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