The insect flight muscle Z disc is the most regularly ordered example of an actin cross-linking and anchoring assembly observed in nature. Its three dimensional structures will be determined by analysis of electron micrographs of tilted specimens using image processing techniques. The insect Z disc is a crystal with p3m projective symmetry. This symmetry will be exploited in the structure analysis to achieve a resolution of about 70 angstroms. Our preliminary studies of Z dics in projection at this resolution have resolved the actin filaments in the Z dics, and the connections between them. The proteins of the Z discs will be located in the structure by specific modification and labelling of Z discs, followed by structure analysis and comparison with the native Z disc. Actin filaments play a key role in the contractile apparatus of muscle cells. In non-muscle cells they have been implicated in cell migration and adhesion, cytokinesis, maintenance of cell shape, and endo- and exocytosis. in all of these systems the actin filaments must be anchored so contractile force can be transmitted through them. Recent bichemical and immunochemical characterization of a variety of such anchoring assemblies have uncovered strong structural homologies. As the most ordered example of this class of structures, the Z disc will be a valuable source of structural information of general applicability to actomyosin dependent cell motility.
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