Single particle analysis of filamentous and highly elongated macromolecular assemblies

The application of single particle techniques to the three-dimensional analysis of electron microscope images of elongated or filamentous macromolecular assemblies is evaluated, taking as an example the muscle thin filament. Although the thin filament contains local helical symmetry, because of the inherent variable twist along it, the helical coherence does not extend for large enough distances to allow the symmetry to be used for full reconstruction of the tropomyosin/troponin repeat along the filament. The muscle thin filament therefore represents a general case of a filamentous object in that it is not possible to exploit symmetry in a full analysis. Due to the nature of the imaging process in the electron microscope, only projections of the thin filament around its long axis are available without tilting the grid. Crucially, projection images around a single axis do not provide enough information to assign Euler angles ab initio using current methods. Tests with a model thin filament structure indicated that an out-of-plane tilt of approximately 20 degrees was needed for ab initio angular assignment of sufficient accuracy to calculate a 3D structure to a resolution of approximately 25 A. If no out-of-plane views are available, an alternative approach is to use a prior 3D model as a reference for the initial angle assignment. Tests with the thin filament model indicated that reasonably accurate angular assignment can be made using a reference containing actin, but lacking the regulatory proteins tropomyosin and troponin. We also found that an adaptation of the exact filtered back projection method is required to allow the correct weighting of projection images in which the particle has a very large axial ratio. This adaptation resulted in significant improvements in the reconstruction.