3D reconstruction by combining data from sections cut oblique to different unit cell axes

Insights into Actin-Myosin Interactions within Muscle from 3D Electron Microscopy

Much has been learned about the interaction between myosin and actin through biochemistry, in vitro motility assays and cryo-electron microscopy (cryoEM) of F-actin, decorated with myosin heads. Comparatively less is known about actin-myosin interactions within the filament lattice of muscle, where myosin heads function as independent force generators and thus most measurements report an average signal from multiple biochemical and mechanical states. All of the 3D imaging by electron microscopy (EM) that has revealed the interplay of the regular array of actin subunits and myosin heads within the filament lattice has been accomplished using the flight muscle of the large water bug Lethocerus sp. The Lethocerus flight muscle possesses a particularly favorable filament arrangement that enables all the myosin cross-bridges contacting the actin filament to be visualized in a thin section. This review covers the history of this effort and the progress toward visualizing the complex set of conformational changes that myosin heads make when binding to actin in several static states, as well as the fast frozen actively contracting muscle. The efforts have revealed a consistent pattern of changes to the myosin head structures as determined by X-ray crystallography needed to explain the structure of the different actomyosin interactions observed in situ.

3D reconstruction and processing of volumetric data in cryo-electron tomography

A software package is presented that was primarily developed for electron tomography in biological research. It comprises routines for preprocessing micrographs, CTF-correction of images of untilted and tilted specimens, alignment of tilt series, 3D reconstruction, spatial averaging of paracrystalline specimens, alignment of single particles or components of larger macromolecular assemblies, correlation averaging, multivariate statistical analysis, classification, and volume reassembly, in which molecular components in raw tomograms are replaced by averaged versions with higher signal-to-noise ratio. The emphasis for image analysis is placed on the processing of large sets of individual molecular volumes. The design philosophy of the software is both simplicity and interoperability, i.e. to write standalone programs for simple tasks that are combined through shell scripting to provide more complex functionality, and to communicate with other software via common image formats. This paper gives an overview of the current state of the programs and some applications to cryo-electron tomography.

Three-dimensional distortion correction applied to tomographic reconstructions of sectioned crystals

Unaveraged tomograms of embedded and sectioned insect flight muscle show not only structural variation within the individual unit cells but also defects in the filament lattice such as lack of straightness of the filaments and wrinkling of the sections. Adaptation of conventional 2D crystal image processing methods usually average out these structural variations and lattice defects thereby reducing the resolution and information content of the 3D reconstructions. We present here a computational method to correct the reconstructed 3D object for these distortions to make it suitable for further analysis. The correction is an extension of conventional 2D cross-correlation techniques applied to a 3D image. These procedures determine the unit cell positions of the distorted crystal and use a spline-fitting approach to calculate a mapping function from the observed positions that is subsequently used to reinterpolate the image onto a regular grid. The resulting reinterpolated image is corrected for both filament straightening as well as section wrinkling.