SPRING - an image processing package for single-particle based helical reconstruction from electron cryomicrographs.
Desfosses, A., Ciuffa, R., Gutsche, I. & Sachse, C.
J Struct Biol. 2013 Nov 20. pii: S1047-8477(13)00302-X. doi:10.1016/j.jsb.2013.11.003.
Helical reconstruction from electron cryomicrographs has become a routine technique for macromolecular structure determination of helical assemblies since the first days of Fourier-based three-dimensional image reconstruction. In the past decade, the single-particle technique has had an important impact on the advancement of helical reconstruction. Here, we present the software package SPRING that combines Fourier based symmetry analysis and real-space helical processing into a single workflow. One of the most time-consuming steps in helical reconstruction is the determination of the initial symmetry parameters. First, we propose a class-based helical reconstruction approach that enables the simultaneous exploration and evaluation of many symmetry combinations at low resolution. Second, multiple symmetry solutions can be further assessed and refined by single-particle based helical reconstruction using the correlation of simulated and experimental power spectra. Finally, the 3D structure can be determined to high resolution. In order to validate the procedure, we use the reference specimen Tobacco Mosaic Virus (TMV). After refinement of the helical symmetry, a total of 50,000 asymmetric units from two micrographs are sufficient to reconstruct a subnanometer 3D structure of TMV at 6.4 Aresolution. Furthermore, we introduce the individual programs of the software and discuss enhancements of the helical reconstruction workflow. Thanks to its user-friendly interface and documentation, SPRING can be utilized by the novice as well as the expert user. In addition to the study of well-ordered helical structures, the development of a streamlined workflow for single-particle based helical reconstruction opens new possibilities to analyze specimens that are heterogeneous in symmetries.
Cartwheel architecture of Trichonympha basal body.
Guichard, P., Desfosses, A., Maheshwari, A., Hachet, V., Dietrich, C., Brune, A., Ishikawa, T., Sachse, C. & Gonczy, P.
Science. 2012 Aug 3;337(6094):553. Epub 2012 Jul 12.
Centrioles and basal bodies are essential for the formation of cilia, flagella, and centrosomes. They exhibit a characteristic ninefold symmetry imparted by a cartwheel thought to contain rings of SAS-6 proteins. We used cryoelectron tomography to investigate the architecture of the exceptionally long cartwheel of the flagellate Trichonympha. We found that the cartwheel is a stack of central rings that exhibit a vertical periodicity of 8.5 nanometers and is able to accommodate nine SAS-6 homodimers. The spokes that emanate from two such rings associate into a layer, with a vertical periodicity of 17 nanometers on the cartwheel margin. Thus, by using the power of biodiversity, we unveiled the architecture of the cartwheel at the root of the ninefold symmetry of centrioles and basal bodies.
Paired beta-sheet structure of an Abeta(1-40) amyloid fibril revealed by electron microscopy.
Sachse, C., Fandrich, M. & Grigorieff, N.
Proc Natl Acad Sci U S A. 2008 May 27;105(21):7462-6. Epub 2008 May 15.
Alzheimer's disease is a neurodegenerative disorder that is characterized by the cerebral deposition of amyloid fibrils formed by Abeta peptide. Despite their prevalence in Alzheimer's and other neurodegenerative diseases, important details of the structure of amyloid fibrils remain unknown. Here, we present a three-dimensional structure of a mature amyloid fibril formed by Abeta(1-40) peptide, determined by electron cryomicroscopy at approximately 8-A resolution. The fibril consists of two protofilaments, each containing approximately 5-nm-long regions of beta-sheet structure. A local twofold symmetry within each region suggests that pairs of beta-sheets are formed from equivalent parts of two Abeta(1-40) peptides contained in each protofilament. The pairing occurs via tightly packed interfaces, reminiscent of recently reported steric zipper structures. However, unlike these previous structures, the beta-sheet pairing is observed within an amyloid fibril and includes significantly longer amino acid sequences.
High-resolution electron microscopy of helical specimens: a fresh look at tobacco mosaic virus.
Sachse, C., Chen, J.Z., Coureux, P.D., Stroupe, M.E., Fandrich, M. & Grigorieff, N.
J Mol Biol. 2007 Aug 17;371(3):812-35. Epub 2007 Jun 4.
The treatment of helical objects as a string of single particles has become an established technique to resolve their three-dimensional (3D) structure using electron cryo-microscopy. It can be applied to a wide range of helical particles such as viruses, microtubules and helical filaments. We have made improvements to this approach using Tobacco Mosaic Virus (TMV) as a test specimen and obtained a map from 210,000 asymmetric units at a resolution better than 5 A. This was made possible by performing a full correction of the contrast transfer function of the microscope. Alignment of helical segments was helped by constraints derived from the helical symmetry of the virus. Furthermore, symmetrization was implemented by multiple inclusions of symmetry-related views in the 3D reconstruction. We used the density map to build an atomic model of TMV. The model was refined using a real-space refinement strategy that accommodates multiple conformers. The atomic model shows significant deviations from the deposited model for the helical form of TMV at the lower-radius region (residues 88 to 109). This region appears more ordered with well-defined secondary structure, compared with the earlier helical structure. The RNA phosphate backbone is sandwiched between two arginine side-chains, stabilizing the interaction between RNA and coat protein. A cluster of two or three carboxylates is buried in a hydrophobic environment isolating it from neighboring subunits. These carboxylates may represent the so-called Caspar carboxylates that form a metastable switch for viral disassembly. Overall, the observed differences suggest that the new model represents a different, more stable state of the virus, compared with the earlier published model.