The possibilities and prospects of obtaining high-resolution information (below 30 A) on biological material using the electron microscope. Some comments and reports inspired by an EMBO workshop held at Gais, Switzerland, October 1973

Single-Particle Reconstruction of Biological Molecules-Story in a Sample (Nobel Lecture)

Pictures tell a thousand words: The development of single-particle cryo-electron microscopy set the stage for high-resolution structure determination of biological molecules. In his Nobel lecture, J. Frank describes the ground-breaking discoveries that have enabled the development of cryo-EM. The method has taken biochemistry into a new era.

From envelopes to atoms: The remarkable progress of biological electron microscopy

The electron microscope has, in principle, provided a powerful method for investigating biological structures for quite sometime, but only recently is its full potential being realized. Technical advances in the microscopes themselves, in methods of specimen preparation, and in computer processing of the recorded micrographs have all been necessary to underpin progress. It is now possible with suitable unstained specimens of two-dimensional crystals, helical or tubular structures, and icosahedral viruses to achieve resolutions of 4Å or better. For nonsymmetrical particles, sub-nanometer resolution is often possible. Tomography is enabling detailed pictures of subcellular organization to be produced. Thus, electron microscopy is now starting to rival X-ray crystallography in the resolution achievable but with the advantage of being applicable to a far wider range of biological specimens. With further improvements already under way, electron microscopy is set to be a centrally important technique for understanding biological structure and function at all levels-from atomic to cellular.

Single-particle reconstruction of biological macromolecules in electron microscopy--30 years

This essay gives the autho's personal account on the development of concepts underlying single-particle reconstruction, a technique in electron microscopy of macromolecular assemblies with a remarkable record of achievements as of late. The ribosome proved to be an ideal testing ground for the development of specimen preparation methods, cryo-EM techniques, and algorithms, with discoveries along the way as a rich reward. Increasingly, cryo-EM and single-particle reconstruction, in combination with classification techniques, is revealing dynamic information on functional molecular machines uninhibited by molecular contacts.

Immunoelectron microscopy of enzymes, multienzyme complexes, and selected other oligomeric proteins

The collective term "immunoelectron microscopy" subsumes a number of techniques in which the biological material is decorated with specific antibodies, prior to being visualized in the electron microscope. In this article, we have reviewed literature on immunoelectron microscopy that focusses on the analysis of the molecular architecture of proteins, in particular of enzymes and of multienzyme complexes. Molecular immunoelectron microscopy has been remarkably successful with multi-subunit enzymes of complex quaternary structures, and in many cases the data have been the basis for the eventual development of detailed three-dimensional molecular models. The elucidation of subunit composition and juxtaposition of a given enzyme, an important accomplishment in itself, has in turn stimulated and guided discussions on the catalytic mechanism; illustrative examples include F1 ATPase and citrate lyase, among others. Here we have chosen a variety of enzymes, multienzyme complexes, and non-enzymatic proteins to demonstrate the versatility of immunoelectron microscopy, to illustrate methodological prerequisites and limitations, and to discuss significance and implications of individual immunoelectron microscopy studies.

Immunocytochemistry of the cell nucleus

This electron microscopic review addresses in situ immunocytochemistry of the mammalian cell nucleus with special reference to the use of autoantibodies, which are the major source of antinuclear antibodies. The localization of many key nuclear antigens is documented and immunocytochemical data are related to the major functional processes of transcription and processing of RNA and to replication of DNA.

Electron microscopy and image processing applied to the study of protein structure and protein-protein interactions

We review the application of electron microscopy and image processing at the molecular level to an ever increasing range of biological specimens. Although recent advances have been due in part to development of more sophisticated instrumentation and/or processing algorithms, widespread application of the well-known techniques of image enhancement and structure reconstruction has depended on new strategies of in vitro crystallization and polymerization, some of which are outlined here. We also discuss the use of stoichiometric labeling and/or "cocrystallization" in identifying the different subunits in multisubunit complexes and in studying protein-protein interactions.

Specificity of stain distribution in electron micrographs of protein molecules contrasted with uranyl acetate

The mechanism of contrast enhancement of protein molecules by negative staining with uranyl acetate has been investigated by analysing electron micrographs of microcrystals of the human immunoglobulin Dob. Digitally filtered micrographs were compared systematically with idealized reference images which were constructed computationally, starting from knowledge of the primary sequence and three-dimensional crystal structure of this IgG molecule. By separately modelling negative staining as bulk exclusion of heavy metals, and positive staining as the specific decoration of charged amino acid residues, and then combining these simulated images, we were able to assess quantitatively the amount of positive staining present in micrographs of ostensibly 'negatively stained' proteins. At a resolution of 2 nm, we find that the experimental images do indeed exhibit predominantly negative staining, the best matches being obtained by simulations which also include a minor contribution (10-40%) of positive staining. We have also compared two independent measures for the significant resolution present in images of periodic biological specimens: (i) the outermost visible orders of optical diffraction patterns, and (ii) the band-limited resolutions of the idealized simulations when they most closely match the experimental images. These criteria observe close correspondence, thus vindicating the traditional practice of inferring resolution from the optical diffraction spectra of indirectly represented (stained) objects.

Three-dimensional structure of proteins determined by electron microscopy

Recent developments in specimen preparation and image processing techniques have made it possible to determine the three-dimensional structure of proteins by electron microscopy. Periodic supramolecular aggregates of the protein under investigation are requiring to minimize radiation damage and to maximize the signal-to-noise ratio of structural detail. Useful information about the fine structure of the protein (e.g. binding sites for interacting molecules, antigenic determinants) can often be obtained by stoichiometric labeling of the ordered arrays with interacting molecules or antibody fragments, and computing difference maps from the reconstructions of the labeled and native structures. The use of this approach to molecular structure determination of proteins will be discussed in light of our work with bacteriophage and actin.

Fidelity of structure representation in electron micrographs of negatively stained protein molecules

We have investigated the fidelity of structure representation in electron micrographs of negatively stained proteins by conducting a systematic evaluation of such micrographs in terms of a known molecular structure, solved by x-ray crystallography. Microcrystals of immunoglobulin G Dob were used as specimens in this comparison between micrograph images, optimized by computer image processing, and reference images derived computationally from the crystal structure. To an effective resolution of 2 nm, we observed a remarkably good correlation between the experimental images and their idealized counterparts, which are unaffected by those factors--electron irradiation and dehydration--that are thought to be primarily responsible for perturbation of protein structure during electron microscopy. Separate structural features resolved in these micrographs do not, in general, correspond to specific components of individual molecules but arise instead from complex superpositions involving several overlapping molecules.

Special specimen preparation methods for image processing in transmission electron microscopy: a review

One of the important developments in quantitative electron microscopy has been the application of optical and computer imaging methods to electron micrographs. In general these techniques of image analysis have been applied to electron micrographs from isolated biological structures prepared in the presence of various negative stains. To make full use of image processing techniques there are obvious advantages in preparing suitable specimens containing large areas of repeating features. However, the number of naturally occurring biological specimens exhibiting crystalline or paracrystalline features suitable for high resolution electron microscopy and subsequent image analysis is relatively small.Some recent experiments on the in vitro formation of crystalline and paracrystalline arrays from highly concentrated and purified isometric, filamentous and rod-like viruses is reviewed. The problems associated with the preparative procedures for producing two-dimensional and three-dimensional crystalline arrays are discussed together with the possibility of extending the negative staining-carbon film method for studying the gradual dissociation or assembly of viral components.

Fine structure of crystalline inclusions in B-cells of the islets of Langerhans in the alligator

The ultrastructure of crystalline beta granules of the islets of Langerhans in the alligator has been investigated. From optical diffraction analysis and serial sectioning, the existence of four distinct types of crystalline inclusions was established in ultrathin sections. The first type is the most frequent and is interpreted as a rhombohedron with a base, the ortho-hexagonal unit-cell edges being a equal to 18.9 nm, c equal to 23.0 nm. The second type of crystal (not observed in serial sections) is found compatible with a rhomb-dodecahedron which indexes on a cubic cell with a equal to 9.6 nm. The third type of crystal was assigned to dipyramids. Dipyramids are extremely rare, and only two diffraction patterns were obtained; their crystal system could not be determined. Prisms, which are second in abundance, represent the fourth type of crystal. Spacings as well as the symmetry differ from those of the above three crystal types and indicate a tetragonal cell with a equal to 4.2 nm, c equal to 14.2 nm. The data for the prismatic crystals are strikingly similar to those of proinsulin and may represent the first case of agreement between crystals (i) formed in vitro and studied by X-ray diffraction and (ii) those investigated in situ by electron microscopy.

Getting started with a scanning transmission electron microscope coupled to a small computer

A scanning transmission electron microscope (STEM) equipped with a laser-heated gun was coupled to a small computer. Several alterations to the commercially obtained parts of this system are described. On-line operation procedures were developed aiming to reduce the amount of radiation of the specimen area of interest. The resulting system is capable of recording information at the 1.0 to 1.5 nm resolution level by taking advantage of the efficiency of a STEM in recording the information and in controlling the irradiation conditions. These features are important in the study of biological material.

Ion etching of tobacco mosaic virus and T4 bacteriophage

A technique for the etching of biological materials by accelerated nitrogen ions is described. Upon exposure to such a beam, tobacco mosaic virus becomes thinner in diameter and the head of T4 becomes smaller. Experiments on radioactively labeled T4 suggest that the particle's protein is more susceptible to removal by the incident ions, and that the residual material is predominantly nucleic acid.

Averaging of low exposure electron micrographs of non-periodic objects

The investigation concerns the possibility of extending to non-periodic objects the low exposure averaging techniques recently proposed for non-destructive electron microscopy of periodic biological objects. Two methods are discussed which are based on cross-correlation and are in principle suited for solving this problem.