Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 212

Virus particles are among the most elegant and visually pleasing structures found in nature, as illustrated by the images presented in this chapter. Now that many structures of particles or their components have been examined, we can appreciate the surprisingly diverse architectures they exhibit. Nevertheless, the simple principles of their construction proposed more than 60 years ago remain pertinent: with few exceptions, the capsid shells that encase and protect nucleic acid genomes are built from a small number of proteins arranged with helical or icosahedral symmetry. This feature is characteristic of even some of the largest viruses yet described, indicating that not only genetic economy but also optimized and regular interactions among structural units dictate virus architecture.

The detailed views of nonenveloped virus particles provided by X-ray crystallography emphasize just how well these protein shells provide protection of the genome during passage from one host cell or organism to another. They have also identified several mechanisms by which identical or nonidentical subunits can interact to form icosahedrally symmetric structures, and protein-protein interactions that stabilize larger virus particles with icosahedral symmetry. More-elaborate virus particles, which may contain additional protein layers, a lipid envelope carrying viral proteins, and enzymes or other proteins necessary to initiate the infectious cycle, pose greater challenges to the structural biologist. Indeed, for many years we possessed only schematic views of these structures, deduced from negative-contrast electron microscopy and biochemical or genetic methods of analysis. In previous editions, we noted the power and promise of continuing refinements in methods of cryo-EM (or cryo-electron tomography), image reconstruction, and difference imaging. These techniques have attained atomic- or near-atomic-level resolution, providing remarkable views of large viruses with multiple components, viral envelopes, and, in some cases, the organization of genomes within particles. The structural descriptions of ever-increasing numbers of viruses representing diverse families have also allowed unique insights into evolutionary relationships among seemingly disparate viruses or viral proteins.

These extraordinary advances notwithstanding, important challenges remain, most obviously the visualization of structures that do not exhibit simple symmetry (or are not constructed from components that do). These structures include many genomes and the particles of some large viruses (e.g., poxviruses). The giant viruses, such as mimiviruses and pithoviruses, some with particles so large that they can be seen by light microscopy, also pose new technical challenges and intimate that unanticipated structural principles remain to be elucidated.