Introduction Functions of the Virion Nomenclature Methods for Studying Virus Structure

        Building a Protective Coat Helical Structures Capsids with Icosahedral Symmetry Other Capsid Architectures

        Packaging the Nucleic Acid Genome Direct Contact of the Genome with a Protein Shell Packaging by Specialized Viral Proteins Packaging by Cellular Proteins

        Viruses with Envelopes Viral Envelope Components Simple Enveloped Viruses: Direct Contact of External Proteins with the Capsid or Nucleocapsid Enveloped Viruses with an Additional Protein Layer

        Large Viruses with Multiple Structural Elements Particles with Helical or Icosahedral Parts Alternative Architectures

        Other Components of Virions Enzymes Other Viral Proteins Cellular Macromolecules

        Mechanical Properties of Virus Particles Investigation of Mechanical Properties of Virus Particles Stabilization and Destabilization of Virus Particles

        Perspectives

        References

        Study Questions

      LINKS FOR CHAPTER 4

In order to create something that functions properly—a container, a chair, a house—its essence has to be explored, for it should serve its purpose to perfection; i.e., it should fulfill its function practically and should be durable, inexpensive and beautiful.

      WALTER GROPIUS

       Neue Arbeiten der Bauhauswerkstätten, Bauhaus Book no. 7, 1925

Introduction

Virus particles are elegant assemblies of viral, and occasionally cellular, macromolecules. They are marvelous examples of architecture on the molecular scale, with forms perfectly adapted to their functions. Virus particles come in many sizes and shapes (Fig. 4.1; also see Fig. 1.7) and vary enormously in the number and nature of the molecules from which they are built. Nevertheless, they fulfill common functions and are constructed according to general principles that apply to them all. These properties are described in subsequent sections, which include examples of the architectural detail characteristic of members of different virus families, and nonstructural components of virus particles needed for initiation of infectious cycles.

Functions of the Virion

Virus particles have been selected during evolution for effective transmission of the nucleic acid genome from one host cell to another within a single organism or among host organisms (Table 4.1). A primary function of an infectious virus particle (called the virion) is protection of the genome, which can be damaged irreversibly by a break in the nucleic acid or by mutation during passage through hostile environments. During its travels, a virus particle may encounter a variety of potentially lethal chemical and physical agents, including proteolytic and nucleolytic enzymes; extremes of pH, humidity, or temperature; and various forms of natural radiation. In all virus particles, the nucleic acid is sequestered within a sturdy barrier formed by extensive interactions among the viral proteins that comprise the protein coat. Such protein-protein interactions can maintain surprisingly stable capsids: many virus particles composed of only protein and nucleic acid survive exposure to large variations in the temperature, pH, or chemical composition of their environment. For example, when dried onto a solid surface, human rotavirus (a major cause of gastroenteritis) loses <20% of its infectivity in 30 days at room temperature, whereas the infectivity of poliovirus (a picornavirus) is reduced by some 5 orders of magnitude within