4 Structure
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
LINKS FOR CHAPTER 4
Video: Interview with Dr. Michael Rossmann http://bit.ly/Virology_Rossmann
Movie 4.1: Virus-based piezoelectric generator http://bit.ly/Virology_piezo
Movie 4.2: Cryo-EM reconstruction of the adenovirus type 5 capsid http://bit.ly/Virology_AD5Cap
Sizing up adenovirus http://bit.ly/Virology_Twiv101
The Big Picture Book of Viruses http://www.virology.net/Big_Virology/BVHomePage.html
ViralZone http://viralzone.expasy.org/
Viruses in the extreme http://bit.ly/Virology_5-28-15
Virus particle explorer http://viperdb.scripps.edu/
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