Principles of Virology. Jane Flint
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cellular pathways in viral reproduction and can reveal signatures of virus-induced lethality or immune protection.
Figure 2.1 The viral infectious cycle. The infectious cycle of poliovirus is shown as an example, illustrating the steps common to most viruses: attachment and entry, translation, genome replication, particle assembly, and release. See Appendix, Fig. 22, for explanation of abbreviations. Inset: Lipid components of the plasma membrane. The membrane consists of two layers (leaflets) of phospholipid and glycolipid molecules. Their fatty acid tails converge to form the hydrophobic interior of the bilayer; the polar hydrophilic head groups (shown as balls) line both surfaces.
Entering Cells
Viral infection is initiated by a collision between the virus particle and the cell, a process that is governed by chance. A virion may not infect every cell it encounters: it must first come in contact with the tissues that contain cells to which it can bind. Such cells are normally recognized by means of the specific interaction of a virus particle with a cell surface receptor. These cellular molecules do not exist for the benefit of viruses: they all perform functions for the cell. Virus-receptor interactions can be either promiscuous or highly selective, depending on the virus and the distribution of the cell receptor. The presence of such receptors determines whether the cell will be susceptible to the virus. However, whether a cell is permissive for the reproduction of a particular virus depends on other, intracellular components found only in certain cell types. Cells must be both susceptible and permissive if an infection is to be successful. Virus entry into cells is the topic of Chapter 5.
Viral RNA Synthesis
Although the genomes of viruses come in a number of configurations, they share a common requirement: they must be efficiently copied into mRNAs for the synthesis of viral proteins and progeny genomes for assembly. The synthesis of RNA molecules in cells infected with RNA viruses is a unique process that has no counterpart in the cell (see Chapter 6). With the exception of retroviruses, all RNA viruses encode an RNA-dependent RNA polymerase to catalyze the synthesis of both mRNAs and genomes. For the majority of DNA viruses and retroviruses, synthesis of viral mRNA is accomplished by RNA polymerase II, the enzyme that produces cellular mRNA (see Chapter 7). Much of our current understanding of the mechanisms of cellular transcription comes from study of the transcription of viral templates.
Viral Protein Synthesis
All viruses are parasites of translation: their mRNAs must be translated by the host’s cytoplasmic protein-synthesizing machinery (see Chapter 11). However, viral infection often results in modification of the host’s translational apparatus so that viral mRNAs are translated selectively. The study of such modifications has revealed a great deal about mechanisms of protein synthesis. Analysis of viral translation has also led to the discovery of new mechanisms, such as internal ribosome binding and leaky scanning, that have been subsequently found to occur in uninfected cells.
Viral Genome Replication
Replication of viral genomes requires the cell’s synthetic machinery in addition to viral proteins (see Chapters 6, 7, and 9). The cell provides nucleotide substrates, energy, enzymes, and other proteins. Transport systems are required because the cell is compartmentalized: essential components might be found only in the nucleus, the cytoplasm, or within subcellular organelles. Study of the mechanisms of viral genome replication has established fundamental principles of cell biology and nucleic acid synthesis.
Assembly of Progeny Virus Particles
The various components of a virus particle, the nucleic acid genome, capsid protein(s), and in some cases envelope proteins, are often synthesized in different cellular compartments. Their trafficking through and among the cell’s compartments and organelles requires that they be equipped with the proper homing signals (see Chapter 12). Components of virus particles must be assembled at some central location, and the information for assembly must be preprogrammed in these molecules (see Chapter 13). The primary sequences of viral structural proteins contain sufficient information to specify assembly; this property is exemplified by the remarkable in vitro assembly of tobacco mosaic virus from coat protein and RNA (Box 2.1). Successful virus reproduction depends on redirection of the host cell’s metabolic and biosynthetic capabilities, signal transduction pathways, and trafficking systems (see Chapter 14).
Viral Pathogenesis
Viruses command our attention because of their association with animal and plant diseases. Viral pathogenesis is the process by which viruses cause disease. The study of viral pathogenesis requires investigating not only the relationships of viruses with the specific cells that they infect but also the consequences of infection for the host organism. The nature of viral disease depends on the effects of viral reproduction on host cells, the responses of the host’s defense systems, and the ability of the virus to spread in and among hosts (Volume II, Chapters 1 to 5).
EXPERIMENTS
In vitro assembly of tobacco mosaic virus
The ability of the primary sequence of viral structural proteins to specify assembly is exemplified by the coat protein of tobacco mosaic virus. Heinz Fraenkel-Conrat and Robley Williams showed in 1955 that purified tobacco mosaic virus RNA and capsid protein assemble into infectious particles when mixed and incubated for 24 h. When examined by electron microscopy, the particles produced in vitro were found to be identical to the rod-shaped particles produced from infected tobacco plants (Fig. 1.9B). Neither the purified viral RNA nor the capsid protein alone was infectious. The spontaneous formation of tobacco mosaic particles in vitro from protein and RNA components is the paradigm for self-assembly in biology.
Fraenkel-Conrat H, Williams RC. 1955. Reconstitution of active tobacco mosaic virus from its inactive protein and nucleic acid components. Proc Natl Acad Sci U S A 41:690–698.
Overcoming Host Defenses
Organisms have many physical barriers to protect themselves from dangers in their environment, such as invading parasites. Vertebrates also possess an immune system to defend against anything