parts with helical or icosahedral symmetryThe presence of multiple viral enzymesStructures dedicated to exit of the viral genomeMore than a single membraneA large DNA or RNA genome8 Mature virus particles are said to be metastable.Explain what is meant by this term and why virus particles must be metastablePredict the consequences of a mutation that precludes formation of metastable virus particles during assembly and maturation
9 Which of the following statements about helical symmetry is INCORRECT?Helical viruses can contain RNA or DNA genomesThese genomes can be packaged by either specialized nucleic acid binding proteins or direct contact with coat proteinsAs the structure of a helix is open, the largest known viruses display helical symmetryHelical viruses can be naked or envelopedViral enzymes are present in many helical viruses that reproduce in animal cells
10 You have recently isolated a virus associated with a human disease. The virus particles are spherical, ~100 nm in diameter, and possess an envelope. Preliminary genome sequence analysis suggests the virus is not related to known human viruses.Indicate how you would determine the structure of this virus, and the rationale for your choice of methodWhich protein topology would you be most likely to find in the major capsid protein?
11 Subunits of viral capsid proteins are ~20 to 100 kDa.Explain how large icosahedral capsids can be assembled and how they differ from small capsidsWhat feature(s), other than genetic economy, dictates that all capsids are built from only a very small number of major structural proteins?
5 Attachment and Entry
Attachment of Virus Particles to Cells General Principles Identification of Receptors for Virus Particles Virus-Receptor Interactions
Entry into Cells Virus-induced Signaling via Cell Receptors Routes of Entry Membrane Fusion
Intracellular Trafficking and Uncoating Movement of Viral and Subviral Particles within Cells Uncoating of Enveloped Virus Particles Uncoating of Nonenveloped Viruses
Import of Viral Genomes into the Nucleus The Nuclear Pore Complex Nuclear Localization Signals Nuclear Import of RNA Genomes Nuclear Import of DNA Genomes Import of Retroviral Genomes
LINKS FOR CHAPTER 5
Video: Interview with Dr. Jeffrey M. Bergelson http://bit.ly/Virology_Bergelson
Video: Interview with Dr. Carolyn Coyne http://bit.ly/Virology_Coyne
Bond, covalent bond http://bit.ly/Virology_Twiv210
Breaking and entering http://bit.ly/Virology_Twiv166
A new cell receptor for rhinovirus http://bit.ly/Virology_4-30-15
Blocking HIV infection with two soluble cell receptors http://bit.ly/Virology_2-26-15
Changing influenza virus neuraminidase into a receptor binding protein http://bit.ly/Virology_11-21-13
Inside this very wood hidden the Greeks are waiting, for this machine was made for our castles, to spy on our houses and quickly move against our city or maybe hiding some other deception; don't trust in the horse, Trojans.
LAOCOÖN, VIRGIL’S THE AENEID, 29-19 B.C.E.
Introduction
Because viruses are obligate intracellular parasites, their genome must enter a cell for the viral reproduction cycle to occur. At first sight, the physical properties of the virus particles appear as obstacles to this seemingly simple goal. Virus particles are too large to diffuse passively across the plasma membrane. Furthermore, the viral genome is encapsidated in a protective coat that shields the nucleic acid as it travels through the harsh extracellular environment. These apparent obstacles must all be overcome during the process of viral entry into cells. Infection of cells by many, but not all, viruses requires binding to a receptor molecule on the cell surface. Exceptions include virus particles of yeasts and fungi, which have no extracellular phases, and plant viruses, which are thought to enter cells in which the cell wall has been physically damaged, for example, by insects or farm machinery.
In addition to binding viral particles, cell surface receptor molecules participate in entry, a process that relies on usurpation of normal cellular processes, such as endocytosis, membrane fusion, vesicular trafficking, and transport into the nucleus. The viral genome has to be released from the interior of the virus particle, a process known as uncoating. The receptor plays a role in this process by either initiating conformational changes that prime fusion or uncoating or by directing the virus particle into endocytic pathways, where fusion and uncoating may be triggered by low pH or by the action of proteases. These steps ultimately deliver the viral genome to the site of replication, which can be the cytoplasm, for most RNA-containing viruses, or the nucleus, for most DNA-containing viruses.
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