virions

Examples of virions in the following topics:

• Nature of the Virion

  • A virion is a complete viral particle consisting of RNA or DNA surrounded by a protein shell, constituting the infective form of a virus.
  • In some virions the capsid is further enveloped by a fatty membrane, in which case the virion can be inactivated by exposure to fat solvents such as ether and chloroform.
  • Other virions have a capsid consisting of an irregular number of surface spikes, with the nucleic acid loosely coiled within.
  • Virions of most plant viruses are rod-shaped; the capsid is a naked cylinder (lacking a fatty membrane) within which lies a straight or helical rod of nucleic acid.
  • Virion capsids are formed from identical protein subunits called capsomeres.

• Viral Replication and Gene Expression

  • This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: "late" gene expression is, in general, necessary for structural or virion proteins.
  • No viral proteins can be made until viral messenger RNA is available; thus, the nature of the RNA in the virion affects the strategy of the virus: In plus-stranded RNA viruses, the virion (genomic) RNA is the same sense as mRNA and so functions as mRNA.
  • One of these includes RNA-dependent RNA polymerase (RNA replicase), which copies the viral RNA to form a double-stranded replicative form, in turn this directs the formation of new virions.
  • The newly synthesized positive-sense RNA molecules can serve as templates for translation (7) or associate with capsid precursors to undergo encapsidation and induce the maturation cleavage of VP0 (8), which ultimately generates progeny virions.
  • Lysis of the infected cell results in release of infectious progeny virions (9).

• Viral Morphology

  • A virion consists of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell.
  • An interesting feature of viral complexity is that host and virion complexity are uncorrelated.
  • Some of the most intricate virion structures are observed in bacteriophages, viruses that infect the simplest living organisms: bacteria.
  • Enveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins.
  • Other envelope proteins include the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions.

• Single-Stranded DNA Bacteriophages

  • Virion release may involve host lysis, but alternatively productive infection may occur by budding from the host membrane.
  • These groups differ in their hosts, genome structure, and virion composition.
  • Each virion has 60 copies each of the F, G, and J proteins and 12 copies of the H protein.

• Batch Culture of Bacteriophages

  • As phage virions do not move independently, they must rely on random encounters with the right receptors when in solution within blood, lymphatic circulation, irrigation, soil water, or other environments..
  • Released virions are described as free, and, unless defective, are capable of infecting a new bacterium.
  • In contrast to virion release, phages displaying a lysogenic cycle do not kill the host but, rather, become long-term residents as prophage.

• The Lytic and Lysogenic Cycles of Bacteriophages

  • When infection of a cell by a bacteriophage results in the production of new virions, the infection is said to be productive.
  • With lytic phages, bacterial cells are broken open (lysed) and destroyed after immediate replication of the virion.
  • Viruses that infect plant or animal cells may also undergo infections where they are not producing virions for long periods.
  • In a process called latency, these viruses can exist in nervous tissue for long periods of time without producing new virions, only to leave latency periodically and cause lesions in the skin where the virus replicates.

• Replication of Double-Stranded DNA Viruses of Animals

  • Polyomavirus virions are subsequently endocytosed and transported first to the endoplasmic reticulum where a conformational change occurs; then by an unknown mechanism the virus is exported to the nucleus.
  • The virion also has a unique "spike" or fiber associated with each penton base of the capsid that aids in attachment to the host cell via the receptor on the surface of the host cell.
  • Following binding of viral envelope glycoproteins to cell membrane receptors, the virion is internalized and dismantled, allowing viral DNA to migrate to the cell nucleus.
  • Poxviridae viral particles (virions) are generally enveloped (external enveloped virion- EEV), though the intracellular mature virion (IMV) form of the virus, which contains different envelope, is also infectious.
  • The virion is exceptionally large—around 200 nm in diameter and 300 nm in length.

• Steps of Virus Infections

  • Some infected cells, such as those infected by the common cold virus known as rhinovirus, die through lysis (bursting) or apoptosis (programmed cell death or "cell suicide"), releasing all progeny virions at once.
  • Many animal viruses, such as HIV (Human Immunodeficiency Virus), leave the infected cells of the immune system by a process known as budding, where virions leave the cell individually.
  • The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and to assemble new virions.
  • The last stage of viral replication is the release of the new virions produced in the host organism.
  • RNA and proteins are made and assembled into new virions.

• Double-Stranded DNA Viruses: Pox Viruses

  • Along its approximate 195-kbp double-stranded DNA genome, VACV encodes approximately 200 proteins, ranging in function from viral RNA and DNA synthesis and virion assembly to modulation of host immune defenses.
  • The most abundant and simplest infectious form of the poxvirus particle, the mature virion (MV), consists of the viral DNA genome encased in a proteinaceous core and an outer lipoprotein membrane with approximately 60 and 25 associated viral proteins, respectively.
  • Late gene products primarily consist of structural proteins needed for progeny virion assembly, as well as those enzymes destined for incorporation into progeny virions, and used for early gene expression during the next round of infection.
  • In addition, during transit through the cytoplasm, a subset of progeny MVs acquires two additional membrane bilayers, one of which is lost during exocytosis of the particle, to yield the less abundant enveloped virion (EV).

• Replicative Cycle of HIV

  • The final step of the viral cycle, assembly of new HIV-1 virions, begins at the plasma membrane of the host cell.
  • The Gag (p55) and Gag-Pol (p160) polyproteins also associate with the inner surface of the plasma membrane along with the HIV genomic RNA as the forming virion begins to bud from the host cell.
  • Maturation occurs either in the forming bud or in the immature virion after it buds from the host cell.
  • The various structural components then assemble to produce a mature HIV virion.
  • The mature virion is then able to infect another cell.