PARVOVIRUSES | AT A GLANCE

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INTRODUCTION

PARVOVIRUSES | AT A GLANCE

  • The smallest known viruses.
  • Virions in the range 18–26 nm in diameter.
  • The name was derived from the Latin word parvus which means small.
  • Family – Parvoviridae
  • Subfamily – Parvovirinae (vertebrate viruses) and the Densovirinae (invertebrate viruses )

 

PARVOVIRUSES | AT A GLANCE

PARVOVIRUS VIRON STRUCTURE

  • Simple structure with the ssDNA
  • Genome enclosed within a capsid
  • Icosahedral symmetry
  • The capsid is built from 60 protein molecules
  • The proteins are numbered in order of their size.
  • Each of the protein species contains an eight-stranded β-barrel structure.
  • The virion is roughly spherical.
  • With surface protrusions and canyons
  • A protrusion with a pore at the center is present at each of the vertices of the icosahedron.

PARVOVIRUSES | AT A GLANCE

GENOME

  • Genomes composed of linear ssDNA
  • Size range 4–6 kb
  • Several short complementary sequences are present at each end of a DNA molecule.
  • They can base pair to form a secondary structure.
  • Inverted terminal repeats (ITRs) are present in some parvovirus genomes.
  • The sequence at one end is complementary and in the opposite orientation to, the sequence at the other end.
  • The ends have identical secondary structures as the sequences are complementary.
  • Parvoviruses with ITRs during replication generate equal numbers of (+) and (−) strands of DNA.
  • In the case of (-) DNA, the genes for non-structural proteins are towards the 3’end and the structural protein genes are towards the 5’end.

 

PARVOVIRUSES | AT A GLANCE

PARVOVIRUS REPLICATION

  • Its small genome can encode only a few proteins.
  • The virus depends on its host cell or another virus to provide important proteins
  • Some of these proteins are available only during the S phase of the cell cycle during DNA synthesis.
  • Hence, this restricts the opportunity for parvovirus replication to the S phase.
  • The virion attaches to receptors on the surface of a  host cell.
  • Enters the cell by endocytosis and is released from the endosome into the cytoplasm.
  • Associates with microtubules and is transported to a nuclear pore.
  • The parvovirus virion is too small to pass through a nuclear pore.
  • The capsid proteins of some parvoviruses undergo nuclear localization signals.
  • The single-stranded virus genome is converted to dsDNA inside the nucleus by a cell DNA polymerase.
  • As a result of base pairing, the ends of the genome are double-stranded.
  • At the 3’end the –OH group acts as a primer to which the enzyme binds.
  • The virus genes are transcribed by the cell RNA polymerase II.
  • The primary transcript(s) undergo various splicing events.
  • Two size classes of mRNA are produced.
  • The non-structural proteins are encoded by larger mRNAs.
  • The smaller mRNAs encode the structural proteins.
  • The non-structural proteins are phosphorylated.
  • The DNA is replicated by a mechanism called rolling-hairpin replication after the conversion of the ssDNA genome to dsDNA.
  • This mechanism sets parvoviruses apart from other DNA viruses.
  • Capsids are constructed from the structural proteins and filled by a copy of the virus genome, either a (+) DNA or a (−) DNA as required.

PARVOVIRUSES | AT A GLANCE

REFERENCE

  1. https://pubmed.ncbi.nlm.nih.gov/29261104/
  2. https://pubmed.ncbi.nlm.nih.gov/28410597/
  3. https://www.academia.edu/30888358/John_Carter_Virology_Principles_and_Applications.pdf