Human immunodeficiency virus 2

VIRION

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Enveloped, spherical to pleomorphic in shape, 80-100 nm in diameter.

GENOME

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Monopartite, linear, dimeric, ssRNA(+) genome of about 9 kb, with a 5'-cap and a 3'poly-A tail. There are two long terminal repeats (LTRs) of about 600nt long at the 5' and 3' ends. The LTRs contain the U3, R, and U5 regions. There are also a primer binding site (PBS) at the 5'end and a polypurine tract (PPT) at the 3'end.

GENE EXPRESSION

The integrated provirus utilizes the promotor elements in the 5' LTR to drive transcription. This gives rise to the unspliced full length mRNA that will serve as genomic RNA to be packaged into virions or used as a template for translation of gag and gag-(pro)pol (1 ribosomal frameshift) polyproteins. The uncompletely spliced mRNAs encode env that is cleaved into SU and TM envelope proteins, and the accessory proteins vif, vpx, and vpr. Completely spliced mRNAs encode Rev, Tat and Nef accessory proteins. Rev escorts unspliced and uncompletely spliced RNAs out of the nucleus of infected cells.

REPLICATION

NUCLEAR

Lytic replication:

  1. Virus attaches to host CD4 receptors through the SU glycoprotein (gp120), with subsequent interaction with a chemokine coreceptor.
  2. TM glycoprotein (gp41) mediates fusion with ?the host cell membrane or ?the endosomal membrane like HIV-1?. Nucleocapsid is released into the cytoplasm, with partial uncoating.
  3. ssRNA(+) genome is copied into a linear dsDNA molecule by the reverse transcriptase.
  4. Nuclear entry of the viral dsDNA which is covalently and randomly integrated into the cell's genome by the integrase (=provirus).
  5. Transcription of provirus by Pol II produces viral spliced and unspliced RNAs.
  6. Translation of spliced viral RNAs produces tat, rev, and nef proteins.
  7. Rev mediates nuclear export of the uncompletely spliced RNAs.
  8. Translation of unspliced viral RNAs produces Env, Gag and Gag-Pol polyproteins.
  9. Assembly of the virion at the host cellular membrane and packaging of the viral RNA genome.
  10. Budding through the plasma membrane and release of the virions.
  11. Proteolytic processing of the precursors polyproteins by viral protease and maturation of the virions.

Latent replication : replication as a provirus integrated in the host chromosome.