Human Immunodeficiency Virus

Question 1

Attachment

Answer 1

• Virion spike gp120 (SU) binds to CD4 receptor on host cell

Question 2

Penetration and Uncoating

Answer 2

• Binding of gp120 to co-receptor (CXCR4, CCR5) triggers conformational change in the spike
• Gp41(TM) extends to impale host cell plasma membrane
• Virion envelope and plasma membrane fuse
• Nucleocapsid core enters host cell cytoplasm

Question 3

Reverse Transcription and Integration

Answer 3

• Reverse transcription occurs inside viral nucleocapsid cores in cytoplasm of newly infected cell
• Reverse transcriptase converts (+)ssRNA into greater than full-length linear dsDNA with LTRs forming the preintegration complex
  - Preintegration complex has NLS so that HIV can infect non-dividing cells
• Linear dsDNA provirus integrates into host chromosome using integrase and gaps are repaired by cellular enzymes

Question 4

“Immediate” Early Gene Expression

Answer 4

• Initial transcription of proviral DNA depends only on host cell transcription factors and DdRp II that bind to LTR sequences
• Primary transcript capped, polyadenylated, and most are spliced
• Translation of spliced mRNA produces Tat regulatory protein

Question 5

Early Gene Expression

Answer 5

• Tat protein imported into nucleus, activates LTR transcription
• Primary transcripts capped, polyadenylated, and most are spliced before export to cytoplasm
• Translation of spliced mRNAs produces accessory proteins ENV, Tat and Rev

Question 6

Late Gene Expression

Answer 6

• Tat protein continues to activate transcription from LTR
• Rev protein has NLS and NES and undergoes cytoplasmic shuttling
• Rev protein promotes nuclear export of unspliced viral RNA
• Rev binds to Rev Response Element in viral RNA
• NES on Rev causes rapid nuclear export of bound viral RNA
• Unspliced, viral genomic RNA translated on free, cytoplasmic ribosomes to produce GAG and GAG-POL polyproteins
  
  • GAG-POL produced by -1 frameshift mechanism
  • GAG and GAG-POL not cotranslationally cleaved
  • Migrate to plasma membrane for assembly and budding
• Spliced ENV mRNA translated on rough ER
  
  • ENV polyprotein co-translationally extruded into RER lumen
  • Glycolysation occurs on both TM and SU beads
  • ENV polprotein cleaved by furin in trans-Golgi to separate gp120 (SU) and gp41 (TM) subunits
  • Spikes traffic to plasma membrane for assembly and budding

Question 7

Assembly and Release

Answer 7

• GAG and GAG-POL polyproteins associate with viral RNA at plasma membrane where spikes have accumulated
• Polyprotein cleavage drives budding

Question 8

Maturation

Answer 8

• Occurs after release ONLY

- Cleavage by viral protease to give nucleocapsid its distinctive cone shape.