HIV Flashcards
What genus is HIV?
What family is HIV?
Lentivirus
Retroviridae
What are the 3 steps characteristic of a retrovirus?
- RT of genome RNA
- Transfer of proviral DNA to nucleus
- Integration of provirus into host chromatin
What are the 9 genes of HIV?
gag, pol, env, vif, vpu, vpr, nef, rev, tat
What is the receptor for HIV?
CD4
CCR5
CXCR4
on T helper cells
What is the role of cyclophilin A?
- enables HIV to reverse transcribe.
What are the 2 active sites on a RT enzyme?
RNase active site
Polymerase active site
What are the 3 steps of HIV virus integration?
- Processing
- Joining
- Repair
What is the function of the Tat gene?
- essential for infection
- transactivator of transcription
- binds to TAR
How does Tat affec gene expression?
Translation - may increase translation of HIV mRNA
Transcription - increased rate of initiation
Increased efficiency of elongation so increased processitivity of RNA Pol II
What occurs in the presence of Tat?
RNA polymerase complex is converted into a fully processive mode.
High proportion of initiation events leads to full length transcription.
What occurs in the absence of Tat?
HIV LTR assembles a poorly processive RNA polymerase complex - once clear of the promoter it frequently drops off producing truncated RNA.
What is TAR?
Where is it found?
Tat response element
Found immmediately downstream of txn start site
Describe how TAR is linked to Tat.
Tat works via a heterologous RNA binding domain which won’t act unless TAR is present.
What is Rev?
regulator of virion protein expression. Also essential for HIV replication.
What happens in the absence of Rev?
cytoplasmic mRNA for Gag, Pol and Env are reduced.
mRNA for Tat, Nef are unaffected.
What type of effect does Rev have?
Post transcriptional effect.
What sequences are necessary for Rev function?
Sequences mapped to the Env coding region.
What is the RRE
Rev response element, RRE is necessary for Rev to activate mRNA accumulation of an intron containing construct.
Describe the function of the RRE
confers Rev-dependent export on mRNAs that contain it and would otherwise be retained.
How does the RRE work?
Binds Rev at a high-affinity site in the RRE and Rev then multmerises on the RNA.
What is the Rev activation domain?
A nuclear export signal which binds the exporting Crm1 and exits dependent on RAN.
How does Rev relate to splicing
Export competes with splicing but splicing inhibition isnt the mechanism of Rev action.
Describe the time course of translation of HIV proteins.
Early proteins: Tat and Rev and Nef
Tat are rev are transported to the nucleus where they act.
Late proteins: Gag, Pol, Env, accessory proteins vif, vpu, vpr. The expression of these depends on Tat and Rev
How is Pol HIV protein produced?
Ribosomal frameshifting
How does frameshifting occur??
Stem loop structure forms in HIV RNA, causes ribosome to pause on a slipperry sequence
10% of the time, the ribosome slips back on nucleotide then continues to translate Gag-Pol fusion protein
How is env protein produced?
- from 4kb species.
- co translationally inserted into the ER membrane.
Where does glycosylation of gp120 region occur?
In the ER
Where is the site of HIV particle assembly?
Env clusters in regions of the membrane high in lipids - lipid rafts.
What is the packaging signal
Psi,
What 2 signals do HIV RNAs contain
- Dimerisation signal
- Packaging signal
How are Gag-RNA complexes transported to regions of the membrane containing Env proteins to form the immature virus particle?
Cellular proteins called ESCRT complexes
At which stage does HIV become an infectious virion
After the structural rearrangement
What is the role of Vif?
inhibits host cell APOBEC3G, an antiviral enzyme which causes hypermutation of the HIV genome.
What is the role of Vpu?
inhibits host cell tetherin, a cell surface molecule preventing the release of viral particles
What is the role of Nef?
causes the downregulation of MHC Class I and CD4 from the cell surface.
