HIV Genome Structure Replication (Franco Falcone) Flashcards
What is HIV-1 Entry and Tropism?
HIV mainly infects CD4+ T cells and will lyse with them during a productive infection. This leads to progressive reduction in CD4+ T cells during infection (because of their lysis).
Which cells are infected by HIV-1 but not lysed?
Natural killer cells CD8+ killer T cells Macrophages Cells of the nervous system Dendritic cells
How would you describe the structure of HIV?
A glycoprotein, 120 Mr Enveloped Protease p10 Integrase p32 Matrix Reverse Transcriptase ssRNA Transmembrane Capsid
What is the role of the nucleocapsid?
Interacts with the RNA strands
What is GP41?
The transmembrane
It is contact with the matrix of the virus
What problems does HIV face?
There is only space for a limited number of nucleotides in the capsid, limited number of genes can be encoded.
Genome is RNA and needs to be reverse transcribed to DNA by reverse transcriptase. RT has low fidelity: if the RNA genome is too long there is a high likelihood of lethal mutations for the virus.
What does low fidelity mean re HIV reverse transcriptase?
If the RNA genome in HIV is too long then it is likely that mistakes in transcription will be made. These mutations could be lethal to the virus.
How does the use of polyprotein precursors reduce the likelihood of mistakes made by reverse transcriptase?
A large precursor protein from which smaller proteins are generate by proteolytic cleavage. Polyproteins allow for more compact genome by eliminating the need for genetic features such as enhancers, promotors which would be needed to express each gene individually.
What are the three genes that HIV has that encode for polyproteins?
Gag - for group specific antigen (capsid and matrix)
Pol - for polymerase (reverse transcriptase and integrase)
Env - for envelope (for attachment and fusion)
How can the capsid be fashioned to make replication easier for the virus re reverse transcriptase?
The unusual capsid structure is roughly conical shaped and formed by approximately 1000 repeats of the same p24 capsid protein, organised in hexamers, dimers and a few pentamers.
What does HIV-1 show tropism for? (More detailed)
Enters cells via a cellular receptor and a co-receptor.
Major cellular receptor: CD4 present on T lymphocytes (also present in lower density on macrophages).
Co-receptor on T cells - CXCR4
Co-receptor on macrophages - CCR5
When does HIV infect T cells or macrophages? What is the difference?
Early stage infection HIV infects macrophages (M-tropic or R5 subtypes). During later infection its preference switches to CD4+ T cells (T cell tropic or X4 subtypes).
What co-receptor changes leads to the rapid progression to AIDS?
From macrophase CCR5 co-receptors
to T cell/lymphocytes CXCR4
What is Enfuvirtide?
A fusion inhibitor
Inhibits the process of fusion and so prevents the virus from infecting the cell by acting as a biomimetic peptide.
Expensive, inconvenient and reserved for multiple resistant HIV strains (salvage therapy).
What are ‘Entry Inhibitors’?
They can stop viral infection by blocking receptors on the host cell membrane;
- gp120-CD4 binding e.g. Ibalizumab monoclonal antibody
- gp120-co-receptor binding e.g. Maraviroc
- gp42-mediated membrane function e.g. Enfuvirtide
How can a person have natural resistance to HIV infection?
A defective CCR5 co-receptor on macrophage cells means that the HIV virus cannot bind to the target host cell
Why does the HIV capsid not disintegrate immediately upon entering the host cell?
If it happens too early then the infectious cycle would be aborted.
fusion –> reverse transcriptase complex –> pre-integration complex
How do Reverse Transcriptase Inhibitors treat HIV?
The absence of Rt in the host cell makes it an ideal therapeutic target.
How do ‘Integrase Inhibitors’ work?
Targetting the the integrase viral DNA complex in the pre-integration complex (PIC) blocks the integration of HIV into the host genome therefore inhibiting viral replication.
What is the Gag polyprotein?
The Gag gene encodes for the structural proteins of the viral capsid. These are expressed as a single chain polyprotein which has 5 cleavage sites for the HIV protease.
The polyprotein is inserted into the cell membrane and cleaved in subsequent steps during maturation.
Cleavage by the HIV protease results in the release of matrix protein, capsid protein, protein NC (forms part of the nucleocapsid), and p6 which plays a role in initiating budding. Sp1 and Sp2 are short spacer proteins with unclear function.
What happens in Gag polyprotein is stopped by an anti-viral treatment?
Without the proteolytic cleavage the viral particles have not undergone full maturation and so are not infective.
How do ‘Protease Inhibitors’ work?
HIV relies on its own proteases for maturation to infective particles. Stopping these proteases inhibits the maturation
