Viral Pathogens: Classification, Biology, Disease 1 Flashcards
How are viruses classified?
Based on their structural viral genomes
Baltimore classification
List the possible structure of viral genomes
Single-stranded RNA (ssRNA)
Double stranded RNA (dsRNA)
Single stranded DNA (ssDNA)
double stranded DNA (dsDNA)
DNA genomes can be linear or circular
What is the central dogma of viral biology?
The production of proteins
- Reverse transcription can occur to convert RNA to DNA
- (-) sense RNA can be converted to (+) sense RNA
Describe the structure of HIV
dsRNA
Capsid
Lipid bilayer
Envelope protein or spikes
Protease integrase
Reverse transcriptase
Gag protein inside shell
Matrix
Describe the genome organisation of HIV
Mainly 3 polyproteins
Gag: group scientific antigen: viral core proteins; matrix, capsid
Pol; viral enzymes: protease, reverse transcriptase and integrase
Env: envelope glycoprotein
What two proteins are required for HIV-1 to enter a host cell?
CD4
Chemokine receptor (CCR5/CXCR4)
How does HIV-1 enter a host cell?
- Naive Trimer
- CD4 binding - T20 binding site exposure
- CoR binding - Fusion peptide insertion
- 6-helix bundle formation - Membrane fusion
How is HIV-1 tropic?
Topic for CD4 expressing cells such as T cells and macrophages
Describe what occurs during the early stages of HIV-1 infection
- Virgin fuses with membrane
- Uncoating of viron and reverse transcription
- Incorporation of cellular proteins during intracellular trafficking
- Nuclear entry
What is reverse transcriptase?
A heterodimer of p66 and p51 subunits
Catalytic properties are in p66
P51 serves structural role
What are the 3 next activities of reverse transcriptase?
RNA-dependent RNA polymerase
RNAse H (cleaves RNA from RNA/DNA hybrid)
DNA dependent DNA polymerase
What occurs once the viral genome has entered the nucleus of the host cell?
Reverse transcription
HIV DNA genome is integrated into the host DNA via viral intagrase enzyme
What occurs during reverse transcription?
Reverse transcriptase converts viral RNA into DNA
What happens during retroviral integration?
- Retroviral DNA and host cell DNA get bent until both terminals are in close proximity
- Host DNA is cut by viral integrase enzyme
- Host DNA then binds to viral DNA
- Repair process occurs, making sure the cuts are resolved
What is the role of LEDGF/P75 in Reverse transcription?
Binds to HIV-1 integrase and facilitates targeting to chromatin
How does HIV DNA bind to host DNA?
HIV promoter contains binding sites for transcription factors that are present in T-lymphocytes
Eg. Transcription factors Lef and Nf-kb
How do viruses ensure that they’re RNA are given preferential treatment?
Viral TAT protein will bind specifically to viral RNA and enhance the production of RNA
POSITIVE FEEDBACK MECHANISM
How is unspliced viral RNA exported out of the nucleus?
Rev protien mediates the export of spliced and unspliced viral RNA
- Rev is produced by the transcription and translation of the genome
- Rev re enters the nucleus and binds to RRE and interacts with CRM1
- This promotes the movement of viral RNA out of the nucleus and splicing of into-containing viral mRNAs
Describe what happens after translation occurrs for HIV
Genomic RNA enters the cytoplasm
Genomic RNA forms a kissing loop complex when two sequences in the RNA recognise each-other
Dimerisation of of the unspliced viral RNA allows packing of two genomes
What the two ways in which viral protein can be made from mRNA?
Gag-pol protein is generated by -1 ribosomal frame shifting induced by a “slippery” sequence and a RNA hairpin structure
How does Gag associate with the plasma membrane mediated?
Myristolaytion of glycines in the MA domain of Gag
How is Gag traffic towards the plasma membrane?
Using the association of Tsg101
How are the viral proteins organised at the plasma membrane in preparation for budding off?
ESCRT machinery is hijacked by HIV to perform membrane abscission during viral release
Abscission is the organisation of all proteins and RNA together in a new capsid.
New capsids are pushed out of plasma membrane
How are proteins released from Gag or Gag-pol polyproteins?
Proteases cut the protiens at specific location to release smaller proteins
What’s the structural differences between a immature and a mature capsid?
Immature contains proteases - Polyprotiens are yet to be cleaved
