Lecture 5: DNA virus overview and polyomavirus Flashcards
why don’t small DNA viruses encode entire replication systems
no DNA virus encodes everything they need
- encodes proteins that orchestrate the host
- papillomaviridae, polyomaviridae, parvoviridae
where does the polymerase come from
small DNA viruses
- no DNA virus encodes everything they need
- encodes proteins that orchestrate the host
- papillomaviridae, polyomaviridae, parvoviridae
large DNA viruses
- encode their own replication system
- herpes, adenoviridae, poxviridae
basic rules of DNA replication in eukaryotic cells
- DNA synthesized by template-directed incorporation of nucleotides into 3’-OH of DNA chain
- always synthesized 5’ to 3’ (semiconservative)
- replication origins
- catalyzed by DNA dependent DNA polymerase and accessory proteins
- always primer dependent
host cell nuclear functions
- DNA replication (DNA polymerases, helicases, RNA primase, ligase, DNA binding proteins
- RNA transcription (initiation factor, RNA polymerase II)
- RNA processing (capping/splicing)
all DNA viruses encode an initiation protein that
- bind to ORI region
- recruit host DNA replication proteins
all DNA viruses require ______ for DNA synthesis
RNA primer
replication fork vs strand displacement (primer)
replication fork
- in both directions
- papillomavirus, polyomavirus, herpesvirus, retroviral proviruses
strand displacement
- one direction
- adenoviruses (protein), parvoviruses (DNA hairpin), poxvirus (DNA hairpin)
describe the “end replication” problem
all linear DNA shortens when replicated
- RNA primers added and removed during replication
- primers located at 5’ end of linear DNA not replaced
what is the “solution” for DNA viruses
- circular genomes
- circularized linear genomes: inverted repeats
- protei primers
- hairpin loops
- rolling circle
- reverse transcription
28 mn no envelope capsid: icosahedral (T=1) baltimore class 2 linear, ssDNA (hairpin termini) segments: 1 genes: 6 mRNAs genome size: 5kb members: B19, FPV, CPS unique traits: integrate into host DNA
parvoviruses
replicate in cells that normally cycle and frequently enter S phase
autonomous parvoviruses
replicate in cells infected with a helper virus which indices entry of the cell into S phase
-may enter latent phase by integrating in host genome if helper virus is absent
dependent parvoviruses
gene products in viruses
multiple proteins from one open reading frame by staggering promoter
- alternative splicing od mRNA
- post-translational proteolytic cleavage
- internal translation initiation
describe parvovirus replication
extremely host dependent
- dependent on host cell DNA rep system
- require actively dividing cells (s phase)
- rbc precursors
- cancer cells (lethal)
regulatory proteins (NS1 and Rep78/68
- bind to viral DNA- powerful transcription activator for recruiting host polymerase
- freeze cell cycle in s phase
- helicase activity unwinds circular
- endonuclease activity
45 nm no envelope capsid: icosahedral (T=7) baltimore class 1 circular, dsDNA; "minichromosome" segments: 1 genes: 6-7 proteins genome size: 5.3 kb members: simian virus 40, BK and JC virus unique traits: T antigens= oncogenes, tumorigenic in animals
polyomaviruses
polyomavirus structure
72 pentomers (no hexamers) inner: nucleosome with viral DNA wrapped around, VP2/VP3, VP1: outer
describe the mouse polyomavirus: divergent transcription, early/late proteins, differential splicing
divergent transcription- complementary strands transcribed in opposite directions
early proteins: regulatory genes transcribed first
- sm, middle, and lg T antigens
- stimulate the cell cycle to enhance viral DNA replication
late proteins: transcription of structural genes is delayed
-capsid proteins
differential splicing of a common mRNA transcript maximizes coding capacity
all mRNAs are coded at the polyadenylation signal region
- functions by indirectly enhancing the transcription of cyclin D1
- binds and inactivates protein phosphatase 2A
- end result is binding of AP-1 to cyclin D1 transcription enhancer
small T antigen
how do small T antigen function by indirectly enhancing the transcription of cyclin D1
activates the cell cycle and brings host cell into S phase
why do small T antigen bind and inactivate protein phosphatase 2A
allows phosphorylation of MSP kinase pathway
- activates signaling pathways
- cell metabolism stimulated and initiates the cell cycle
middle T antigen
how are middle T antigen pathway signals activated
- middle T antigen anchored in cell membrane
- associates with several protein tyrosine kinases
- initiates cascade of signaling events
tyrosine kinase protein that a potent oncogene, activator of cell cycle that has the ability to over stimulate cell growth but normally regulates the cell
C-Src
tyrosine kinase proteins attached to middle T antigen
-C-Src, activated MAP kinase, phosphatidyl inositol 3 phosphate, inositol triphosphate (middle T antigen serves as dock for them)
- RB binding site and J domain dissociates the retinoblastoma protein (pRb) from E2F transcription factors
- blocks protein p53, preventing activation of genes that black cell growth/induce cell death (apoptosis)
- binds to viral DNA replication origin and recruits cellular DNA replication proteins
Large T antigen
what happens if E2F transcription factors are bound to pRb
if pRb is bound then cell won’t divide
what kind of cells can NOT survive without p53?
tumor cells
describe the MAP kinase pathway
MAP kinase (inactive)
- MAP kinase kinase + ATP = MAP kinase +P (activated)
- phosphorylates and activates AP-1 (Fos/Jun)
- AP-1 binds to cyclin D1 enhancer, stimulates transcription
OR
-small T antigen binds to protein phosphatase 2A and inactivates PP2A = inactivated MAP K
- identified in 1960
- potent DNA tumor virus
- natural host: rhesus monkey
- persistent infections in kidneys
- urine transmission
- opportunistic pathogen
simian virus 40 (SV40)
describe productive SV40 infections
- completes infection cycle in permissive cells
- requires actively dividing culture cells
- cell lysis w/in 72 hrs
- lg T antigen binds, allowing for replication
- no cell transformation occurs (rhesus monkey to rhesus monkey)
describe abortive SV40 infections
- cells transform ONLY non permissive cells (non natural host and injections of high titers into post-natal animals)
- early genes expressed, DNA replication is aborted and not late proteins expressed (no virus production)
- small portion of cell transformed (~10/10^5)
- increases cellular DNA replication/ cell division
- early gene region of viral genome integrated into host genome
- purely lab phenomenon
first polio vaccine
Salk vaccine
what happened with the SV40 in polio vaccine
SV40 in polio vaccine, made it more resistant to activation
vaccinated people with polio killing AND SV40 virus, not knowing SV40 can cause cancer
-10-30 millions exposed to SV40
significant risk of SV40 and cancer
risk/benefit of vaccines
-theres always a risk when producing vaccines so you weigh risks and benefits
