ICL 1.3: RNA Viruses - Poliovirus Flashcards
which property is unique to viruses?
A. they are small in size B. they replicate only intracellularly C. their genome can be made of RNA D. infection involves receptor binding and endocytosis E. outer lipid membrane
C. their genome can be made of RNA
what is the name of the viral structure composed of the genome enclosed in a protein coat?
nucleocapsid
what are the characteristics of the Picornaviridae family?
single stranded RNA virus
positive sense
naked
which polymerases does viral RNA replication use?
RNA dependent RNA polymerase
RNA dependent DNA polymerase
if the first function is nucleic acid synthesis, polymerases must be virion-associated
**virion-associated polymerases are packed into the particle itself to be used immediately after entry
what determines the replication strategy of a virus?
it’s genome!
what are the two classes of RNA viruses?
- strand RNA viruses
2. - strand RNA viruses
what are + strand RNA viruses?
their genome is mRNA that can be directly translated into protein by host ribosomes!***
so the first thing that happens when the viral RNA enters the cell is translation into proteins
this means that the RNA genome by itself is infectious!! it will initiate a complete replication cycle all on its own because it can be directly made into proteins by host ribosomes
what are - strand RNA viruses?
their genome is a complement of viral mRNA so it can NOT be directly translated into protein – instead they serve as templates for mRNA synthesis after entry
first, mRNA has to be synthesized by virion-associated polymerases because host polymerases only read DNA and don’t recognize RNA
so the naked genome of -strand RNA viruses is NOT infectious
which families of RNA viruses are naked virions?
- Picornaviruses
- Caliciviridae
- Hepeviridae
they’re all icosahedral symmetry
which families of RNA viruses are enveloped virions?
- Togaviruses
- Flaviviruses
- Coronaviruses
they’re all pleomorphic symmetry
which virus family is poliovirus part of?
picornaviruses
aka a naked, positive strand RNA virus with icosahedral symmetry
there are 3 polio subtypes: 1,2,3
does poliovirus have virion-associated polymerase?
nope
since it’s + strand, there is no virion-associated polymerase and the RNA genome is translated directly by host ribosome
what happens during the attachment phase of replication for polio?
the polio capsid canyon region specifically binds to host cell at CD155 = PVR
PVR = poliovirus receptor
this happens at 0 - 4°C in the lab so it’s a super strong binding, no energy is needed
which cells does polio infect?
only cells with CD155!
this means that there is a very narrow range for polioviruses as only humans and some primates are hosts and only some human cell types express CD155
so this means that the presence of the host receptor, called PVR for poliovirus receptor, on the surface of cells is a major determinant of host range!!
which proteins are in the capsid of poliovirus?
VP1-4
what is the structure of PVR?
PVR - poliovirus receptor
it’s an immunoglobulin
what is the canyon of a poliovirus?
it’s the specific part of the poliovirus capsid that binds to the host PVR (CD155)
what does the WIN compound do?
it’s an inhibitor that blocks the poliovirus from binding to host cells
it does this by binding to the canyon region of the capsid
however, viruses mutate to evade this inhibition
what happens during the uncoating phase of replication for polio?
a conformational change happens to the virion from binding the PVR leads to genome entry into the cytoplasm
the naked RNA binds ribosomes for translation
the RNA genome has a VPg protein that covalently attaches at the 5-end but it’s NOT a cap
what is the function of the VPg?
VPg = virion genome protein
it’s a protein that’s part of the poliovirus RNA genome
it’s a virus-encoded protein that’s covalently attached to the 5’ end of poliovirus virion RNA
VPg is a structural protein that acts as a primer for both - and + strand RNA synthesis!
what happens during the synthesis phase of replication for polio?
nucleic acids and proteins are synthesized!
- one long polyprotein is synthesized from host ribosomes
- the polyprotein is cleaved into P1, P2 and P3
- P1 is cleaved into VP0, VP1 and VP3
VP0 is cleaved into VP4 and VP2
VP1-4 make up the capsid of the virion!
- P2 turns into 21 protease
- P3 is cleaved into VPg and a polymerase
where does poliovirus RNA synthesis occur?
the cytoplasm!!!
how does RNA synthesis occur for the poliovirus?
- (+) genome RNA is copied into complementary (-) RNA templates with VPg-covalently coupled to the 5’end of each RNA
VPg acts as the primer for initiation of each RNA strand, both - and +!!
- then the (-) RNA is copied many times to produce a large excess of (+) RNA
each (+) RNA strand will have a VPg coupled to its 5’ end
while RNA replication is happening, there exists a replicative intermediate (RI) which is partially dsRNA that is made of the the template (-) RNA strand, polymerase and nascent (+) product strands
what is RI?
RI = replicative intermediate
part of the poliovirus synthesis phase of replication
it is partially dsRNA that is made of the the template (-) RNA strand, polymerase and nascent (+) product strands
what’s the difference between (+) and (-) RNA virus replication?
in (+) RNA viruses, the (+) RNA functions as a mRNA, genome and template; its (-) RNA functions only as a template
in (-) RNA viruses the (-) RNA is the genome and template for (+), while its (+) strand functions as mRNA and template for (-) RNA
what happens during the poliovirus assembly phase?
a virion is made of 60 copies of VP1, VP2, VP3, VP4 capsid proteins and one (+) RNA molecule
P1 precursor is cleaved into VP0, VP1 and VP3, 5 copies of these form a pentamer structure –> 12 pentamers (n=60) form a capsid shell
a provirion has (=) RNA and VP0 which is then cleaved into VP2 and VP4 which makes it an infectious virion
infectious virions accumulate in the cytoplasm as paracrystalline inclusion bodies
what is a provirion for the poliovirus?
a provirion has (+) RNA and VP0
VP0 is then cleaved to VP2 + VP4 in process of maturation to an infectious virion
what happens during the poliovirus release phase?
as virions are assembled, infectious virions accumulate in the cytoplasm as paracrystalline inclusion bodies
since polio is a naked virus, once there’s a ton of virions accumulated they will lyse the cell to be released
what are the effects of polio on an infected cell?
- inhibition of host protein synthesis
viral 2A protease cleaves translation cofactors needed to recognize host mRNA cap structures
you need cap recognition of host mRNA for translation to happen so without the translation cofactors, the ribosome won’t recognize the mRNA and you won’t have any protein production
- CPE = rounding-up of cells and condensed nuclei
- cell death due to poliovirus-induced lysis when the virion is released from the cell
what effect does polio have on your body function?
polio infects neurons
since polio is a naked virus, it lyses the cells when its released
so since the virus is killing neurons, it will lead to paralysis of muscles from all the neurons being lost
what are the steps in polio replication?
- polio attachment of canyon region of capsid to CD155 on host cell
- uncoating aided by virus induced pores
- genome release
- translation of virion mRNA in the cytoplasm**
+ve RNA is translated to polyprotein which is digested immediately to VP1, VP2, and VP3 by viral proteases
- VP1, VP2 and VP3 are used to make a dsRNA replication intermediate
- so tons of + strand RNA are being made and then combined with VP1, VP2 and VP3 structural proteins to make a virion
- virus exits cell via cell lysis and death
how do neutralizing antibodies work against polio?
poliovirus VAP is a canyon that binds to the host receptor CD155
VAP = virus associated protein
antibodies recognizing amino acids on canyon walls prevent receptor binding and thus neutralize the virion particle
