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30 Cards in this Set

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What are the characteristics of the Picornvirus genome and its virion?
-Linear
-ssRNA (+)
-7-8.5 kb
-naked (non-enveloped) virus
-iscosahedral symmetry
How many mRNAs does this virus have?
-One mRNA: codes for a single polyptn that is cleaved by viral proteases into smaller ptns in the cell
What are the gene product of this virus?
-3capsid ptns
-1-3 proteinases
-->Cleaves the polyptn encoded by the genome into single fctnal ptns
-6-8 Replication ptn: replication more complicated cuz start with RNA. Also replication isnt free floating, its tethered to the mb of the cytoplasm
What is VPg?
small virus-coded ptn unique to picornavirus.
-Covalently attached to 5'end of RNA genome.
-Serves as prmer for the viral pol to start replication
-prevents viral degradation by acting as a protective cap to the mRNA
Whendoes cleavage of the large precursor polypeptide byviral proteinases take place?
Immediately after translation of the viral polyptn
What are some examples of Picornaviruses?
-Enterovirus (Polio)
-Cardiovirus
-Rhinovirus
What is an acute virus?
Virus that goes into the cell and can kill it quite rapidly by taking over the cellular machinery and using it to make a lot of virions
-Can spread quickly and cause acute infections
-Acute infection are brief but cause a lot of harm
What are Chronic viruses?
-Don't take over host completely(go in and live with the cell)
-They make less progeny and try to hide from the immune system
-Usually don't kll the cell and go away over time
How does the virus enter the cell?
-Have structure on capsid (which has 20 triangles) that bind to the receptors on host cells
-Dif viruses use dif receptors, some use more than 1 receptor
Tropism: specific infectivity of tissues
How is the capsid of a picornavirus formed?
-Have P1 (polyptn) which is cleaved into VP 1/2/3/4
-VP1/2/3 forms a Protomer (5S) which forms a Pentamer (14S) which assembles to the empty capsid (80S)
-The genome is then inserted into it => Provirion (150S)
How do you get an infectious virion?
Cleave VP0 to VP2 + VP4
(Provirion is not infectious cuz you don't want to have infectous viruses in the cell)
How does the virus get its RNA into the cell?
2 ways
1) Endocytosis: virus binds receptor, changes conformation. Induction due to low pH, allows RNA to get out. VPg will make a hole and let the RNA enter the cytoplasm
2) Virus can bind the receptor and VP4 comes off. VP1 changes conformation upon binding and forming a hole thorugh the plasma mb, allowing for RNA to escape
What does the genome look like (5' and 3' ends)?
-5' end: VPg ptn that forms a CAP and is imp for replication
-5' oncoding region: has Internal Ribosomal Entry Site (IRES) which forms a RNA structure that allows non-conventional ribosomal attachment --> imp for translation and replication
-3' end: Poly(A) tail imp for rep
-->dif from humans cuz the virus codes for the poly-A stretch are in the genome already (not added later). Required as a primer for replication cuz VPg acts as a primer and, covalently bound to 2 U's, will bind to the A's
What is the Leader Protein (L) of the polypeptide ptn?
-A protease
-When ptn is made, cleavage of polyptn into P1, P2 and P3
What are the 2 types of ptns?
Structural Proteins
-> P1: VP1-VP4
Non-structural ptns:
-> P2: (2A,B,C) These ptns requied for replication
-> P3: (3A, B, C, D)
-3B is VPg
-3C is the main protease ans is conserved in all viruses
-3D encodes RNA-dep-RNA-POL
How can the infection be studied?
-Look at Rate of Ptn Synthesis
--> use radioactive aa in sol'n and let cells translate ptns. Plot these on Western BLot/x-ray film
--> Found that cellular translation is shut off while viral tanslation is still on, so the virus manages to hijack the E of the cells to direct ptn synthesis only towards viral ptns
What is different about viral translation of picornavirus, as opposed to mammalian translation?
-Have long 5' UTR with IRES in the RNA (.: ribosome will bind mRNA internally, instead of at 5' end)
-Many AUG in the non-coding regions that aren't used for initiation of ptn synthesis
-2 Types of IRES (Type 1: 6 stem loop; Type 2: 12 stem loop)
In tye I IRES, what happens when the PolyRC binds:
1) stem loop 4?
2) Stem loop1?
1) PolyRC binds stem loop 4, allows ribosome binding and translatio can occur
2) PolyRC binds stem loop 1, inhibits translation and activates replication
-also require mammalian ptns
What is the initiator sequence for Type I IRES? Type 2?
Type I: NOT AUG
Type II: AUG
Will two viruses of the same type have the same structural homology even though there is no conservation of sequence?
Yes
Will a circularized picornavirus without a 5' cap be translated?
Only if the virus has an IRES can it be translated without a 5' cap
(For normal ss linear RNA. using the IRES and 5' initiation, you have 2 possibilities for translation of ptn)
How does the picornavirus shut off host translation?
1-Use 2A or L protease to cleave off the eiF-4G (which holds ptns of the eIF-4F complex together)
-part with the eIF-4E cap binding ptn falls off so remaining segment can't bind to the cap and regular cell translation can't be initiated
(Virus replication only needs fragment of eIF-4G with eIF-4A and an IRES for translation of viral RNAs)
2-Can also use 4E-BP1 to get rid of eIF-4E. deP 4E-BP1 that will bind to eIF-4E and take it away from the 5E complex, preventig translation at the 5' cap (this mech done in ECMV/Poliovirus)
What proteases are used to process the poly ptns?
1) just 3C: does first 3 cleavages to get P1/P2/P3
-Cleaves itself, gets other proteases to do other cleavages, doesn't shut off ptn synthesis
2) 3C and 2A
-2A: can cleave itself and host ptn, but can't cleave any other viral ptns
3) 3C, 2A and L?
How does the virus become infective?
Furin (cellular protease) cleaves 1A to 1B as a final step to make the virions infective
How is the Picornavirus genome replicated?
-Start with (+)mRNA--> (-) mRNA --> (+) mRNA
-Start with + and make - cuz these - will make more + strands (get amplification)
What are the 3 forms Picornavirus can be seen under in an infected cell?
- ss(+) mRNA
- Replicative intermediate: (-) strand attached to many (+) strands (- being made from +)
-Replicative form: Finished product: +/- strand hybrid (many + strands being polymerized off of each of the - strands)
Why is VPg imp at the 3' end?
Acts as a primer cuz its linked to 2 U's (allows repl from 5'-->3')
VPg is on the 5' end of the viral genome, but due to folding, it interacts with the 3' end
What is 3D?
What is 3CD imp for?
-Polymerase
-Starts synthesizing the (-) strand as soon as VPg binds th polyA tail
-3CD; imp for replication. Contains a protease (3C) and a polymerase (3D)
What is the 3A/3B comlex for?
Important for attachment to the ER mb for assembly of the virus
What is PCBP?
Poly C Binding Ptn
-Regulates translation and replication depending on which hairpin structure it binds to (1 = replication, 4= translation)