Lecture 3 Flashcards
Main difference in how DNA viruses and RNA viruses assemble virus?
DNA viruses assemble capsid first, then force DNA inside, whereas RNA viruses assemble capsid around RNA.
How does capsid assemble for many viruses involving just coat protein?
Capsid assembles spontaneously. Is a reversible process depending on pH and virus.
Once assembly starts, it is more energetically stable to complete, cascade reaction is caused to complete capsid formation.
How and why does the capsid disassemble?
When virus enters new cell, meaning capsid must be easily disassembled and bonds not too strong (if no method of injecting nucleic acid occurs).
Must also be able to disassemble incase of misfold and incorrect conformation.
How does capsid assemble for many viruses involving coat protein and RNA?
If capsid assembly is dependent on RNA being there, it is a good way to ensure the capsid is encasing the correct thing.
Dependent on RNA to assemble, used like a ‘string’ to tie subunits together.
What virus assembles their capsid using interactions between coat protein and RNA?
Tobacco Mosaic Virus.
What did a 1957 study by Fraenkel-Conrat and Williams show?
That a simple mixture of purified TMV RNA and coat protein are incubated, virus particles form.
Shows the spontaneity of virus assembly.
Shows the forces that drive assembly are weak interactions such as hydrophobic and electrostatic, rarely are covalent bonds involved in holding together subunits.
How does the assembly process ensure that the capsid doesn’t assemble around any old RNA?
Assembly of TMV particles is sequence dependent also. Requires origin of assembly sequence (packaging signal). This causes specific conformational change in coat protein, allowing capsid to properly assemble around viral RNA.
What principle do larger icosahedral DNA viruses employ for assembly to happen, as it does not happen spontaneously?
Formation of scaffolding proteins that form a mild, then get removed and then DNA is packaged.
(formation of empty capsids where DNA is forced in later).
What is maturation of virus?
When virion becomes infectious.
After assembly of the prolapsed, small proteolytic cleavages by maturation proteins lead to increased stability, subtle structural changes etc.
A good antiviral target, e.g. HIV protease inhibition leads virion to be ‘dead-on-arrival’
What are common problems with packaging NA in the capsid?
Negative charge causes electrostatic repulsion.
vNA specificity.
Encapsidation specific for viral nucleic acids over cellular NA background.
What is the Stochastic approach to viral production?
Producing high numbers of visions, but not all of them are viable or infectious. Enough are for it to be viable.
What has molecular biology found useful about Class II ssDNA phage M13?
It can encapsidate a little extra nucleic acid than the genome, useful for recombinant techniques.
Which virus is the simplest example of a multipartite genome in viruses?
Geminivirus (ssDNA) that consists of twinned T=1 icosahedra, packing a monopartite or bipartite genome (one even 9 segments).
How are mass spec and super-sensitive fluorescence techniques used to study viral assembly?
Allow tracking of individual components.
SSFM, nanoparticles tracked during uptake into cell with millisecond time and nanometer spatial resolution.
Provides pathway and location of viruses and nanoparticles.
What is Cryo-EM imaging guilty of?
Symmetry averaging. Where capsid appears roughly icosahedral, but cannot distinguish vertices, washing out any asymmetric features.
