Exam 3 - Lecture 7 Flashcards
What triggers virions to begin assembling and where do they assemble?
An increased concentration of viral proteins triggers virion assembly
- in RNA viral genomes and bacteriophages -> cytosolic assembly
- in DNA viral genomes -> nuclear assembly
How do viruses ensure specificity of genome packing into capsids?
They ensure only the viral genome is packaged at the correct number and that the genome contains a specific packing sequence to be recognized by viral capsid proteins
How do icosahedral viruses deal with size limitations when they are trying to package large genomes? In other words, how do they pack so much genome into a little capsid?
They neutralize the genome before packaging it
Know the mechanism by which helical nucleocapsids are assembled.
1) capsomeres form a small disk, and the genome is spooled through that disk
2) more capsomeres assemble on the disk and along the genome
3) this structure expands until the whole genome is covered, and the final capsid resembles a helical tube with a hollow center
Know the mechanism by which icosahedral procapsids are made and how they package their genome.
capsids self-assemble without the genome into empty shells called procapsids
these procapsids may undergo conformational changes to prepare for genome packaging
Icosahedral procapsids
- What is the portal?
The portal is the opening created by one of the vertices to allow the genome to move into the procapsid and become a nucleocapsid
Icosahedral procapsids
- What are scaffolds, what do they do, and what happens to them during genome packaging?
Scaffolds are protein structures that help to provide structure and stability during assembly
During genome packaging, the scaffold degrades and leaves so that the genome can enter
What are VLPs (know their properties)?
Why are VLPs useful vaccine agents?
VLPs (virus-like particles) are procapsids that have been accidentally released without their genome
They’re noninfectious, and they contain most of the viral antigens, and as such are useful for creating vaccines
Know the different ways in which fibers/spikes become part of naked capsids
spike proteins must get into or onto the nucleocapsid
1) bacteriophages - assemble nucleocapsid first, spikes come in after
2) other viruses, the spikes get integrated as the capsomeres come together
For enveloped viruses, what are the three major parts that must come together during assembly?
Nucleocapsid, matrix proteins, and envelope glycoproteins
Where does budding take place and what determines the site for different viruses?
Depends on where the viral proteins are targeted. ex. if the three components go to the ER, budding will occur there
How do proteins know where to go during budding (what directs them to the right location)?
Proteins have specific amino acid sequences for membrane targeting, or they interact with proteins that do
What role does the matrix play during assembly?
The matrix acts as the link between capsids and viral glycoproteins in the membrane
How are cellular membrane proteins typically excluded from the viral envelope?
Viral glycoproteins will cluster in specific areas of the membrane and in the process, push the host proteins out of that spot
What are the two major steps of budding? What happens in each?
1) accumulation of viral structural proteins on the membrane cause it to push out and form a bulb with a stalk
2) pinching - the membranes of the stalk are brought close enough together that they fuse and release virions
