(Section A: Virology) Lecture 04: Viral Structures Flashcards
Define:
Capsid
Viral container or shell
* The “box” that contains the virus genome
Define:
Virion
A complete infectious particle
Define:
Nucleocapsid
A situation where the capsid also contains the genome
* Close and interacts with the genome
Define:
Envelope
Lipid bilayer membranes enclosing nucleocapsids
True or False:
The virion must be in stable conditions at all times
False, the virion must be both stable and unstable
The stability of a virion varies in the different steps of…
Viral infectious cycle
What are the main functions of the capsid proteins?
- Protection of the genome
- Delivery of the genome
- Other interactions
List:
Important roles the capsid proteins play in:
* Protection of the genome
- Assembly of a stable protective shell
- Specific recognition and packaging of the nucleic acid genome
- In certain cases, interaction with host cell membrane to form the envelope
List:
Important roles the capsid proteins play in:
* Delivery of the genome
- Specific binding to external receptors of the host cell
- Transmission of specific signals that induce uncoating of the genome
- Induction of fusion with host cell membranes
- Interaction with specific components of the host cell to direct transport of the genome to the appropariate site
List:
Important roles the capsid proteins play in:
* Other interactions
Interactions with…
* Cellular components for transport to intracellular sites of assembly
* Cellular components to ensure an efficient infectious cycle
* The host immune system
What are methods that viral structural biology is studied?
- Electron Microscopy
- Cryo-Electron Microscopy
- X-Ray Crystallography
- Nuclear Magnetic Resonance Spectroscopy (NMR)
Define:
Negative staining
Stain sample with electron-dense materials
* Done to absorb electrons
Describe:
Electron Microscopy
- Allows us to understand what the virus looks like
- Potential to “destroy” ultrastructural features
Describe:
Cryo-Electron Microscopy
- Requires NO staining; structure preservation
- Flash freeze samples and image at very cold temperatures
- Use computers to reconstruct images (3D reconstruction)
- Shows ultrastructural details
Describe:
X-ray Crystallography
- Relies on the key ability of viral protein/virion to crystallize
- Shows surface structures (e.g. canyons)
Describe:
Nuclear Magnetic Resonance Spectroscopy (NMR)
- Only looks at the viral proteins
- Used if the virus protein does not crystallize
- Radiation emitted from a nucleus in a magnetic field are measured by spectra
List:
Resolution of Viral Structural Biology Structures by:
* Resolution from more detail to less detail
- X-Ray and NMR
- Cryo-Electron Microscopy
- Electron Microscopy
What is different for NMR from other structural techniques?
NMR will only give viral proteins
* Cryo-EM, EM, and X-Ray Crystallography give full virion structures
Virions are made from —- copies of — proteins
- Many
- Few
True or False:
The viral genome’s coding capacity is limited
True
What is the key to forming a virus shell?
Symmetry
* If protein subunits on the surface are identical, the contacts will be identical generating a symmetrical object
What are the 2 types of symmetry?
- Helical Symmetry
- Polyhedral Symmetry
State:
Rules of Viral Symmetry
- Each subunits had identical bonding contacts with neighbouring subunits
- Bonds between subunits are mostly via non-covalent interactions
Describe:
Helical symmetry
A protein subunit interacts with an identical protein subunit in an identical manner
* Viral genome straight down a central axis
* Protein subunits interacts with viral genome
True or False:
Capsids in helical symmetry are nucleocapsids
True
Are nucleocapsids composed of one type of protein or multiple types?
It can be either (one type or multiple types)
Nucleoprotein associates with…
Viral genomes
Proteins are irregularly shaped, so why do spherical capsids appear nice and round?
Symmetry
* Round capsids have a defined number of proteins
* Proteins are in multiples of 60 (60, 180, 240 etc.)
What type of symmetry do round capsids have?
Icosahedral symmetry
What is an icosahedron? How does the number 60 fit into this?
Icosahedron: Solid with 20 faces; each of the faces is an equilateral triangle
* 20 identical trimers (3)
* 20 times 3 = 60
What types of symmetry occur in icosahedral symmetry?
- 5 fold symmetry: At vertices
- 3 fold symmetry: On faces
- 2 fold symmetry: At edges
True or False:
Bigger viruses are perfectly symmetrical
False
What is the term for almost perfectly symmetrical?
Quasiequivalent
Where are envelopes derived from?
Host membranes
Describe:
- How envelopes are derived?
- Their purpose
- Acquired by a budding nucleocapsid (escaping the host cell)
- Contains host cell receptors, allows interactions with host cell receptors
