Structure Flashcards
Nomenclature used in description of virus structure
What do these terms mean?
protein subunit
structural unit
protein subunit - single, folded polypeptide chain
structural unit - unit from which capsids or nucleocapsids are built; may comprise one protein subunit
or multiple, different protein subunits
Cryo-EM is a new technique, which provides more information about virus structure, than classic x-ray crystallography
How does it work?
Sample is flash-freezed to -160degC.
This prevents water from forming crystals
EM used to visualise structure
Different to x-ray crystallography, which cools structures into crystals, before using x-rays to produce an image
What are the different types of capsid symmetry?
Helical
Icosahedral
Complex - pox virus, retroviruses
What is unique about retrovirus capsid morphology?
Most viruses have helical or icosahedral capsid
Retroviruses have either spherical, cylindrical or conical capsid shape
gammaretrovirus - spherical
betaretrovirus - cylindrical
lentivirus - conical
What is the significance of the size of the capsid?
Size limits the internal volume, and limits the length of the nuclear material
Nuclear material is often highly folded, and condensed into the capsid. This creates high pressure/ electrostatic forces. When uncoating begins in an infected cell, then nuclear material is ejected quickly
EM measurement might be written in Angstroms. What does this mean?
angstrom (Å), unit of length, equal to 10−10 metre, or 0.1 nanometre
old-fashioned use of measuring, before metric system
What components make up the viral envelope?
Host lipids
viral glycoproteins
What is the general structure and orientation of a viral glycoprotein?
small internal domain remains within envelope
membrane spanning domain - an alpha-helix anchors it to envelope. May span envelope and a protein matrix, if present
exterior domain contains oligosaccharides and proteins, primed to bind to a receptor
Some viruses have a matrix protein
What is the function of this?
Can act as a physical barrier to prevent destruction
Provides physical barrier to prevent nucleocapsid from interacting with the viral envelope
Can have ion channels to help regulate virus e.g M2 ion channel in influenza A regulates pH
How does temperature affect viral structure?
Some viruses always maintain same structure
An example of a virus which changes, is dengue virus. It lives at 28degC in mosquitoes. However when it is at 37degC in humans, it undergoes a change
the E protein dimers are tightly packed and icosahedrally ordered. However, the epitopes for binding of antibodies that neutralize the virus at 37°C are either partially or entirely buried, suggesting that the virus particle might undergo temperature-dependent conformational transitions. Indeed, when particles are exposed to temperatures encountered in the mammalian host (e.g., 37°C), they do expand significantly, exposing segments of the underlying membrane, and the E protein interactions are altered
DNA viruses can utilise host enzymes for replication and transcription
Why does vaccinia need to bring its own enzymes?
Most viral replication takes place in nucleus
However, virions of vaccinia virus contain a DNA-dependent RNA polymerase, analogous to cellular RNA polymerases, as well as several enzymes that modify viral RNA transcripts
This complement of enzymes is necessary because transcription
of the viral double-stranded DNA genome takes place in the cytoplasm of infected cells, whereas cellular DNA-dependent RNA polymerases and the RNA-processing machinery are restricted to the nucleus
