Heaphy 5 virology Flashcards
Some different shaped viruses examples
Tobacco mosaic virus. => Rectangles
Calici Virus – 1-2 particles enough to infect => diarrhea => star shape 50nm
Hep A- via faecal –oral 8-9mth orange jaundice => round
HIV => round + lipid bilayer
VSV. Vesicular stomatitis virus => cattle helical coat of proteins => rod envelope.
Bacteriophage T4. => infects e.coli hexagonal head w/ tube body & tails/ base plates. 100nm
Rhinvirus => T=3 related to poli virus, cause of common cold.
Methods to determine structures
X-ray crystallography.
• NMRI for small molecules 30,000 viruses are millions
Beam through molecule => defraction pattern => molecules
• crystallization
a biochemical analysis can determine what constituents are present in what ratio
- by ‘building’ the virus with subunit structures from other techniques such as electron microscopy. => purify individual crystalised proteins.
model build from each subunits -> computers used too.
• Variation in individual particles stops it from being crystallised. i.e. HIV
• Crick &Watson (1956), 1st to suggest that virus capsids
composed of numerous identical protein sub-units arranged either in helical or icosahedral symmetry after seeing Ems.
• helical symmetry
simplest way to arrange multiple subunits
identical subunits:
use rotational symmetry to arrange irregularly shaped proteins around the circumference of a circle to form a disc.
• Multiple discs stacked to form a cylinder,
protein shell
virus genome coated by the protein shell or contained in the hollow centre of the cylinder.
capsid
consists of helix rather than a pile of stacked disks. 20-30nm wide, length variable, 300nm. Sometimes flexible
• Icosahedral structures
building capsid is to arrange protein subunits in the form of a hollow quasi-spherical structure, enclosing the genome within like football shape (encloses genome)
• 20 equilateral triangles arranged into a sphere.
An icosahedron
has 2-3-5 rotational symmetry.
bacteriophage ØX174
Simplist form 60 identical subunits form a capsid. 3 protein subunits per triangular face. Most have more.
⇨ 20 not enough to protect & be stable.
T3 quasi equivalence
Contacts are different between subunits but similar.
Enveloped viruses:
contain lipids
• Membrane envelopes acquired from a cellular structure during release. Matrix layer found in some enveloped viruses. The capsid underlying the envelope may be have helical or icosahedral symmetry.
Intergrase:
Interts => permanently infects cell
Nuclear capsid:
protein that interacts with P15 => helical symmetry
Protein layer (P24):
to protect virus icosahedral symmetry
Why have subunits?
- Major function of the CAPSID. outer shell, of a virus is to protect the fragile nucleic acid genome from:
- Physical damage - Shearing. Even by water, sunlight,
- Chemical damage- UV irradiation leading to chemical modification.
- Enzymatic damage - Nucleases from dead or leaky cells or deliberately made as defence against infection.
- Protein subunits in a virus capsid are multiply redundant, Damage to one subunit does necessarily destroy the infectivity of the whole particle.
Small genes advantage?
=> small proteins => more error resistant
- More contacts => more stable
Spontaneous formation from purified subunits indicated
that particle was in the free energy minimum state i.e. the favoured structure of the components. This intrinsic stability is an important feature of the virus particle.
• Only goes together in one way
• Other advantages?
Self assembly. Easier w/ one/ few subunits
- > Fidelity. smaller protein/gene it means there is less chance of an error occurring….
- Economy. And less waste when it does
- Stability. The larger the number of subunits the more stable the virus particle becomes.
