week 9 - virus structure Flashcards
viruses contain 3 major components
- Genome (nucleic acid)
- Viral capsid or core
- Viral envelope (and in some cases)
viral genomes do not contribute much to virus structure but can facilitate assembly of capsids
main building blocks of viral capsids (cores) and viral envelopes are:
Viral structural proteins
- To protect the genome by assembling a stable protein shell
- Specifically recognised and package the viral genome
- To facilitate the formation of viral enveloped from cellular sources
- All viruses no matter how complex or simple contain a…
capsid or core
most capsids are…
spherical or tubular
capsids and typically built of…
structural units comprised of subunits
Subunit = structural protein (can be 1 or multiple different proteins)
Low coding capacity drives simplicity of form and function
how to build a virus
protein subunits –> structural units –> structural units (more) —> capsid
more virus capsids are:
spherical (3-D spherical objects)
capsids
watson and crick propose:
virus cubic symmetry
There are 5 platonic solids
Each face is identical
The number of contacts for each face is equal
virus structure
caspar and klug 1950s
using X ray diffraction note that viruses have a 5-, 3-, 2-fold symmetry with 12 spikes
virus capsids are…
icosahedrons
only they have 12 vertices
ability to determine how many subunits a virus is composed of
- T = triangulation number (Caspar and Klung 1962)
- Assumption that simple capsids are built from many copies of 1 structural protein
- Assuming that each protein has the same type and number of contacts the maximum number of subunits is 60
- Derived by 5-fold symmetry at each of the 122 vertices
- Simple icosahedral virus is defined as T=1
- T=1 viruses are built from 60 subunits
- T number is the standard used to describe the structure of icosahedral virus
T1 viruses: roughly the same size?
- They are all roughly the same size: 60 subunits (cant be any bigger) imposes a size constraint
to build a bigger virus
- Can have multiple proteins in one structure
e.g.
T=1 x 5 (12 vertices) = 60 subunits
T=3 x5 = 15 (12 vertices) = 180 subunits
T=4 x 5 = 20 (12 vertices) = 240 subunits
Not changing no. of spike proteins but
Increasing no. of subunits
odd shaped viruses
- To obtain curvature cannot use these hex
- Pentagon to provide curvature
odd shaped viruses
examples:
HIV
influenza
poxviruses
giant viruses
odd shaped viruses:
Poxviruses
Don’t know how the core of this is built
* Complex
* Better technology in order to understand
* If know how virus is built can attack
3 different kinds of virus particles
helical
icosahedral
complex
viral enveloped proteins:
come in 3 flavours
o Structural proteins: Give structure
o Binding proteins: To cell surface, Virus can attach
o Fusion proteins: Forms a pore, Genome/capsid delivered into host cell
Typically in triomers or hexomoers
E.g. fusion proteins work better if loads in one place
Larger hole etc.
viral enveloped proteins:
usually…
oligomeric (trimers)
viral enveloped proteins:
termed…
“spike” proteins (these are usually the fusion proteins)
viral enveloped proteins:
often…
glycosylated (virial glycoproteins)
o Sticky, attach, protection
viral enveloped proteins:
integral membrane proteins…
span the viral lipid bilayer membrane
viral enveloped proteins:
often connect to…
internal viral structural proteins (assures correct capsid packaging during assembly)
Viral particles can have many other proteins components
- Enzymes (polymerases (replicate), integrases, proteases (clip virial proteins so become integrated))
- Cell components (histones, lipids, host modulators)
- Cell structural components (actin and intermediate filaments)
viral structure summary:
viral particles come in …
all shapes and sizes
viral structure summary:
there are 3 main building blocks…
genome
capsids
sometimes envelopes
viral structure summary:
the norm is….
icosahedral symmetry
but there is exceptions to this rule
When building a virus one must consider metastability
Two truths
- All virus capsids must be built as stable structures capable of protecting the genome in transit from one host cell or organism to another
- All virus capsids must be readily and efficiently broken down or disassembled to release the viral genome when entering a host cell
Both of these things often occur in the same host cell compartment
THIS IS CALLED THE ASSEMBLY-UNCOATING PARADOX
How is metastability achieved?
- Viruses are built through symmetrical arrangement to assure maximal contact between structural subunits
- The structure is not permanently bound together (not covalent)
- Can be loosened or disassembled by cellular cue during entry
- This allows for release or the genome into host cells
