Lecture 2: virus architecture Flashcards
functions of structural proteins
protection of the genome and delivery of viral genome
how are viral genomes protected
- capsid
- recognition/packaging of nucleic acid genome
- interaction with host cell membranes to form viral envelope
stable protective coat around virus
capsid
possible structures of a virus
viral envelope around, naked, spherical and helical
how is the viral genome delivered
- binds to host cell receptors- very specific (capsid has viral receptors)
- uncoating of the genome
- fusion with cell membrane (now in the cell)
- transport of genome to appropriate cellular site
cellular site for RNA and DNA
RNA- cytoplasm
DNA- nucleus
single, viral-encoded protein
subunit
basic unit of capsid, one or multiple protein subunits
structural unit
surface structures as seen in EM
morphological unit (capsomere)
protein shell around nucleic acid
capsid
nucleic acid: capsid protein assembly
nucleocapsid
lipid bilayer carrying glycoprotein
obtain from host
envelope
complete infectious viral particle
virion
describe virion structure
genome- nucleic acid core
capsid- surrounds genome; viral encoded
envelope- from host cell
complete or infectious
how is a virus metastable
-stable when protecting genome before infection
- unstable when allowing infection
- virus recognizes receptor and triggers endocytosis into host, uncoating its genome
-change of pH and binding to receptor causes uncoating virus particle
how can a virus be stable and unstable
stable
-symmetrical arrangement of identical subunits
unstable
-structure not permanently bonded
- virus particles aren’t at min free energy level; stored potential energy = spring-loaded
- potential energy used for disassembly if cell provides proper signal
nucleocapsid inside the envelope may have __________ symmetry
helical or icosahedral
describe viral envelopes
- host lipid membrane (viral encoded proteins)
- flexible shape (helical- ebola, bullet shape- rabies, pleomorphic- herpes)
saggy and baggy viral envelope
pleomorphic
examples of (-)ssRNA and helical capsids
- Paramyxoviridae (measles and mumps)
- Rhabdoviridae (rabies)
- Orthomyxoviridae (influenza)
- Filoviridae (ebola)
must be present on virus to be infectious
glycoproteins
describe viral envelope glycoproteins
integral mem glycoproteins
ectodomain- attachment, antigenic sites, fusion
internal domain- assembly
oligomeric- spikes
perpendicular (no symmetry) or parallel (symmetry)
describe helical and icosahedral nucleocapsids
helical- unstructured envelopes
icosahedral- structured envelopes
largest known virus
pandoravirus 1000x500 nm
- negative staining (50-75 A resolution, stain background, shape, capsomeres)
- some distortions
EM
- rapid freeing
- specimen preserved
- lower contrast
- improved resolution (8-20 A)
- computer reconstruction
- secondary structure (surface features)
cyro-EM
- naked virions (crystalized, bombard with x-rays)
- highest resolution (2-3 A)- shortest wavelength; atomic level
- measure diffraction patterns
- computer generated images (put back together)
x-ray diffraction
were the first to see viruses put together
watson and crick
watson and cricks rules for building virions
- virions either spherical or rod- shaped
- many copies of similar proteins
- repeated interactions
rule 1- identical bonding contacts b/t subunits
rule 2- bonds usually non-covalent
describe icosahedral symmetry
closed shell/identical subunits
- tetrahedron (4 triangular faces)
- cube (6 square faces)
icosahedron (20 triangular faces)
- most economical w/ least amt of subunits used
- 12 vertices (black pentagons soccer ball)
- 20 triangular faces
smallest # of subunits
- 60 identical subunits
- small viruses (parvovirus)
axis’s
-five fold, two fold, three fold
characteristics of capsids
icosahedral symmetry
equivalence vs quasi equivalence
simplest = equivalent
- parvovirus: singel structure proteins, 60 copies = 60 subunits
- poliovirus: three structural proteins, 60 copies = 180 subunits, least economical
multiple = quasi equivalent
-norwalk virus: single structure, 180 copies = 180 subunits, most economical
multiple of 60 copies***
cluster of 3 subunits
facet
triangulation number equation
T = h^2 + hk + k^2
number of “jumps” between pentamers
triangulation number
number of structural units/faces
T
total number of subunits
60T (smallest is T=1)
permissible T values
1, 3, 4, 7, 13, 16, 25…
12 pentamers + # of hexamers
describe helical structure
elongated tube
-identical subunits, wind around a groove, # of nucleotides/subunit varies
naked capsid (rigid)
enveloped capsid (flexible)
ex: TMV
helical structure equation
P= u X p
P=pitch of helix (height per turn)
u=# subunits per turn
p= displacement b/t subunits
