Lecture 7: Flaviviruses Flashcards
flavivirus structure
- Spherical enveloped particle
- Spherical nucleocapsid (25-30 nm) with icosahedral symmetry
- No projections, “golf ball” like appearance
- envelope glycoproteins also arranged with icosahedral symmetry
- slightly bigger than picornaviruses
- immature viral particles display spikes on surface @ neutral pH, as pH lowers the orientation of the spikes changes and mature particles are smoother
flavivirus genus
- transmitted by arthropods (arboviruses)
- causes several important human
diseases - Virus infects humans, monkeys, birds
pestivirus genus
- causes economically important diseases of cattle, sheep etc.
- No insect vectors
- humans not infected
hepacivirus genus
- contains one single virus, Hepatitis C virus
- No insect vectors
- only infects humans via blood transfusion, IV needles, sexual contact
flavivirus genome
- Linear ‘+’ sense ssRNA genome
- capped at 5’ end, but no poly(A) tail at 3’ end
- All genes translated as a single polyprotein followed by proteolytic cleavage
- genome organization most resembles that of Picornaviruses (+ssRNA)
- Distinct from Togaviruses, despite similar virion morphology, genome size, and transmission via arthropod
flavivirus proteins
cleaved from single polyprotein
3 structural proteins:
- One capsid protein (C)
- Two envelope proteins (M and E)
- Seven non-structural proteins
- viral and cellular proteinases are required for processing into proteins
flavivirus E protein
- directs receptor binding and membrane fusion
- type I membrane protein and found as a dimer, lays parallel to the lipid bilayer (instead of protruding)
- Domain II forms dimer interface and contains the hydrophobic fusion peptide (helps virus fuse with membrane)
- Domain III used for receptor binding
- Domain I joins domains II and III
- class II type fusion protein
attachment and entry of flaviviruses
- No cellular receptor has been clearly identified
- Following attachment, entry is mediated by endocytosis within clathrin-coated vesicles
Alternative entry mechanism: - Virus bound by antibody can enter cells that express immunoglobulin Fc receptors on their surface
- Antibody-dependent enhancement (ADE) causes more severe disease
endocytosis within clathrin-coated vesicles of flaviviruses
- Endocytosed vesicles fuse with endosomes
- undergo acidification
- results in rearrangement of the E dimer into a fusion-active state to reveal fusion peptide
what happens once the flavivirus genome is in the cytosol?
- RNA is bound by ribosomes and translated
- produces polyprotein
- cleaved to produce precursor / functional protein
what happens to the precursor proteins cleaved from the polyprotein ? (flaviviruses)
- Capsid protein precursor (anchC) is inserted into endoplasmic reticulum
- Following the capsid protein is the precursor membrane protein (prM)
- prM associates with E protein in ER to form a heterodimer, protects E from premature conformational change and exposing the fusion peptide
- At a later stage prM is cleaved by cellular furin protease
- Releases ‘pr’ peptide extracellularly
- Leaves M associated with virion
- Yields E homodimer
what does replication of flavivirus RNA require
synthesis of a complementary copy (minus-strand) of the plus-strand
RNA
Capsid protein precursor (anchC)
- inserted into endoplasmic reticulum
- signal sequence (on C terminus) is cleaved in lumen of ER by cellular signal peptidase, releases polyprotein from capsid precursor (anchC)
- N-terminal of signal sequence on cytoplasmic side is later cleaved by viral proteinase (NS2B/NS3A) to release mature C protein
what does the synthesis of non-structural proteins in flaviviruses result in?
establishment of active RNA replicase complexes and then RNA synthesis is carried out on membranes in the cytoplasm
why does synthesis of new plus-strands of RNA in flaviviruses occur
- Translation: making more viral proteins
- Replication: making more RNA copies
- Packaging: making virions
