1. Orthomyxoviruses Orthomyxoviridae
Virion
Genome
Genes and proteins
Viruses and hosts
Diseases
Distinctive characteristics

2. Orthomyxoviruses Orthomyxoviridae
Virion
Enveloped particles, quasi-spherical or filamentous.
Diameter 80–120 nm.
Envelope derived from host cell plasma membrane by budding.
Compact helical nucleocapsids.

3. Orthomyxoviruses Orthomyxoviridae
Genome
Linear ss RNA, negative sense.
Six to eight different segments.
Total genome length 10–15 Kb.

4. Orthomyxoviruses Orthomyxoviridae
Genes and proteins
Each genome segment codes for one or two proteins.
Envelope glycoproteins: hemagglutinin (HA) and neuraminidase (NA).
Integral membrane protein (M2) with ion channel activity.
Matrix protein (M1).
Nucleocapsid protein (NP).
Three RNA polymerase proteins (PA, PB1, and PB2).
Nonstructural protein (NS1).
Minor structural protein (NS2).

5. Orthomyxoviruses Orthomyxoviridae
Viruses and hosts
Influenza types A, B, and C.
Hosts include birds, various mammals (A) and humans (A, B, C).
A separate genus, Thogotovirus, includes viruses transmitted between vertebrates by ticks.

6. Orthomyxoviruses Orthomyxoviridae
Diseases
Symptoms include high fever, sore throat, cough, headache, muscular pain.
Can be fatal in elderly, infants, and chronically ill, often by secondary bacterial infections.
In 1918, an influenza pandemic killed 20 million people worldwide.
Emerging avian influenza virus strains threaten domestic fowl and a possible new human pandemic.

7. Orthomyxoviruses Orthomyxoviridae
Distinctive characteristics
Replicate in the nucleus, unlike most RNA viruses.
Viral mRNA synthesis is primed by stealing capped 5 ends of cellular pre-mRNAs in the nucleus.
Undergo reassortment by exchanging genome segments between related strains.
Reassortment generates new viruses that can cause pandemics because of changed surface antigens.

8. Virion
Influenza viruses cause serious acute disease in humans, and occasional pandemics
Spanish flu
Asian flu 1956
Hong Kong flu 1967
If you really can’t get out of bed, it’s probably influenza!

9. Virion
Influenza virus infections of the respiratory tract can lead to secondary bacterial infections
Loss of ciliated epithelium leads loss of the ability of the respiratory tract to clear viruses or bacteria by mucociliary flow
Viral replication induces production of interferon and cytokines -> inflammatory response, flu symptoms
Secondary bacterial pneumonia causes many of the deaths attributed to influenza virus infection

10. Virion
Fig 23.1 Epithelial cells of the upper respiratory tract infected with influenza virus

11. Genome
Orthomyxoviruses are negative-strand RNA viruses with segmented genomes
Fig Schematic diagram of
influenza virus virion.

12. Genes and proteins
Eight influenza virus genome segments code for a total of 10 different viral proteins

13. Genes and proteins
Hemagglutinin protein binds to cell receptors and mediates fusion of the envelope with the endosomal membrane
Fig Influenza A hemagglutinin protein (HA).

14. Genes and proteins
M2 is an ion channel that facilitates release of nucleocapsids from the virion
Release of viral nucleocapsid from the virion is facilitated by M2 H+ ion channel
Low pH within virion weakens the interaction of M1 protein with nucleocapsid, facilitating release of nucleocapsid into cytoplasm upon membrane fusion
Anti-influenza virus drug amatadine is a specific M2 blocker

15. Genes and proteins
Nucleocapsids enter the nucleus, where mRNA synthesis and RNA replication occur
Fig Helical nucleocapsid of influenza virus.

16. Genes and proteins
Fig Transport of influenza virus RNAs and proteins between nucleus and cytoplasm.

17. Genes and proteins
Capped 5 ends of cellular pre-messenger RNAs are used as primers for synthesis of viral mRNAs
Actinomycin D or a-amanitin blocks influenza virus replication
Fig Production of influenza virus messenger RNAs by cap-stealing.

18. Genes and proteins
Viral mRNAs terminate in poly(A) tails generated by “stuttering” transcription
Two influenza A mRNAs undergo alternative splicing in the nucleus
Segment 7 : M1, M2
Segment 8 : NS1, NS2

19. Genes and proteins
Genome replication begins when newly synthesized NP protein enters the nucleus
Switching from primed to unprimed RNA synthesis depends on the presence of free NP protein
NP does not bind to influenza mRNA presumably they are capped and contain cellular mRNA sequence at their 5’ ends

20. Genes and proteins
Fig Replication of influenza virus genome RNA.

21. Genes and proteins
Nucleocapsids are exported from the nucleus in a complex with matrix protein and NS2
NS2 contains a nuclear export signal
The NS1 protein interferes with polyadenylation of cellular mRNAs
Binds to two host proteins and blocks their functions
cleavage and polyadenylation specificity factor (CPSF)
Poly(A)-binding protein II (PABII)
Synthesis of viral mRNA is not affected

22. Genes and proteins
NS1 also inhibits activation of PKR, an important antiviral pathway induced by interferon
Double stranded RNAs are formed during viral replication
Cells produce double stranded RNA-dependent protein kinase (PKR)
Activated PKR blocks both cellular and viral protein synthesis
NS1 binds ds RNA, reducing its concentration
NS1 could be a potential target for chemotherapeutic agents to suppress influenza virus infection

23. Genes and proteins
Viral envelope proteins assemble in the plasma membrane and direct budding of virions
Neuraminidase cleaves sialic acid, the cellular receptor that binds to HA
Influenza virus strains vary in both transmissibility and pathogenicity

24. Genes and proteins
Genetic variability generates new virus strains that can cause pandemics
Antigenic change (HA, NA) occurs in two ways
Antigenic drift
Slowly but continuously
Results from accumulation of point mutations in antigenic domains of envelope glycoproteins
16 HA subtypes, 9 NA subtypes in type A virus in birds
Antigenic shift
Suddenly but episodically
Results from reassortment of influenza virus genes during mixed infections with different subtypes

25. Genes and proteins
The 1918 pandemic influenza A virus was probably not a reassortant virus
Genome sequences from some previous influenza A virus strains confirm the antigenic shift hypothesis
Highly pathogenic influenza A strains in poultry farms could lead to a new pandemic

27. Key Terms Hemagglutinin a-amanitin Importin Actinomycin D Interferon
Lipid rafts
Mucociliary flow
Neuraminidase
Oseltamivir
Pandemic
Reassortment
Sialic acid
Type I transmembrane
protein
Zanamivir
a-amanitin
Actinomycin D
Amantadine
Antigenic drift
Antigenic shift
Ciliated epithelium
Cytokine
Double-stranded RNA-dependent protein kinase (PKR)
Epidemic
Furin
Fusion peptide
Hemagglutination assay