This document summarizes the properties and life cycle of orthomyxoviruses, focusing on influenza viruses. It describes the structure of influenza viruses including their segmented RNA genome and envelope glycoproteins like hemagglutinin and neuraminidase. It then outlines the stages of the influenza virus replication cycle within the host cell, from attachment and entry, through transcription and replication of the viral genome, to assembly and release of new virus particles. Key features of each viral protein involved in these stages are also summarized.

1. ORTHOMYXOVIRUSES
Edited by
Yousef doozandegan

2. The Influenza Virus
2

3. Properties
• Enveloped, helical nucleocapsid
• Contains segmented negative sense RNA
– 8 pieces for types A and B
– 7 pieces for type C
– 6 pieces for thogotovirus , 8 pieces for Isavirus
• Envelope glycoproteins form two types of
spikes
• respiratory transmission
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4. Genome structure of influenza A
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5. 5

6. Envelope Glycoproteins
• Hemagglutinin ( H antigen)
– Rod-shaped trimers Attack viral protein Required for
viral attachment to sialic acid receptors on
susceptible cells
– Promotes the fusion
– Stimulates formation of protective (neutralizing)
antibody
– Changes due to mutations causing
– Agglutinates erythrocytes in vitro
– Important diagnostic tool
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7. 7

8. haemagglutinin(HA)
The HA must be enzymatically activated in two subunits: H1 and H2
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9. 9

10. Envelope Glycoproteins
• Neuraminidase (N antigen)
– tetramer with enzymatic activity
– cleaves sialic acid glycoproteins
– facilitates the release of the virus from infected cells
– target of drugs oseltamivir (Tamiflu)and zanamivir (Relenza)
– mutational changescause variability
– Stimulates production of protective antibodies
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11. • M2 and a Proton Pump
Amantadine specifically
blocks the pump M2 and
is therefore an inhibitor
of influenza virus
replication.
• The histidine 37 and the
tryptophan 41 side
chains form the bottom
of the pore in the closed
state at neutral pH.
M2 protein
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12. The HEF of Influenza C Viruses
• the major glycoprotein of the C viruses has a receptor-
destroying activity.
• In contrast to the HA of influenza A and B viruses.
• About a 12% sequence identity between HAs and HEF.
• In contrast to the neuraminidase activity of the NA proteins of
influenza A and B viruses.
• has esterase activity.
• sequence similarities between the esterases of influenza C
viruses and some coronaviruses.
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13. 13

14. STAG ES O F VIRAL REPLICATION
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15. Attachment and entry
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16. • Influenza viruses bind to neuraminic acids (sialic acids) on
the surface of cells.
• Human viruses preferentially bind to N-acetylneuraminic
acid attached to the penultimate galactose sugar by an a2,6
linkage (SAa2,6Gal).
• whereas avian viruses mostly bind to sialic acid with an a2,3
linkage.
• Receptor-mediated endocytosis.
• There are four internalization mechanisms:
• (a) via clathrin -coated pits; (b) via caveolae; (c) through
nonclathrin, noncaveolae pathways; and ( d) through
macropinocytosis
• Virus enters cytoplasm in endosome.
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20. Mechanism of clathrin
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21. Mechanism of caveplae
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22. Fusion and uncoating
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23. • Endosome is acidified
• HA conformation change exposes hydrophobic
“fusion peptide”
• Fusion peptide interacts with endosome
membrane and stimulates fusion
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24. 24

25. • Low pH in endosome activates M2 protein: ion
channel
• M2 allows protons to enter virion
• Acid-induced conformational changes disrupt capsid
• Once membranes have fused, RNA is free to enter
cytoplasm
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27. 27

28. Nuclear Import of Ribonucleoproteins
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30. 30

31. Transcription and Replication
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32. 32

33. THE PB2 PROTEIN
• responsible for binding the cap on host pre mRNA
molecules
• bound to cap analog m7GTP reveals that the cap is
sandwiched between phenylalanine 404 and
histidine 357.
• in influenza B and C virus PB2 proteins, the histidine
is replaced with a more traditional tryptophan.
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34. THE PB1 PROTEIN
• catalyzes the sequential addition of nucleotides
during RNA chain elongation.
• RNA-dependent RNA polymerases(RdRp).
• polymerization activity is an S-D-D motif at positions
444 to 446.
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35. THE PA PROTEIN
• endonuclease activity of the polymerase
• The catalytic site involves residues His 41 ,Glu 80,
Asp 108,Glu 119, and Lys 134 and harbors two
Mn(2+) ions.
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37. transcription
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40. 40

41. splicing
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43. The Switch from Transcription to
Replication
• the transcription competent polymerase is structurally
different from the replication-competent polymerase.
• The availability of soluble NP (i.e., not associated with RNPs)
controls the switch between mRNA and cRNA synthesis.
• the accumulation of NEP /NS2 is associated with a decrease in
transcription and an increase in replication, suggesting a
regulatory role.(svRNA) 22 to 27 nt
• accumulation of a newly synthesized free polymerase
complex, which enhances cRNA to vRNA synthesis (and vice
versa) over mRNA synthesis.
• The role of host factors.
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44. replication
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48. Nuclear Export of Ribonucleoproteins
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50. Assembly and Release
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51. • RNA segments assemble with replicase and NP in
nucleus
• M2, HA, NA inserted into membrane
• M1 assembles with RNP and directs exit to cytoplasm
• M1 directs assembly with envelope proteins at
membrane
• Budding through membrane
• Hemagglutinin binds sialic acid on surface as virus buds
out
• Neuraminidase cleaves sialic acid to free progeny
virions
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59. H
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
H11
H12
H13
H14
H15
H16
N
N1
N2
N3
N4
N5
N6
N7
N8
N9
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H N
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