1. Understanding Influenza:
Replication in the host cells
Dr. Sanjeeva Kumar M.
Ph.D Research Scholar
School of Veterinary Medicine & Science
Sutton Bonnington University ofNottingham,UK
2. Overview
• Introduction
• Virus entry,membrane fusion and uncoating of the viral core
• Entry of vRNP into the nucleus
• Virus replication, transcription & Translation
• vRNP export
• Transport of virus proteins to the cell membrane
• Packaging and budding
• Post-translational processing
3. Introduction
Adaptation to new hosts & frequent antigenic alterations -challenging
Better intervention methods, A deeper understanding of the viral infection
process is critical
Influenza possesses an RNA genome of ~12 kilobases (kb) encodes 10–12
proteins
Structurally, each RNP consists of a segment of ss-RNA, with the terminal
sequences of the RNA bound by a trimeric RNA-dependent RNA
polymerase and the remaining sequence bound by multiple copies of
nucleoprotein
4. Virus entry , Fusion and uncoating
HA is a homotrimer-spikes
Preferentially bind to sialic acid
Alpha 2,6 linkage –Human viruses
Alpha2,3 linkage –avian viruses
Swine-both
Clathrin dependent Receptor mediated
endocytosis
Edosome- low pH induces-confirmational
change in HA0-HA2
Low pH-also induces-opening of M2 ion
channel-acidifies the viral core-releases
vRNP from M1 –into cytoplasm
5. Entry of vRNP into Nucleus
• The viral proteins vRNP are PA, PB1 & PB2
• Endosome to Nuclear membrane-Diffusion
• Thru classical nuclear import pathway
• NP-has two NLS signals
• NP –binds –alpha importin-beta importin- nuclear importation
• No reversal bcz of masking of NLS by M1
6. Virus replication & transcription
• Ran-GTP binding to the importins-RNP released
• Synthesis of new RNP –replication of viral genome-transcription-translationVP
encapsidation of genome- is catalysed by polymerase complex subunits (PB1,PB2
& PA)
• RNA replication starts with synthesis of a +ve mRNA copy =cRNA
• Initiated by binding of PB2 to the 5’cap structure of host mRNA
• Endonuclease activity of ‘PA’ snatches the cap structure along with 10-13
nucleotides along with the cap which serves as the primer for viral mRNA synthesis
• The synthesis of viral mRNA is carried out by the PB1 polymerase activity
• Transcription proceeds until the polymerase complex stalls at polyadenylation
signal near the end of the viral RNA
• Newly-synthesised vRNA is encapsidated co-transcriptionally by a viral polymerase
and free NP to form an RNP
7. What causes an RNP to replicate its RNA
rather than transcribe
• Two modes of activity differ in the source of the polymerase
• Transcribing polymerases can function in cis, copying the RNA of their RNP
• Replication, by contrast, requires a free polymerase, such as that assembled from
newly-translated and imported viral proteins
8. vRNP export
• As the infection advances other viral proteins also accumulate in the nucleus. The
matrix protein M1 contains an NLS and its import into the nucleus bind to the
newly-synthesised RNP through interactions with NP
• Mediated by NEP (14 kDa) – small enough to pass thru the nuclear pore
• NEP- has two NES’s- which are recognised by exportin Crm1
• Crm1- activated for nuclear export by binding to Ran-GTP
• (Chromatin-bound Ran guanin by binding to Ran-GTP)
• Export is regulated process by various separate mechanisms-progression of
infection triggers –apoptotic pathway-activating caspase 3 pathway
9. Transport of virus proteins to the cell membrane
• Nuclear export transports RNPs to the perinuclear cytoplasm
• MTOC-microtubule organising centre
• HRB-helps in dessociating from Crm1-Ran GTP complex-
• Interacts with Rab11 and transported to the apical plasma membrane
• Rab11 on RE-vasicular transport or diffusion.
10. Packaging and budding
• As the infection progresses, the apical plasma membrane becomes enriched with
viral proteins, which together co-ordinate the budding of virions around the
complexes of RNPs
• Glycoproteins haemagglutinin (HA) and neuraminidase (NA) span the membrane
and are concentrated in lipid raft microdomains
• The membrane is also spanned by the ion channel M2, which accumulates on the
boundaries of lipid rafts
• On the cytoplasmic face of the membrane, the matrix protein M1 interacts with
the cytoplasmic tails of HA, NA and M2 and with the membrane itself
• Both M1 and M2 interact with RNPs, and HA, NA, when individually over-
expressed, cause the budding of virus-like particles.
11. Post-translational processing
• Several post-translational modifications have been described for IV proteins, Like
• Glycosylation of HA and NA
• Palmitoylation (S-acylation) of HA and M2
• SUMOylation (i.e., conjugation with the small ubiquitin like modifier) of M1 ,NS1
NP PB1 and NEP/NS2
12. References
• “A comprehensive map of the influenza A virus replication cycle”. Yukiko Matsuoka, Hiromi Matsumae,
Manami Katoh, Amie J Eisfeld, Gabriele Neumann, Takeshi Hase,Samik Ghosh, Jason E Shoemaker, Tiago JS
Lopes, Tokiko Watanabe, Shinji Watanabe, Satoshi Fukuyama,Hiroaki Kitano and Yoshihiro Kawaoka. BMC
Systems Biology 2013, 7:97.
• “Studies on the cellular and molecular mechanisms of innate host susceptibility and resistance to
influenza A viruses in chicken and ducks”. Kuchipudi, Suresh Varma. PhD thesis (2010).
http://theses.gla.ac.uk/2167/
• “Influenza A: Understanding the Viral Life Cycle”. Tasleem Samji. YALE JOURNAL OF BIOLOGY AND
MEDICINE 82 (2009), pp.153-159.
• “Transport of the Influenza Virus Genome from Nucleus to Nucleus”. Edward C. Hutchinson * and Ervin
Fodor. Viruses 2013, 5, 2424-2446.