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

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.

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

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).

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.

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.

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.

Virion Influenza viruses cause serious acute disease in humans, and occasional pandemics Spanish flu 1918-1920 Asian flu 1956 Hong Kong flu 1967 If you really can’t get out of bed, it’s probably influenza!

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

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

Genome Orthomyxoviruses are negative-strand RNA viruses with segmented genomes Fig. 23.3 Schematic diagram of influenza virus virion.

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

Genes and proteins Hemagglutinin protein binds to cell receptors and mediates fusion of the envelope with the endosomal membrane Fig. 23.4 Influenza A hemagglutinin protein (HA).

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

Genes and proteins Nucleocapsids enter the nucleus, where mRNA synthesis and RNA replication occur Fig. 23.5 Helical nucleocapsid of influenza virus.

Genes and proteins Fig. 23.6 Transport of influenza virus RNAs and proteins between nucleus and cytoplasm.

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. 23.7 Production of influenza virus messenger RNAs by cap-stealing.

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

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

Genes and proteins Fig. 23.8 Replication of influenza virus genome RNA.

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

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

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

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

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

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