Picornaviruses. Virion. Genome. Genes and proteins. Viruses and hosts. Diseases. Distinctive characteristics

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1 Picornaviruses Virion Genome Genes and proteins Viruses and hosts Diseases Distinctive characteristics

2 Virion Naked icosahedral capsid (T=1) Diameter of 30 nm

3 Genome Linear single-stranded RNA, positive sense kb Viral protein VPg covalently bound to 5 end Short, genome-encoded poly(a) tail at 3 end

4 Genes and proteins All genes are translated as single polyprotein followed by proteolytic cleavage Three to four capsid proteins (VP1-4) One to three proteinases Six to eight replication proteins

5 Viruses and hosts Picornaviruses cause a variety of human and animal diseases including poliomyelitis and the common cold Pico(small) RNA- containing viruses Nine genera, including Enterovirus, Rhinovirus, Cardiovirus, Hepatovirus Hundreds of known viral species Infect humans, mammals, birds, fish, etc.

6 Viruses and hosts Selected members of the picornavirus family

7 Diseases Cause hepatitis, myocarditis, meningitis, common cold and poliomyelitis in humans Causes Foot-and-mouth disease in animals

8 Poliovirus: A model picornavirus for vaccine development and studies of replication Poliomyelitis is virtually absent from the developed world because of the Salk and Sabin vaccines

9 Poliovirus became a model system for the replication of RNA viruses and allowed for the observation of: RNA-dependent RNA-polymerase Translation and subsequent cleavage of large polyproteins Internal ribosome entry site

10 Virion Picornavirus virions bind to cellular receptors via depressions or loop regions on their surface Capsid contains 60 copies each of proteins VP1, VP2, VP3 and VP4 VP1-4 all fold into a jelly roll, composed of an 8-stranded β-barrel

11 Virion Cell receptors and co-receptors used by some picornaviruses

12 Genome RNA may pass through pores formed in cell membranes by capsid proteins Some picornaviruses have been shown to enter cell by receptor-mediated endocytosis followed by capsid dissociation

13 Other picornaviruses inject RNA directly across plasma membrane after conformational change in capsid

14 Translation initiates on picornavirus RNAs by a novel internal ribosome entry mechanism Host cell proteins bind to the internal ribosome entry site (IRES) and recruit ribosomes Cap-independent translation Allows for virus to abolish cap-dependent translation in cell

15 Initiation of protein synthesis

16 Essential features of picornavirus IRES elements Composed of six stem loops (Type 1 IRES) or twelve stem loops (Type 2 IRES)

17 Interaction of picornavirus IRES elements with host cell proteins La autoantigen, polypyrimidine-tract binding protein, poly(rc) binding protein 2, and the product of the unr gene Multiple eukaryotic initiation factors

18 Picornavirus proteins are made as a single precursor polyprotein that is autocatalytically cleaved by viral proteinases Polyprotein first cleaved into precursors P1, P2 and P3 P1 then cleaved into capsid proteins P2 and P3 cleaved into non-structural proteins

19 Picornaviruses make a variety of proteinases that cleave the polyprotein and some cellular proteins Proteinases L, 2A and 3C L is found on the N-terminus of only Foot-andmouth disease virus polyprotein 2A only has proteinase activity in entero- and rhinoviruses All picornaviruses have 3C, its precursor 3CD, is also a proteinase

20 Replication of picornavirus RNAs is initiated in a multiprotein complex bound to proliferated cellular vesicles Involves proteins from P2 and P3 regions A full length negative RNA strand is made and then used as a template for positive-strand synthesis Vesicles serve as a nucleation site

21 Proposed mechanism for replication of picornavirus RNA

22 RNA synthesis is primed by VPg covalently bound to uridine residues Uridylylation of VPg allows it to hybridize to the poly(a) tail and serve as a primer for (-) strand synthesis RNA synthesis is completed by 3D

23 Viral and cellular proteins bind to both ends of the RNA, bringing them together VPg uridylylation occurs using cis-acting replication elements (cre) and is then used as a primer for (+) strand RNA synthesis

24 Virion assembly involves cleavage of VP0 to VP2 plus VP4 VP0, VP1 and VP3 assemble into protomers Five protomers self-assemble into 14S pentamers Pentamers and RNA assemble into provirion VP0 is cleaved into VP2 and VP4 to form the mature virion

25 Assembly of the picornavirus virion

26 Inhibition of host cell macromolecular functions Shutoff of host cap-dependent translation and RNA synthesis Induction of cytoplasmic vesicles Alteration of intracellular transport pathways

27 Fundamental concepts Picornaviruses display wide ranges of species and cell susceptibilities for viral infections Picornaviruses encode their proteins as a large precursor polyprotein which is cleaved by viral proteinases Synthesis of proteins occurs via an internal ribosome entry site (IRES) Picornaviruses carry out RNA replication on cytoplasmic membranes Many picornaviruses inhibit cellular macromolecular functions

28 Key terms Cis-acting replication element (cre) Hepatitis Internal ribosome entry site (IRES) Jelly roll β-barrel Myocarditis Poliomyelitis Protomers Receptor-mediated edocytosis