(+) Strand RNA viruses (Schein) - 5/16/16

Question 1

Families of (+)-strand RNA viruses

Answer 1

Picornaviridae
Caliciviridae
Flaviviridae
Togaviridae
Coronaviridae

Question 2

Common Features of (+)-strand RNA viruses

Answer 2

Replicate in cytoplasm (b/c have NO DNA unlike DNA viruses which replicate in nucleus)

Genomic RNA serves as a message and is translated

Genomic RNA = infectious

Virions do not contain any enzymes

Viral proteins are translated as polyproteins (so once RNA enters cell, a long protein is translated)

Question 3

Poliovirus (Prototype Picornavirus)

Answer 3

• Small, naked icosahedral virus: 28 nm
• (+)-sense non-segmented RNA genome (one long piece of RNA)
• Virus contains 4 different capsid proteins for core: VP1, VP2, VP3, and VP4 (VP4 is internal)
• 3 serological types which infect humans (which means you can make a vaccine for it)

Question 4

Special features of the viral RNA

Answer 4

Genetically encoded polyA sequence at the 3’ end

No 5’ cap on the RNA

Instead, small virus encoded peptide, VPg covalently bound to the 5’ end of the viral RNA

• 5 UTR = very long and contains IRES (internal ribosomal entry site)
• In mammalian cells, ribosome comes along and sees the cap and scans until it sees AUG –> then starts translation
• But along comes poliovirus and infects: ribosome see IRES and attaches onto it and starts translation (IRES will capture ribosome without need of 5’ cap… bypasses it and starts translating viral protein)

Question 5

Replication of poliovirus

Answer 5

Attachment to a specific receptor

Entry via receptor-mediated endocytosis

Viral RNA gets across membrane, gains access to cytosol, where it binds to ribosomes

Question 6

Translation and processing of viral proteins

Answer 6

Viral genome contains a single, long open reading frame –> therefore, viral proteins are synthesized as one long polyprotein

Two virus-encoded proteases self-cleave polyprotein giving rise to all the structural and nonstructural proteins

Only viral proteins can be encoded because of IRES - all of our proteins are being shut off

Question 7

Once viral proteins are made…

Answer 7

One is an RNA-dependent RNA polymerase (RDRP)

Incoming (+) sense RNA serves as template for synthesis of (-) strand RNA, which in turn serves as template for (+) strand RNA

Whole cycle takes 5-6 hours

Question 8

Features common to the Flaviviridae and Togaviridae

Answer 8

Non-segmented (+) strand RNA genome

Viruses are enveloped (enter cell through receptor-mediated endocytosis and there are vesicles which then fuse… capsomere is liberated –> they get their membranes from internal membranes and then they bud out at the surface)

Most, but not all viruses in these two families are arthropod-borne

Question 9

Replication strategies:

Flavivirus vs. Togavirus

Answer 9

Flavivirus:

• One ORF-like Picornavirus (one polyprotein)
• Structural genes on 5’ end

Togavirus:

• Two open reading frames (Subgenomic = structural genes only; both polyproteins)
• Structural genes on 3’ end