EM of purified papillomavirus particles

Papovaviruses

Papovaviridae• Name from: Papilloma Polyoma Vacuolating agent• Suffix “oma” means swelling or tumor• Two subfamilies: Papillomavirinae,

Polyomavirinae• Not very important as lethal human pathogens,

but cause warts and in some cases cancer• The most common viral STD• Highly restricted host ranges

Major papovavirus diseases

• Papillomavirus:– Most important of papovaviruses– More than 100 distinct strains identified to date– Cause common and genital warts, sometimes associated with

cancer• Polyomaviruses:

– JC Virus• Infect only humans• Common, causes disease only in immunosuppressed

– BK Virus• Also found in immunosuppressed• Both JC and BK associated with cancer

– Role of SV40 in human cancer still debated

Papovaviridae properties

• Simple T=7 particles, 45-55 nm, no envelope• Small dsDNA genomes, 5-7 kbp• 3 capsid proteins subunits• Host-derived histones account for 20% of viral

protein

Polyomavirus Papillomavirus

Genome

•double-stranded •circular •DNA •~5kbp •Uses overlapping genes and both strands of DNA to pack all 6 genes into a tiny space •genome ~5000 nucleotides

•double-stranded •circular •DNA •~8kbp •Uses overlapping genes and one strand of DNA to pack at least 12 genes into 8kbp •Genome ~ 8000 nucleotides

 

Morphology

•Noneveloped •~45 nm diameter •icosohedral, skew, T= 7 •3 capsid proteins

•Nonenveloped •~52-55nm diameter •icosohedral, skew, T= 7 •2 capsid proteins

Other Differen

ces •Subfamily specific antigens •Papillomavirus can't be grown in culture, while polyomaviruses are often grown in culture

Pathoge

nesis Most are asymptomatic, although can be oncogenic in hamsters

Cause various types of warts, epidermodysplasia verruciformis

Representative Viruses

JC Virus•Associated with Progressive multifocal leukoencephalopathy, found mainly in the elderly and immunocompromised

BK Virus•Results in mild respiratory illness in kids •Found in some tumors

Simian Virus 40A completely sequenced animal virus used frequently as a cloning vector.

At least 62 strains of Human Papillomaviruses•Widespread •Cause growths or warts •Many are associated with cancer

   

Summary from Robert Siegel, Stanford U.

Papillomavirus properties

• No tissue culture system available, so relatively poorly studied

• Productively infect only fully differentiated squamose epithelial cells (dead end cells)

• Result in warts, sometimes become malignant• All ORFs located on one strand; all transcripts

from that strand• 70% of genome for transformation and plasmid

maintenance

Viral capsid

Minor capsidcomponent

transformation

Transcriptionaltransactivation

episomalpersistence

transformation

high copy

Papillomavirus genome organization

Linear representation of circular genome

A. EM of particles and B. Circular representation of Human papillomavirus genome. Note that transcription proceeds from only one of the two strands.

Polyomavirus properties

• SV40 the best known• Infectious closed circular DNA 5.2-5.3 kbp• Infection:

– Establish lytic infection in permissive cells, may transform non-permissive cells

• Entry– By receptor-mediated endocytosis– Virions enter nucleus for genome expression and

replication

Polyomavirus properties

• Transcription of T-antigens before replication– Large T

• Induces cellular DNA synthesis• Required for transformation and maintenance off

transformed state• Required for viral DNA synthesis (has ATPase and

helicase activities• Regulates its own synthesis and that of other T

antigens

– Small T• Regulation of viral DNA synthesis

– Middle T• Required for transformation

Polyomavirus replication

• Occurs 12-15 hours post-infection

• Proceeds bidirectionally from origin

• Large T is the only viral protein involved in DNA replication

• Other replication-associated proteins are from host

Polyomavirus late transcription

• Late mRNA:– transcribed after DNA replication– transcribed from the strand complementary to

the strand used for early RNA transcription– transcribed from progeny, not parental genomes– transcribed in much greater amounts than early– encodes three structural proteins, by differential

splicing

Polyomavirus assembly and release

• Assembly– Takes place in the nucleus– Viral genome complexed with histones

encapsidated into preformed particles– Particle assembly process is mediated by host

chaperone sp70 and Large T

• Release– Initially by exocytosis of virus-containing

vesicles– Later by cell death and lysis

A. SV40 has a distinctive T=7 structure made up of three proteins. Circular dsDNA inside is complexed with histone proteins. B. Genome organization is compact; transcription bidirectional from ORI. Transcription of early T-antigens is from input DNA; transcription of late structural proteins is from newly synthesized DNA.

A. EM of replicating SV40 DNA, showing unwinding during bidirectional replication.

B. Schematic of bidirectional replication from single origin of replication (Ori)

A. Continuous DNA synthesis from Ori, RNA primed (primase)5’>3’B. Discontinuous DNA synthesis toward Ori, also RNA primed, also 5’>3” (Fig

9.2, Principles of Virology)

Semidiscontinuous DNA synthesis from SV40 bidirectional origin

SV40 infection cycle

1. Entry and release of core2. Entry of DNA/nucleosome

complex to nucleus3-5.Synthesis and alternative

splicing of early (T) transcripts; proteins synthesized

6-7.LT imported back to nucleus where it initiates DNA synthesis with host enzymes and potentiates late gene transcription

8-12. Late gene mRNAs synthesized, spliced, exported, translated, and products imported back to nucleus for particle assembly and release by unknown mechanism