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Herpesviruses

Herpesviruses

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Herpesviruses

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  1. Herpesviruses

  2. 100 nm Herpesvirus structures are unique, with tegument layer present and genomic DNA wrapped around core

  3. Herpesviridae • Large family of large, complex viruses • Infect vertebrate hosts • Three subfamilies: • Alphaherpesvirinae, 2 genera • Betaherpesvirinae, 3 genera • Gammaherpesvirinae, 2 genera • Very important as human pathogens • Cause cold sores, genital herpes, chicken pox, shingles, mononucleosis and many other diseases • Infection is for life – herpesviruses become latent in hosts, then reactivate

  4. Irregular structures often seen in micrographs are artifacts of distortion from negative staining Cryo-EM shows regular, external structure preserved Nucleocapsid contains core of protein wrapped in genomic DNA

  5. Herpesvirus particles • Genome: Single large segment of dsDNA, ~ 3% of particle weight (124-235 kbp) • Core: Nucleic acid wrapped around cylindrical structure 25-30 nm • Capsid: T=16 icosahedron composed of 162 capsomeres (150 hexamers and 12 pentamers), capsid diameter 100-110 • Tegument: poorly defined material between capsid and envelope, contains alpha trans-inducing factor (α-TIP) necessary for activation of α genes and virion host shutoff protein (VHS) • Envelope: Derived from nuclear membrane, surrounds tegument, has spike glycoproteins – virion diameter 120-200 nm

  6. Major Herpesvirus structural proteins

  7. Herpesviruses • HHV1, Herpes simplex 1 • Cold sores, epithelial and neuronal cells • HHV2, Herpes simplex 2 • Sexually transmitted disease (STD), also as above, teratogenic, can be fatal in newborns • HHV3, Varicella-zoster • Chicken pox, shingles • HHV4, Epstein Barr virus • Mononucleosis, Burkitt’s lymphoma, lymphoid tissue only • HHV5, Cytomegalovirus • Salivary gland tropic, teratogenic, can be fatal in newborns • HHV6, Roseolovirus • Childhood rash, multiple sclerosis? • HHV8 • Associated with Kaposi’s sarcoma HSV 6,7,8 all identified after 1990, after HIV

  8. Biology of herpesviruses • All specify a large array of enzymes involved in nucleic acid metabolism • Virus DNA synthesis and capsid assembly in nucleus • Production of infectious progeny is accompanied by destruction of infected cell • A single virus can cause several diseases • Herpesviruses remain latent in the host for life, and can be reactivated to cause lesions at or near the initial infection site

  9. HHV genomes Vary in size, structure, positions of repeats

  10. HHV 1 genome occurs as approximately equal quantities of four isomers Short and long regions of viral genome are inverted relative to each other Other herpesviruses may have 1, 2, or 4 isomeric DNA forms

  11. Two models for DNA replication of herpesvirus; details are still poorly understood • DNA nicked • DNA replication is discontinuous • RNA primed • by rolling circle mechanism? Recent studies on HHV1 replication do not support a rolling circle model, but rather a complex set of concatameric intermediates

  12. Human Herpesvirus 1 particles and genome organization

  13. Herpesvirus infection cycle 1-4. Membrane fusion, release of tegument and VHS. 5-8. Nucleocapsids transported on microtubules to nuclear membrane, ejects DNA which circularizes. 9-10. Transcription, export, translation of immediate early genes by cellular Pol II 11-13. Transcription, translation of early genes; genome replication in nucleus. 14-18. Transcription, translation of late (structural) protein genes; import to nucleus, nucleocapsid assembled and stuffed with DNA. 19-23. Nucleocapsid/tegument buds into ER lumen; acquires envelope, egress by fusion.

  14. Herpesvirus packaging • Phage-like process • Empty particle with single portal assembled first • Genomic DNA stuffed through portal • pac1 and pac2 signals on genomic DNA are required for insertion of DNA through portal and cleavage of unit length DNA once headful is achieved

  15. Occurs in neurons, steps 1-7 probably like productive infection; normal viral transcription blocked (leaky), latency-associated RNAs transcribed, spliced, 2kb LAT in the form of lariat transported to cytoplasm, translated to LAT proteins LAT genes are clustered; micro RNAs regulate LAT mRNA expression Herpesvirus latency

  16. Phylogenetic relationships among orthopoxviruses

  17. Relationships among large dsDNA viruses Closest relatives of poxviruses are baculoviruses, viruses the infect insects Closest relatives of herpesviruses are phycodnaviruses, viruses the infect algae