Herpesviruses

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

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

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

Herpesvirus particles Genome: Single large segment of dsDNA, ~ 3% of particle weight ( kbp) Core: Nucleic acid wrapped around cylindrical structure nm Capsid: T=16 icosahedron composed of 162 capsomeres (150 hexamers and 12 pentamers), capsid diameter 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 nm

Major Herpesvirus structural proteins

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

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

HHV genomes Vary in size, structure, positions of repeats

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

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

Human Herpesvirus 1 particles and genome organization

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

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

Herpesvirus latency 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

Phylogenetic relationships among orthopoxviruses

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