Herpesviruses Virion Genome Genes and proteins Viruses and hosts Diseases Distinctive characteristics
Virion Enveloped icosahedral capsid (T=16), diameter 125 nm Diameter of enveloped virion 200 nm Capsid contains six proteins, envelope, contains at least ten glycoproteins Material between capsid and envelope, called the tegument, contains at least 14 viral proteins
Genome Linear double-stranded DNA, 125-250 kb Contains unique regions, inverted repeat elements, and terminal direct repeat sequences
Genes and proteins Herpes simplex virus type 1: 90 different transcriptional units Transcribed by cellular RNA pol II Most mrnas are unspliced Most transcriptional units encode only one protein
Genes and proteins Genes are expressed in three temporal classes: α (immediate early): early gene activation (six genes) β (early): DNA replication, late gene activation γ (late): virion proteins, regulatory proteins
Viruses and hosts Herpesviridae from Greek herpein (creep), referring to spreading of lesions Three subfamilies: alpha, beta and gammaherpesviruses Nine human herpesviruses, including herpes simplex virus, varicella-zoster virus, Epstein-Barr virus, cytomegalovirus Over 100 known herpesviruses from many animal species
Diseases Chickenpox, mononucleosis, pneumonia, hepatitis, encephalitis Recurrent eye, mouth and genital sores Kaposi s sarcoma, Burkitt s lymphoma, nasopharyngeal carcinoma Life-threatening infections in immunesuppressed individuals Neonatal infections, birth defects
Diseases Herpesviruses are important human pathogens Mild diseases like cold sores, genial lesions and chickenpox In newborns and immunosuppressed, can cause encephalitis, pneumonia, and hepatitis Grouped into subfamilies based on cell tropism
Viruses and hosts
Diseases Most herpesviruses can establish latent infections After initial infections, viral DNA is harbored in a latent state in neurons, B or T lymphocytes Reactivation of latent virus leads to recurrent disease
Distinctive characteristics Herpes simplex virus genomes contain both unique and repeated sequence elements Contains covalently linked components, long (L) and short (S) containing: unique sequences U L and U S inverted repeats
Map of herpes simplex virus genes Genome
Virion The icosahedral capsid is enclosed in an envelope along with tegument proteins 150 hexamers and 12 pentamers make up the capsid Envelope contains 10 different glycoproteins The tegument lies between the capsid and envelope
Distinctive characteristics Entry by fusion is mediated by envelope glycoproteins and may occur at the plasma membrane or in endosomes Binds to heparin sulfate proteoglycans or alternative receptors nectin1 and herpesvirus entry mediator (HVEM) a member of the tumor necrosis factor (TNF) receptor family
Distinctive characteristics Entry and uncoating of herpes simplex virus
Distinctive characteristics Viral genes are sequentially expressed during the replication cycle Transcription occurs in nucleus by RNA pol II Immediate early genes first few hours after infection Early genes 4 to 8 hours after infection. Late genes after the onset of DNA replication
Distinctive characteristics Transcriptional map of the b and c regions of herpes simplex virus. Transcripts of the six α genes
Genes and proteins Tegument proteins interact with cellular machinery to activate viral gene expression and to degrade cellular mrna Vp16 forms a complex with Oct-1 and HCF-1 which then binds to response elements upstream of the promoters of α genes Vhs mediates the selective degradation of both cellular and viral mrna
Genes and proteins Structure of promoter regions of different classes of herpes simplex virus genes
Genes and proteins Immediate early (α) genes regulate expression of other herpesvirus genes
Genes and proteins β gene products enable viral DNA replication
Distinctive characteristics DNA replication initially proceeds in a bidirectional fashion from a replication origin Replicating DNA in infected cells is found in multimers called concatemers After bidirectional replication, replication shifts to rolling circle mechanism
Distinctive characteristics Establishment of bidirectional DNA replication U L 42 increases the processivity of the polymerase
Distinctive characteristics Rolling circle replication subsequently produces multimeric concatemers of viral DNA
Distinctive characteristics DNA replication leads to activation of γ 1 and γ 2 genes Encode structural proteins, assembly and packaging proteins Expression is regulated by several α genes and a protein kinase encoded by U L 13 (activates cyclin-dependent kinase cdc2)
Distinctive characteristics Viral nucleocapsids are assembled on a scaffold in the nucleus
Distinctive characteristics Envelopment and egress: three possible routes Nucleocapsids are assembled in the nucleus and bud through the nuclear membrane, acquiring an envelope and tegument proteins in the lumen Three distinct theories account for the mechanism by which virions exit the cell
Distinctive characteristics Envelopment and egress of herpesvirus virions
Genes and proteins Many viral genes are involved in blocking host responses to infection Vhs degrades cellular mrna, including the proteins associated with activation of interferon. ICP27 inhibits splicing of mrnas ICP47 blocks the translocation of peptides and inhibits antigen presentation by major histocompatability (MHC) class I ICP34-5 reverses the effect of RNA-dependent protein kinase (PKR)
Genes and proteins ICP0 dissociates histone acetylases from chromatin in ND10 structures and acts as a ubiquitin ligase to degrade components of ND10 Anti-apoptosis gene :
Distinctive characteristics Herpes simplex virus establishes latent infections in neurons Virus infects skin or mucosal cells then infect nerve endings and undergo retrograde transport along axons to the cell bodies of dorsal root neurons Establish latentcy
Distinctive characteristics Latency-associated transcripts include stable introns May function by inhibiting ICP0 and ICP34.5 expression Physiological stress reactivates latent viruses
Distinctive characteristics Epstein-Barr virus was discovered in lymphomas in African children Primary infection in childhood and results in asymptomatic infections Primary infection in adolescence and adulthood causes infectious mononucleosis EBV is linked to several cancers including Burkitt s lymphoma, nasopharyngeal carcinoma, and Hodgkin s lymphoma
Distinctive characteristics Epstein-Barr virus infects mucosal epithelial cells and B lymphocytes Replicates transiently in oral mucosa, releasing virions into saliva Infection of B cells (by attaching to CD21, a complement receptor, and HLA class II) leads to long term latent infection
Distinctive characteristics Overview of the events during infection by Epstein-Barr virus and stages of latency
Genome Linearized genome of Epstein-Barr virus and transcripts expressed during latent infection
Distinctive characteristics Epstein-Barr virus expresses a limited set of proteins in latently infected B lymphocytes Epstein-Barr nuclear antigens (EBNAs0 Latent membrane proteins (LMPs)
Distinctive characteristics Epstein-Barr virus nuclear antigens direct limited replication of the viral genome and activate viral and cellular genes EBNA-1 directs one replication of the viral genome and the segregation of genomes into progeny cells during cell division EBNA-2 and EBNA-LP activate transcription of LMP genes and cellular genes
Distinctive characteristics Latent membrane proteins mimic receptors on B lymphocytes LMP-1 mimics CD40 LMP-2 mimics B cell receptor
Fundamental concepts Most herpesviruses establish latent infections in their hosts after an initial primary infection Latent herpesviruses may be reactivated regularly Herpes simplex virus genomes have inverted repeat sequences flanking unique sequence elements, as well as direct terminal repeated sequences. The herpesvirus capsid is surrounded by a tegument that contains numerous viral proteins that function upon entry Herpesviruses enter the cell by fusion of their envelope with cell membranes. After fusion, nucleocapsids are transported to the cell nucleus where they release their DNA into the nucleus
Fundamental concepts Viral DNA is circularized once it enters the nucleus All herpesvirus mrnas are made by cellular RNA pol II; transcription factors regulate the transcriptional program into several temporal classes called α (immediate early) β (early), γ 1 and γ 2 (late) A tegument protein called Vhs degrades host cell as well as viral mrna α gene products regulate viral and cellular gene expression β gene products direct viral DNA replication A rolling circle mechanism produces multimeric concatemers of viral DNA that are cleaved into genome length fragments during packaging
Fundamental concepts Herpesvirus nucleocapsids appear to bud through the inner nuclear membrane to acquire an envelope, but the precise mechanism by which they acquire their final envelope and exit the cell is uncertain A number of herpesvirus gene products suppress host cell functions, including apoptosis Herpes simplex viruses establish latent infections in neurons Epstein-Barr virus causes mononucleosis and in certain regions of the world, malignant lymphomas EBV infects B lymphocytes and establishes a variety of latent infections EBV encodes two small RNAs which inhibit PKR
Key terms Apoptosis B lymphocyte Complement Concatemer Cyclin-dependent kinase Dorsal root neurons Encephalitis Heparin sulfate Hepatitis Interferons Latency Latent infection Lumen Major histocompatability complex proteins Nectin NFκB PKR Processivity Proteasomes Proteoglycans Retrograde transport Rolling circle mechanism T lymphocyte Tegument Topoisomerase Tropism Tumor necrosis factor Ubiquitin ligase