Presentation on theme: "Hepadnaviruses - Hepadnaviridae"— Presentation transcript:
1 Hepadnaviruses - Hepadnaviridae VirionGenomeGenes and proteinsViruses and hostsDiseasesDistinctive characteristics
2 Hepadnaviruses - Hepadnaviridae VirionSpherical enveloped particle.Diameter 42 nm.Icosahedral capsid, T=4.Abundant smaller spherical and filamentous forms lack nucleocapsid and are not infectious.
3 Hepadnaviruses - Hepadnaviridae GenomeCircular dsDNA with a ss gap on one strand, 3.2 Kb.
4 Hepadnaviruses - Hepadnaviridae Genes and proteinsmRNAs transcribed by cellular RNA polymerase II from five promoters.Four overlapping reading frames, seven viral proteins.Three surface proteins: LS, MS, SS.Core proteins: C, E.Polymerase (reverse transcriptase) protein: P.Regulatory protein: X.
5 Hepadnaviruses - Hepadnaviridae Viruses and hostsHepatitis B viruses of human, chimpanzee, duck, gibbon, gorilla, ground squirrel, heron, orangutan,snow goose, woodchuck, woolly monkey.
6 Hepadnaviruses - Hepadnaviridae DiseasesHepatitis: incubation period 30–180 days.Transmission by blood and sexual contact.Acute disease can be mild or severe.Chronic or associated diseases: cirrhosis, liver cancer, serum sickness.Treatment: interferon, lamivudine.Prevention: vaccination with recombinant hepatitis B surface antigens; injection with anti-hepatitis B immunoglobulin after exposure to virus.
7 Hepadnaviruses - Hepadnaviridae Distinctive characteristicsUnusual partly ss, partly ds circular genome results from incomplete replication in cell.Pregenome RNA is both an mRNA and a template for synthesis of genome DNA.Reverse transcriptase (polymerase protein) generates genome DNA from pregenome RNA within capsid during virus assembly.Makes large amounts of noninfectious spherical and filamentous particles.Has a satellite virus: hepatitis delta virus (see Chapter 29, viroids).
8 Virion At least seven distinct viruses cause human hepatitis The discovery of hepatitis B virus1963, unknown protein related to type B hepatitis1973, Virus-like particles in the serum of type B hepatitis patients (Dane particle)1979, DNA genome cloned and sequenced
9 VirionDane particles are infectious virions; abundant noninfectious particles lack nucleocapsidsFig Structure of virions of hepatitis B virus.
10 VirionThe viral genome is a circular, partly single-stranded DNA with overlapping reading framesPartly double-stranded with a single-stranded region of variable lengthNeither DNA strands forms a covalently closed circle(-) strand joined at 5’ end to P protein(+) strrand joined at 5’ end to short capped RNA5’ ends of (-) strand and (+) strand genomic DNA are joined by short complementary regions named DR2Four partly overlapped ORFs : C, P, S, X
11 GenomeThe 5 ends of minus-strand (inside circle) and plus-strand (outside circle) genomic DNA are joined by short complementary regions named DR2 (gray boxes). The 5 end of the minus strand is attached to the viral polymerase protein (orange sphere) and the 5 end of the plus strand is attached to a short, capped RNA (black line).DNA polymerase extends the plus strand, filling in the ss gap.Nucleases remove the 5 ends of both strands.DNA ligase joins the 3 and 5 ends of each strand, forming a fully covalently closed double-stranded circular DNA.Fig Structure of hepatitis B virus genome, and its conversion to a covalently closed, circular DNA.
12 Genes and proteinsFig Coding and signaling regions on the hepatitis B genome.
13 Genes and proteinsNucleocapsids enter the cytoplasm via fusion and are transported to the nucleusNo currently available in vitro culture cell linesInitial steps of virus entry are poorly understoodCleavage of large hepatitis B surface Ag exposes fusion peptide, resulting in viral and host cell membrane fusionReleased nucleocapsid into cytoplasm is transported to nuclear membraneViral genome is converted within nucleus into covalently closed circular form by host enzymes
14 Genes and proteinsTranscription of viral DNA gives rise to several mRNAs and a pregenome RNAa. Circular DNA is transcribedby host RNA polymerase II, which recognizes five promoters (preC, pg, preS1, S, and X) to generate five classes of capped mRNAs that share a common 3 polyadenylated end.b. Open reading frames and translation products are shown above each RNA. pg RNA also serves as a template for virus replication.Fig Transcription ofhepatitis B virus DNA.
15 Genes and proteins The roles of hepatitis B virus proteins X protein Implicated in HCCUp-regulate c-src, Ras/raf/MAPk, SAPK, PKC etcInteract with and sequester p53Inhibit apoptosisSurface proteins.Involved in envelope formationCore and E proteins.Core : major component of necleocapsidE protein : may suppress host immune systemPolymerase protein.RNA/DNA-dependent DNA polymerase (reverse transcriptase)Ribonuclease H
16 Genes and proteinsThe pregenome RNA is packaged by interaction with polymerase and core proteinpgRNA serves as mRNA for C and P, and also used as template to produce DNA genome5’ e stem-loop is functional in packaging, and only in the pg RNAP protein recognizes and interacts with the e stem-loop, initiating both encapsidation and reverse transcription of pg RNA
17 Genes and proteinsGenome replication occurs via reverse transcription of pregenome RNAStep 1. Initiation of the reverse transcription process begins when P binds to the 5 stem-loop.Steps 2 and 3. Reverse transcription progresses for only a few base-pairs beyond the 5 stem-loop, after which the enzyme and the newly synthesized DNA are translocated onto an identical sequence in the direct repeat region 1 (DR1) near the 3 end of the pregenome RNA.
18 Genes and proteinsGenome replication occurs via reverse transcription of pregenome RNASteps 4 and 5. The P protein then makes a complete (minus strand) copy of the remainder of the pregenome RNA by extending the DNA chain to the 5 end of the RNA template.Steps 6 and 7. The translocated RNA fragment serves as a primer for subsequent synthesis of plus-strand DNA.
19 Genes and proteinsFig Reverse transcription of pregenome RNA to form hepatitis B virus genome DNA.
21 Genes and proteinsFig Schematic comparison of replication cycles of retroviruses and hepadnaviruses.
22 Genes and proteinsVirions are formed by budding in the endoplasmic reticulumFig Assembly of nucleocapsids and virions of hepatitis B virus.
23 Genes and proteinsHepatitis B virus can cause chronic or acute hepatitis, cirrhosis, and liver cancerHepatitis B virus is transmitted by blood transfusions, contaminated needles, and unprotected sexA recombinant vaccine is availableAntiviral drug treatment has real but limited successInterferon-aLamivudine (3T3)Interfere with DNA polymerase activity of P protein