Using a unique hepatocellular model system designed to support viral growth, we demonstrate that hepatitis B virus (HBV) has remarkably slow infection kinetics. Establishment of the episomal transcription template and the persistent form of the virus, so called covalently closed circular DNA, as well as viral transcription and protein expression all take a long time. Once established, HBV maintains a stable pool of covalently closed circular DNA via intracellular recycling of HBV genomes and through infection of naïve cells by newly formed virions.

Prion diseases are fatal transmissible neurodegenerative diseases affecting many mammalian species. The normal prion protein (PrP) converts into a pathological aggregated form, PrP Sc , which is enriched in the ␤-sheet structure. Although the high resolution structure of the normal PrP was determined, the structure of the converted form of PrP remains inaccessible to high resolution techniques. To map the PrP conversion process we introduced disulfide bridges into different positions within the globular domain of PrP, tethering selected secondary structure elements. The majority of tethered PrP mutants exhibited increased thermodynamic stability, nevertheless, they converted efficiently. Only the disulfides that tether subdomain B1-H1-B2 to subdomain H2-H3 prevented PrP conversion in vitro and in prion-infected cell cultures. Reduction of disulfides recovered the ability of these mutants to convert, demonstrating that the separation of subdomains is an essential step in conversion. Formation of disulfide-linked proteinase K-resistant dimers in fibrils composed of a pair of single cysteine mutants supports the model based on domain-swapped dimers as the building blocks of prion fibrils. In contrast to previously proposed structural models of PrP Sc suggesting conversion of large secondary structural segments, we provide evidence for the conservation of secondary structural elements of the globular domain upon PrP conversion. Previous studies already showed that dimerization is the rate-limiting step in PrP conversion. We show that separation and swapping of subdomains of the globular domain is necessary for conversion. Therefore, we propose that the domain-swapped dimer of PrP precedes amyloid formation and represents a potential target for therapeutic intervention.Prion diseases, also called transmissible spongiform encephalopathies, are neurodegenerative diseases affecting a variety of mammalian species from mink to cow, with human being no exception. In these diseases, cellular prion protein (PrP C ) 5 converts into the aggregated form PrP Sc , which is the main component of the infectious agents, the prions (1). PrP C is a glycosylphosphatidylinositol-anchored protein found on the membranes of neurons and many other cells (2). The N-terminal half of the protein, which is devoid of a defined tertiary structure (3), is followed by a C-terminal globular domain composed of three ␣-helices (H1, H2, H3) and a short antiparallel ␤-sheet (composed of two strands, B1 and B2) (Fig. 1A) (4). High resolution structures of the C-terminal domain of PrP from different species revealed the conservation of the protein fold (5). In contrast to the availability of structural information on PrP C , the characteristics of PrP Sc make it inaccessible to high-resolution structural techniques (x-ray crystallography and high resolution NMR). PrP Sc is characterized by the increased content of the ␤-secondary structure in comparison to PrP C (6 -8). Epitope accessibility (9 -12), deuterium exchange (13-15), limited proteolysis and mas...

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Hepatitis B virus (HBV) persists by depositing a covalently closed circular DNA (cccDNA) in the nucleus of infected cells that cannot be targeted by available antivirals. Interferons can diminish HBV cccDNA via APOBEC3-mediated deamination. Here, we show that overexpression of APOBEC3A alone is not sufficient to reduce HBV cccDNA that requires additional treatment of cells with interferon indicating involvement of an interferon-stimulated gene (ISG) in cccDNA degradation. Transcriptome analyses identify ISG20 as the only type I and II interferon-induced, nuclear protein with annotated nuclease activity. ISG20 localizes to nucleoli of interferon-stimulated hepatocytes and is enriched on deoxyuridine-containing singlestranded DNA that mimics transcriptionally active, APOBEC3Adeaminated HBV DNA. ISG20 expression is detected in human livers in acute, self-limiting but not in chronic hepatitis B. ISG20 depletion mitigates the interferon-induced loss of cccDNA, and co-expression with APOBEC3A is sufficient to diminish cccDNA. In conclusion, non-cytolytic HBV cccDNA decline requires the concerted action of a deaminase and a nuclease. Our findings highlight that ISGs may cooperate in their antiviral activity that may be explored for therapeutic targeting.