Chromosome-protein interactions in polyomavirus virions

Immune sensing of mouse polyomavirus DNA by p204 and cGAS DNA sensors

The mechanism by which DNA viruses interact with different DNA sensors and their connection with the activation of interferon (IFN) type I pathway are poorly understood. We investigated the roles of protein 204 (p204) and cyclic guanosine-adenosine synthetase (cGAS) sensors during infection with mouse polyomavirus (MPyV). The phosphorylation of IFN regulatory factor 3 (IRF3) and the stimulator of IFN genes (STING) proteins and the upregulation of IFN beta (IFN-β) and MX Dynamin Like GTPase 1 (MX-1) genes were detected at the time of replication of MPyV genomes in the nucleus. STING knockout abolished the IFN response. Infection with a mutant virus that exhibits defective nuclear entry via nucleopores and that accumulates in the cytoplasm confirmed that replication of viral genomes in the nucleus is required for IFN induction. The importance of both DNA sensors, p204 and cGAS, in MPyV-induced IFN response was demonstrated by downregulation of the IFN pathway observed in p204-knockdown and cGAS-knockout cells. Confocal microscopy revealed the colocalization of p204 with MPyV genomes in the nucleus. cGAS was found in the cytoplasm, colocalizing with viral DNA leaked from the nucleus and with DNA within micronucleus-like bodies, but also with the MPyV genomes in the nucleus. However, 2'3'-Cyclic guanosine monophosphate-adenosine monophosphate synthesized by cGAS was detected exclusively in the cytoplasm. Biochemical assays revealed no evidence of functional interaction between cGAS and p204 in the nucleus. Our results provide evidence for the complex interactions of MPyV and DNA sensors including the sensing of viral genomes in the nucleus by p204 and of leaked viral DNA and micronucleus-like bodies in the cytoplasm by cGAS.

A polyomavirus peptide binds to the capsid VP1 pore and has potent antiviral activity against BK and JC polyomaviruses

In pursuit of therapeutics for human polyomaviruses, we identified a peptide derived from the BK polyomavirus (BKV) minor structural proteins VP2/3 that is a potent inhibitor of BKV infection with no observable cellular toxicity. The thirteen-residue peptide binds to major structural protein VP1 with single-digit nanomolar affinity. Alanine-scanning of the peptide identified three key residues, substitution of each of which results in ~1000 fold loss of binding affinity with a concomitant reduction in antiviral activity. Structural studies demonstrate specific binding of the peptide to the pore of pentameric VP1. Cell-based assays demonstrate nanomolar inhibition (EC₅₀) of BKV infection and suggest that the peptide acts early in the viral entry pathway. Homologous peptide exhibits similar binding to JC polyomavirus VP1 and inhibits infection with similar potency to BKV in a model cell line. Lastly, these studies validate targeting the VP1 pore as a novel strategy for the development of anti-polyomavirus agents.

The encapsidation of polyomavirus is not defined by a sequence-specific encapsidation signal

Mouse polyomavirus (MPyV) is considered a potential tool for the application of gene therapy; however, the current knowledge of the encapsulation of DNA into virions is vague. We used a series of assays based on the encapsidation of a reporter vector into MPyV pseudovirions to identify putative cis-acting elements that are involved in DNA encapsidation. None of the sequences that were derived from MPyV have been shown to solely enhance the encapsidation of a reporter vector in the assay. The frequency of encapsidation strongly correlated with the total intracellular amount of the vector after transfection. The encapsidation of target DNA into the pseudovirions was shown to be non-specific, and the packaging of non-replicated DNA was observed. We propose that the actual concentration of target DNA at the sites of virion formation is the primary factor that determines its selection for encapsidation.

How viruses use the endoplasmic reticulum for entry, replication, and assembly

To cause infection, a virus enters a host cell, replicates, and assembles, with the resulting new viral progeny typically released into the extracellular environment to initiate a new infection round. Virus entry, replication, and assembly are dynamic and coordinated processes that require precise interactions with host components, often within and surrounding a defined subcellular compartment. Accumulating evidence pinpoints the endoplasmic reticulum (ER) as a crucial organelle supporting viral entry, replication, and assembly. This review focuses on the molecular mechanism by which different viruses co-opt the ER to accomplish these crucial infection steps. Certain bacterial toxins also hijack the ER for entry. An interdisciplinary approach, using rigorous biochemical and cell biological assays coupled with advanced microscopy strategies, will push to the next level our understanding of the virus-ER interaction during infection.

PARP-1 interaction with VP1 capsid protein regulates polyomavirus early gene expression

Poly(ADP-ribose)polymerases are involved in fundamental cellular events as well as they seem to be associated to some viral infection process. In this work, the poly(ADP-ribose)polymerase-1 (PARP-1) role in the polyomavirus life cycle has been investigated. Early viral transcription was reduced by competitive inhibitors of PARPs in Swiss 3T3 cells and almost abolished in PARP-1 knockout fibroblasts and in wild-type fibroblasts when PARP-1 was silenced by RNA interference. In vivo chromatin immunoprecipitation assays showed that poly(ADP-ribosyl)ation (poly(ADP-ribose)) facilitates the release of the capsid protein viral protein 1 (VP1) from the chromatin of infecting virions. In vitro experiments demonstrated that VP1 stimulates the enzymatic activity of PARP-1 and binds non-covalently both protein-free and PARP-1-bound poly(ADP-ribose). Our studies suggest that PARP-1 promotes the complete VP1 displacement from viral DNA favouring the viral early transcription.