Simian virus 40 T antigen can transcriptionally activate and mediate viral DNA replication in cells which lack the retinoblastoma susceptibility gene product

Functional reorganization of promyelocytic leukemia nuclear bodies during BK virus infection

BK virus (BKV) is the causative agent for polyomavirus-associated nephropathy, a severe disease found in renal transplant patients due to reactivation of a persistent BKV infection. BKV replication relies on the interactions of BKV with many nuclear components, and subnuclear structures such as promyelocytic leukemia nuclear bodies (PML-NBs) are known to play regulatory roles during a number of DNA virus infections. In this study, we investigated the relationship between PML-NBs and BKV during infection of primary human renal proximal tubule epithelial (RPTE) cells. While the levels of the major PML-NB protein components remained unchanged, BKV infection of RPTE cells resulted in dramatic alterations in both the number and the size of PML-NBs. Furthermore, two normally constitutive components of PML-NBs, Sp100 and hDaxx, became dispersed from PML-NBs. To define the viral factors responsible for this reorganization, we examined the cellular localization of the BKV large tumor antigen (TAg) and viral DNA. TAg colocalized with PML-NBs during early infection, while a number of BKV chromosomes were adjacent to PML-NBs during late infection. We demonstrated that TAg alone was not sufficient to reorganize PML-NBs and that active viral DNA replication is required. Knockdown of PML protein did not dramatically affect BKV growth in culture. BKV infection, however, was able to rescue the growth of an ICP0-null herpes simplex virus 1 mutant whose growth defect was partially due to its inability to disrupt PML-NBs. We hypothesize that the antiviral functions of PML-NBs are inactivated through reorganization during normal BKV infection.

BK virus (BKV) is a human pathogen that causes severe diseases, including polyomavirus-associated nephropathy in kidney transplant patients and hemorrhagic cystitis in bone marrow transplant recipients. How BKV replication is regulated and the effects of a lytic BKV infection on host cells at the molecular level are not well understood. Currently, there is no specific antiviral treatment for BKV-associated disease, and a better understanding of the complete life cycle of the virus is necessary. Here, we report the interplay between BKV and one of the regulatory structures in the host cell nucleus, promyelocytic leukemia nuclear bodies (PML-NBs). Our results show that BKV infection reorganizes PML-NBs as a strategy to inactivate the negative functions of PML-NBs.

Interaction of retinoblastoma protein family members with large T-antigen of primate polyomaviruses

The retinoblastoma gene product pRb and other members of the Rb family of pocket proteins have a central role in the regulation of cell cycle progression. Soon after its discovery, pRb was found to interact with the transforming oncoproteins of DNA tumor viruses and this led to rapid advances in our understanding of the mechanisms of viral transformation and cell cycle progression. DNA viruses of the polyomavirus family have small, circular, double-stranded DNA genomes contained within non-enveloped icosahedral capsids and are highly tumorigenic in experimental animals. At least three types of polyomavirus infect humans: JC virus (JCV), BK virus (BKV) and Simian Vacuolating virus-40. The early region of these viruses encodes the transforming proteins large T-antigen and small t-antigen, which are involved in viral replication and also promote transformation of cells in culture and oncogenesis in vivo. Binding of T-antigen to pRb promotes the activation of the E2F family of transcription factors, which induce the expression of cellular genes required for S phase. In the context of lytic infection, this cell cycle progression is necessary for viral replication because polyomaviruses rely on S phase-specific host factors for their DNA synthesis. In the context of cellular transformation and tumorigenesis, T-antigen/pRB interaction is an indispensable event.

Human polyomavirus BK (BKV) transiently transforms and persistently infects cultured osteosarcoma cells

Human polyomavirus BK (BKV) DNA and proteins have been detected in a number of bone tumours. We therefore investigated whether BKV infection might initiate transformation of human anchorage-dependent osteosarcoma cells in vitro. Infection of the osteosarcoma cell line U-2OS with a naturally occurring BKV strain resulted in soft agarose competent cell clones. In a subclone, designated U-2OS15E, approximately 10-20% of the cells contained episomal BKV genomes. A corresponding proportion of cells expressed BKV proteins and produced viral progeny. This proportion was not increased by BKV superinfection. Furthermore, U-2OS15E cells were resistant to SV40 infection. The transformed status of U-2OS15E cells lasted only for a few passages. However, the persistently infected cells produced infectious virions for more than 300 generations. In addition to representing a model system for persistent BKV infection, the uninfected and persistently BKV-infected cell cultures are useful tools for control and calibration of in situ BKV nucleic acid and protein detection methods.

Simian virus 40 large T antigen interacts with human TFIIB-related factor and small nuclear RNA-activating protein complex for transcriptional activation of TATA-containing polymerase III promoters

The TATA-binding protein (TBP) is common to the basal transcription factors of all three RNA polymerases, being associated with polymerase-specific TBP-associated factors (TAFs). Simian virus 40 large T antigen has previously been shown to interact with the TBP-TAFII complexes, TFIID (B. Damania and J. C. Alwine, Genes Dev. 10:1369-1381, 1996), and the TBP-TAFI complex, SL1 (W. Zhai, J. Tuan, and L. Comai, Genes Dev. 11: 1605-1617, 1997), and in both cases these interactions are critical for transcriptional activation. We show a similar mechanism for activation of the class 3 polymerase III (pol III) promoter for the U6 RNA gene. Large T antigen can activate this promoter, which contains a TATA box and an upstream proximal sequence element but cannot activate the TATA-less, intragenic VAI promoter (a class 2, pol III promoter). Mutants of large T antigen that cannot activate pol II promoters also fail to activate the U6 promoter. We provide evidence that large T antigen can interact with the TBP-containing pol III transcription factor human TFIIB-related factor (hBRF), as well as with at least two of the three TAFs in the pol III-specific small nuclear RNA-activating protein complex (SNAPc). In addition, we demonstrate that large T antigen can cofractionate and coimmunoprecipitate with the hBRF-containing complex TFIIIB derived from HeLa cells infected with a recombinant adenovirus which expresses large T antigen. Hence, similar to its function with pol I and pol II promoters, large T antigen interacts with TBP-containing, basal pol III transcription factors and appears to perform a TAF-like function.

TAF-like function of SV40 large T antigen

The simian virus 40 (SV40) early gene product large T antigen promiscuously activates simple promoters containing a TATA box or initiator element and at least one upstream transcription factor-binding site. Previous studies have suggested that promoter activation requires that large T antigen interacts with both the basal transcription complex and the upstream-bound factor. This mechanism of activation is similar to that proposed for TBP-associated factors (TAFs). We report genetic and biochemical evidence suggesting that large T antigen performs a TAF-like function. In the ts13 cell line, large T antigen can rescue the temperature-sensitive (ts) defect in TAF(II)250. In contrast, neither E1a, small t antigen, nor mutants of large T antigen defective in transcriptional activation were able to rescue the ts defect. These data suggest that transcriptional activation by large T antigen is attributable, at least in part, to an ability to augment or replace a function of TAF(II)250. In addition, we show that large T antigen interacts in vitro with the Drosophila TAFs (dTAFs) dTAF(II)150, dTAF(II)110, and dTAF(II)40, as well as TBP. The relevance of these in vitro results was established in coimmunoprecipitation experiments using extracts of SV40-infected alpha3 cells that express an epitope-tagged TBP. Large T antigen was coimmunoprecipitated by antibodies to epitope-tagged TBP, endogenous TBP, hTAF(II)100, hTAF(II)130, and hTAF(II)250, under conditions where holo-TFIID would be precipitated. In addition, large T antigen copurified and coimmunoprecipitated with phosphocellulose-purified TFIID from SV40-infected alpha3 cells. Large T antigen also coprecipitated with anti-TBP antibody from extracts of ts13 cells expressing wild-type large T antigen under conditions where the ts defect in TAF(II)250 was rescued. In contrast, a transactivation mutant of large T antigen, which was unable to rescue the ts defect, failed to coprecipitate. We conclude from these data that transcriptional activation of many promoters by large T antigen results from its performing a TAF-like function in a complex with TFIID.