Characterization of two novel YY1 binding sites in the polyomavirus late promoter

Mouse polyomavirus infection induces lamin reorganisation

The nuclear lamina is a dense network of intermediate filaments beneath the inner nuclear membrane. Composed of A-type lamins (lamin A/C) and B-type lamins (lamins B1 and B2), the nuclear lamina provides a scaffold for the nuclear envelope and chromatin, thereby maintaining the structural integrity of the nucleus. A-type lamins are also found inside the nucleus where they interact with chromatin and participate in gene regulation. Viruses replicating in the cell nucleus have to overcome the nuclear envelope during the initial phase of infection and during the nuclear egress of viral progeny. Here, we focused on the role of lamins in the replication cycle of a dsDNA virus, mouse polyomavirus. We detected accumulation of the major capsid protein VP1 at the nuclear periphery, defects in nuclear lamina staining and different lamin A/C phosphorylation patterns in the late phase of mouse polyomavirus infection, but the nuclear envelope remained intact. An absence of lamin A/C did not affect the formation of replication complexes but did slow virus propagation. Based on our findings, we propose that the nuclear lamina is a scaffold for replication complex formation and that lamin A/C has a crucial role in the early phases of infection with mouse polyomavirus.

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.

Transcription factor YY1: structure, function, and therapeutic implications in cancer biology

The ubiquitous transcription factor Yin Yang 1 (YY1) is known to have a fundamental role in normal biologic processes such as embryogenesis, differentiation, replication, and cellular proliferation. YY1 exerts its effects on genes involved in these processes via its ability to initiate, activate, or repress transcription depending upon the context in which it binds. Mechanisms of action include direct activation or repression, indirect activation or repression via cofactor recruitment, or activation or repression by disruption of binding sites or conformational DNA changes. YY1 activity is regulated by transcription factors and cytoplasmic proteins that have been shown to abrogate or completely inhibit YY1-mediated activation or repression; however, these mechanisms have not yet been fully elucidated. Since expression and function of YY1 are known to be intimately associated with progression through phases of the cell cycle, the physiologic significance of YY1 activity has recently been applied to models of tumor biology. The majority of the data are consistent with the hypothesis that YY1 overexpression and/or activation is associated with unchecked cellular proliferation, resistance to apoptotic stimuli, tumorigenesis and metastatic potential. Studies involving hematopoetic tumors, epithelial-based tumors, endocrine organ malignancies, hepatocellular carcinoma, and retinoblastoma support this hypothesis. Molecular mechanisms that have been investigated include YY1-mediated downregulation of p53 activity, interference with poly-ADP-ribose polymerase, alteration in c-myc and nuclear factor-kappa B (NF-kappaB) expression, regulation of death genes and gene products, and differential YY1 binding in the presence of inflammatory mediators. Further, recent findings implicate YY1 in the regulation of tumor cell resistance to chemotherapeutics and immune-mediated apoptotic stimuli. Taken together, these findings provide strong support of the hypothesis that YY1, in addition to its regulatory roles in normal biologic processes, may possess the potential to act as an initiator of tumorigenesis and may thus serve as both a diagnostic and prognostic tumor marker; furthermore, it may provide an effective target for antitumor chemotherapy and/or immunotherapy.

Inhibition of polyomavirus ori-dependent DNA replication by mSin3B

When tethered in cis to DNA, the transcriptional corepressor mSin3B inhibits polyomavirus (Py) ori-dependent DNA replication in vivo. Histone deacetylases (HDACs) appear not to be involved, since tethering class I and class II HDACs in cis does not inhibit replication and treating the cells with trichostatin A does not specifically relieve inhibition by mSin3B. However, the mSin3B L59P mutation that impairs mSin3B interaction with N-CoR/SMRT abrogates inhibition of replication, suggesting the involvement of N-CoR/SMRT. Py large T antigen interacts with mSin3B, suggesting an HDAC-independent mechanism by which mSin3B inhibits DNA replication.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Production of polyomavirus structural protein VP1 in yeast cells and its interaction with cell structures

The gene for mouse polyomavirus major structural protein VP1 was expressed in Saccharomyces cerevisiae from the inducible GAL7 promoter. VP1 pseudocapsids were purified from cell lysates. Their subpopulation contained fragments of host DNA, which, in contrast to those of VP1 pseudocapsids produced in insect cells, did not assemble with cellular histones into pseudonucleocores. VP1 pseudocapsids accumulated in the yeast cell nuclei. A strong interaction of VP1 with tubulin fibres of the mitotic spindle was observed. The fibres of spindles were larger in diameter, apparently due to tight VP1 binding. Substantial growth inhibition of yeast cells producing VP1 was observed.

A mutation in the DE loop of the VP1 protein that prevents polyomavirus transcription and replication

Natural mutants of the DE loop of the Polyomavirus (Py) major coat protein VP1 have been previously shown to display an altered host specificity (L. Ricci, R. Maione, C. Passananti, A. Felsani, and P. Amati, 1992, J. Virol. 66, 7153-7158). To better understand the role of this outfacing loop of the VP1 protein in Py infectivity, we constructed and characterized a Py mutant (Py M17) harboring a deletion of 7 AA within the tip of the DE loop. The mutant virions obtained after DNA transfection were unable to replicate and initiate early transcription in fibroblast cells. Complementation experiments performed to rescue the deficient M17 replication by means of wt functions revealed the cis-dominance of the mutation. In situ cell fractionation experiments demonstrated that the Py mutant, like the Py wt, enters the cells, reaches the nucleus and that both the viral DNA and VP1 protein are found tightly bound to the nuclear matrix. These data suggest that the VP1 protein, associated to the viral DNA, conditions early viral gene expression and that the DE loop of the protein must be involved in this process.

The polyomavirus major capsid protein VP1 interacts with the nuclear matrix regulatory protein YY1

Polyomavirus reaches the nucleus in a still encapsidated form, and the viral genome is readily found in association with the nuclear matrix. This association is thought to be essential for viral replication. In order to identify the protein(s) involved in the virus-nuclear matrix interaction, we focused on the possible roles exerted by the multifunctional cellular nuclear matrix protein Yin Yang 1 (YY1) and by the viral major capsid protein VP1. In the present work we report on the in vivo association between YY1 and VP1. Using the yeast two-hybrid system we demonstrate that the VP1 and YY1 proteins physically interact through the D-E region of VP1 and the activation domain of YY1.

Transcription factor YY1 is a vaccinia virus late promoter activator

Vaccinia virus has a DNA genome, yet replicates in the cytoplasmic compartment of the cell. We previously described the identification of a cellular protein having high affinity for vaccinia virus late promoter DNA. Sequence substitutions in the vaccinia I1L promoter were used to define a 5-nucleotide block at the transcription initiation site as essential for interaction with the protein. Within this sequence is the recognition motif for the nuclear transcription factor YY1. This factor regulates a multitude of cellular promoters, as an activator of transcription, as a repressor, or as an initiator element-binding protein. Antibodies directed against YY1 were used to show that YY1 copurified with the vaccinia late promoter-binding protein and was present in late promoter-protein complexes in gel supershift assays. Bacterially expressed YY1 also bound specifically to late promoter DNA. A dinucleotide replacement within the YY1 recognition motif directly adjacent to the transcription start site severely reduced the affinity of YY1 for the I1L promoter in vitro and impaired I1L promoter-dependent transcription in vivo. The intracellular localization of YY1 was shown by immunofluorescence microscopy to shift from primarily nuclear to the cytoplasm after vaccinia infection. These results indicate that YY1 has a positive role in the regulation of vaccinia virus late gene transcription and suggest that poxviruses have adapted cellular initiator elements as a means of regulating viral gene expression. This is the first identifiable cellular protein implicated in poxvirus transcription.