Large T antigen-rich viral DNA replication loci in SV40-infected monkey kidney cells

Human papillomavirus DNA replication compartments in a transient DNA replication system

Many DNA viruses replicate their genomes at nuclear foci in infected cells. Using indirect immunofluorescence in combination with fluorescence in situ hybridization, we colocalized the human papillomavirus (HPV) replicating proteins E1 and E2 and the replicating origin-containing plasmid to nuclear foci in transiently transfected cells. The host replication protein A (RP-A) was also colocalized to these foci. These nuclear structures were identified as active sites of viral DNA synthesis by bromodeoxyuridine (BrdU) pulse-labeling. Unexpectedly, the great majority of RP-A and BrdU incorporation was found in these HPV replication domains. Furthermore, E1, E2, and RP-A were also colocalized to nuclear foci in the absence of an origin-containing plasmid. These observations suggest a spatial reorganization of the host DNA replication machinery upon HPV DNA replication or E1 and E2 expression. Alternatively, viral DNA replication might be targeted to host nuclear domains that are active during the late S phase, when such domains are limited in number. In a fraction of cells expressing E1 and E2, the promyelocytic leukemia protein, a component of nuclear domain 10 (ND10), was either partially or completely colocalized with E1 and E2. Since ND10 structures were recently hypothesized to be sites of bovine papillomavirus virion assembly, our observation suggests that HPV DNA amplification might be partially coupled to virion assembly.

Association of the human papillomavirus type 11 E1 protein with histone H1

The E1 and E2 proteins are the only virus-encoded factors required for human papillomavirus (HPV) DNA replication. The E1 protein is a DNA helicase responsible for initiation of DNA replication at the viral origin. Its recruitment to the origin is facilitated by binding to E2, for which specific recognition elements are located at the origin. The remaining replication functions for the virus, provided by the host cell's replication machinery, may be mediated by further interactions with E1 and E2. Histone H1 was identified as an HPV type 11 (HPV-11) E1-binding protein by far-Western blotting and by microsequence analyses of a 34-kDa protein purified by E1 affinity chromatography. E1 also bound in vitro to H1 isolated under native conditions in association with intact nucleosomes. In addition, E1 and H1 were coimmunoprecipitated by an E1 antiserum from a nuclear extract prepared from cells expressing recombinant E1. Bound H1 was displaced from HPV-11 DNA by the addition of E1, suggesting that E1 can promote replication initiation and elongation by alteration of viral chromatin structure and disruption of nucleosomes at the replication fork. Furthermore, a region of the HPV-11 genome containing the origin of replication was identified which had weaker affinity for H1 than that of the remaining genome. This result suggests that the presence of a DNA structure at or near the HPV origin facilitates initiation of DNA replication by exclusion of H1. These results are similar to those of studies of simian virus 40 DNA replication, in which a large T antigen-H1 interaction and an H1-resistant region at the origin of DNA replication have also been demonstrated.

The nuclear matrix and virus function

Replication of the small DNA tumor virus, simian virus 40 (SV40), is largely dependent on host cell functions, because SV40, in addition to virion proteins, codes only for a few regulatory proteins, the most important one being the SV40 large tumor antigen (T-antigen). This renders SV40 an excellent tool for studying complex cellular and viral processes. In this review we summarize and discuss data providing evidence for virtually all major viral processes during the life cycle of SV40 from viral DNA replication to virion formation, being performed at or within structural systems of the nucleus, in particular the chromatin and the nuclear matrix. These data further support the concept that viral replication in the nucleus is structurally organized and demonstrate that viruses are excellent tools for analyzing the underlying cellular processes. The analysis of viral replication at nuclear structures might also provide a means for specifically interfering with viral processes without interfering with the corresponding cellular functions.

Simian virus 40 T-antigen DNA helicase is a hexamer which forms a binary complex during bidirectional unwinding from the viral origin of DNA replication

The role of simian virus 40 (SV40) large tumor antigen (T antigen) as a DNA helicase at the replication fork was studied. We found that a T-antigen hexamer complex acts during the unidirectional unwinding of appropriate DNA substrates and is localized directly in the center of the fork, contacting the adjacent double strand as well as the emerging single strands. When bidirectional DNA unwinding, initiated at the viral origin of DNA replication, was analyzed, a larger T-antigen complex that is simultaneously active at both branch points of an unwinding bubble was observed. The size and shape of this helicase complex imply that the T-antigen dodecamer complex, assembled at the origin and active in the localized melting of duplex DNA, is subsequently also used to continue DNA unwinding bidirectionally. Then, however, the dodecamer complex does not split into two hexamer subunits that track along the DNA; rather, the DNA is threaded through the intact complex, with the concomitant extrusion of single-stranded loops.

Structural topography of simian virus 40 DNA replication

Applying an in situ cell fractionation procedure, we analyzed structural systems of the cell nucleus for the presence of mature and replicating simian virus 40 (SV40) DNA. Replicating SV40 DNA intermediates were tightly and quantitatively associated with the nuclear matrix, indicating that elongation processes of SV40 DNA replication proceed at this structure. Isolated nuclei as well as nuclear matrices were able to continue SV40 DNA elongation under replication conditions in situ, arguing for a coordinated and functional association of SV40 DNA and large T molecules at nuclear structures. SV40 DNA replication also was terminated at the nuclear matrix. While the bulk of newly synthesized, mature SV40 DNA molecules then remained at this structure, some left the nuclear matrix and accumulated at the chromatin.

Nuclear subcompartmentalization of simian virus 40 large T antigen: evidence for in vivo regulation of biochemical activities

Simian virus 40 large T antigen (large T) in the early and the late phases of infection differs significantly in its sequence-specific DNA-binding and ATPase activities, indicating that different large-T populations participate in virus-specific events at various stages of the infectious cycle. To further characterize these large-T populations, we have analyzed nuclear subclasses of large T, isolated from their in vivo location, for their biochemical activities. We show that chromatin- and nuclear matrix-associated large-T molecules exhibit different simian virus 40 control region (ORI) DNA-binding and ATPase activities. The association of large T with a certain nuclear substructure, therefore, subcompartmentalizes large-T molecules exerting different biochemical activities. Nuclear subcompartmentalization thus may provide a higher-order level for the regulation of biochemical activities of large T in vivo.

Intranuclear distribution of SV40 large T-antigen and transformation-related protein p53 in abortively infected cells

The intranuclear localization of SV40 T-antigen (T-Ag) and the cellular protein p53 was studied in SV40 abortively infected baby mouse kidney cells using two complementary methods of ultrastructural immunocytochemistry in combination with preferential staining of nuclear RNP components and electron microscope autoradiography. Both proteins were revealed in association with peri- and interchromatin RNP fibrils containing the newly synthesized hnRNA. In addition, T-Ag and p53 remained bound, at least in part, to the residual internal nuclear matrix following nuclease and salt extractions of infected cells. The localization of T-Ag was different in SV40 lytically infected monkey kidney cells since, in addition to hnRNP fibrils, the viral protein was also associated with cellular chromatin. However, when lytic infection was performed in conditions of blocked viral DNA replication, T-Ag was no longer associated with the cellular chromatin but remained bound to the hnRNP fibrils. We conclude that the transforming and lytic functions of T-Ag can be distinguished by different subnuclear distributions. The significance of the association of T-Ag and p53 with hnRNP fibrils and the internal nuclear matrix is discussed in relation to the role of these structures in the control of cellular mRNA biogenesis.

Specific interaction of simian virus 40 large T antigen with cellular chromatin and nuclear matrix during the course of infection

We analyzed the subnuclear distribution of the simian virus 40 (SV40) large tumor (large T) antigen during the course of viral infection. Three distinct nuclear subclasses were detected in SV40 lytically infected TC7 cells (large T antigen in the nucleoplasm, at the cellular chromatin, and at the nuclear matrix). During the course of infection the relative subnuclear distribution of large T antigen changed significantly at about the switch from the early to late phase of infection: at early times postinfection, large T antigen was present mainly in the nucleoplasm and at the cellular chromatin, and nuclear-matrix-associated large T antigen was barely detectable. Concomitant with the onset of viral DNA replication, the amount of nuclear-matrix-associated large T antigen increased drastically. During the further course of infection large T antigen accumulated at the cellular chromatin and nuclear matrix, paralleling the increase in viral DNA synthesis. The biological significance of this correlation was corroborated by analysis of cells infected with the SV40 mutant tsA58 at permissive (32 degrees C) and restrictive (39 degrees C) temperatures. tsA58 large T antigen failed to initiate viral DNA replication in infected cells kept at the restrictive temperature and also failed to associate with the cellular chromatin and nuclear matrix. By blocking viral DNA synthesis with aphidicolin, an inhibitor of DNA polymerase alpha, we were able to show that the accumulation of large T antigen at these structures does not result from the binding of large T antigen to viral chromatin but reflects an association with cellular components of the chromatin and nuclear matrix of infected cells.

SV40 large T-antigen and transformation related protein p53 are associated in situ with nuclear RNP structures containing hnRNA of transformed cells

The localization of SV40 large T-antigen (T-Ag) and the cellular protein p53 in the nuclei of mouse and human SV40-transformed cells and of a methylcholanthrene-transformed mouse cell line, was studied. Their detection by ultrastructural immunocytochemistry with specific monoclonal antibodies employed two complementary methods used in parallel. These consisted of indirect immunoperoxidase labelling carried out before embedment on Triton-permeabilized cells, or indirect immunogold labelling applied to thin sections of cells embedded in Lowicryl K4M. The results indicate that in SV40-transformed cells both proteins are chiefly localized on peri- and interchromatin RNP fibrils. This shows that they occur in structures involved in the synthesis and processing of hnRNA. The nucleoli and chromatin did not appear to be labelled. In methylcholanthrene-transformed cells the protein p53 (in the absence of large T-Ag) was also detected on peri- and interchomatin fibrils. Taken together with recent results which demonstrated that, during lytic infection, T-Ag was associated chiefly with cellular chromatin (Harper, F, Florentin, Y & Puvion, E, Exp cell res 161 (1985) 434) [33], our experiments provide evidence that the transforming function of SV40 large T-Ag is dissociable from its function in SV40 lytic infection in terms of its subnuclear distribution.