A polyoma mutant that encodes small T antigen but not middle T antigen demonstrates uncoupling of cell surface and cytoskeletal changes associated with cell transformation

Live Cell Microscopy of Murine Polyomavirus Subnuclear Replication Centers

During polyomavirus (PyV) infection, host proteins localize to subnuclear domains, termed viral replication centers (VRCs), to mediate viral genome replication. Although the protein composition and spatial organization of VRCs have been described using high-resolution immunofluorescence microscopy, little is known about the temporal dynamics of VRC formation over the course of infection. We used live cell fluorescence microscopy to analyze VRC formation during murine PyV (MuPyV) infection of a mouse fibroblast cell line that constitutively expresses a GFP-tagged replication protein A complex subunit (GFP-RPA32). The RPA complex forms a heterotrimer (RPA70/32/14) that regulates cellular DNA replication and repair and is a known VRC component. We validated previous observations that GFP-RPA32 relocalized to sites of cellular DNA damage in uninfected cells and to VRCs in MuPyV-infected cells. We then used GFP-RPA32 as a marker of VRC formation and expansion during live cell microscopy of infected cells. VRC formation occurred at variable times post-infection, but the rate of VRC expansion was similar between cells. Additionally, we found that the early viral protein, small TAg (ST), was required for VRC expansion but not VRC formation, consistent with the role of ST in promoting efficient vDNA replication. These results demonstrate the dynamic nature of VRCs over the course of infection and establish an approach for analyzing viral replication in live cells.

Murine polyomavirus DNA transitions through spatially distinct nuclear replication subdomains during infection

The replication of small DNA viruses requires both host DNA replication and repair factors that are often recruited to subnuclear domains termed viral replication centers (VRCs). Aside from serving as a spatial focus for viral replication, little is known about these dynamic areas in the nucleus. We investigated the organization and function of VRCs during murine polyomavirus (MuPyV) infection using 3D structured illumination microscopy (3D-SIM). We localized MuPyV replication center components, such as the viral large T-antigen (LT) and the cellular replication protein A (RPA), to spatially distinct subdomains within VRCs. We found that viral DNA (vDNA) trafficked sequentially through these subdomains post-synthesis, suggesting their distinct functional roles in vDNA processing. Additionally, we observed disruption of VRC organization and vDNA trafficking during mutant MuPyV infections or inhibition of DNA synthesis. These results reveal a dynamic organization of VRC components that coordinates virus replication.

Reducing persistent polyomavirus infection increases functionality of virus-specific memory CD8 T cells

Mouse polyomavirus (MuPyV) causes a smoldering persistent infection in immunocompetent mice. To lower MuPyV infection in acutely and persistently infected mice, and study the impact of a temporal reduction in viral loads on the memory CD8 T cell response, we created a recombinant MuPyV in which a loxP sequence was inserted into the A2 strain genome upstream of the early promoter and another loxP sequence was inserted in cis into the intron shared by all three T antigens. Using mice transgenic for tamoxifen-inducible Cre recombinase, we demonstrated that reduction in MuPyV load during persistent infection was associated with differentiation of virus-specific CD8 T cells having a superior recall response. Evidence presented here supports the concept that reduction in viral load during persistent infection can promote differentiation of protective virus-specific memory CD8 T cells in patients at risk for diseases caused by human polyomaviruses.

RNA processing in the polyoma virus life cycle

Not only is gene regulation in polyoma interesting, but it has also proven to be highly informative and illustrative of a number of novel concepts in gene regulation. Of special interest and importance are the mechanisms by which this virus switches from the expression of early gene products to late gene products after the onset of viral DNA replication. This switch is mediated at least in part by changes in transcription elongation and polyadenylation in the late region, and by the formation and editing of dsRNA in the nucleus. In this review we will summarize the regulation of RNA synthesis and processing during polyoma infection, and will point out in particular those aspects that have been most novel.

Polyomavirus small T antigen controls viral chromatin modifications through effects on kinetics of virus growth and cell cycle progression

Minichromosomes of wild-type polyomavirus were previously shown to be highly acetylated on histones H3 and H4 compared either to bulk cell chromatin or to viral chromatin of nontransforming hr-t mutants, which are defective in both the small T and middle T antigens. A series of site-directed virus mutants have been used along with antibodies to sites of histone modifications to further investigate the state of viral chromatin and its dependence on the T antigens. Small T but not middle T was important in hyperacetylation at major sites in H3 and H4. Mutants blocked in middle T signaling pathways but encoding normal small T showed a hyperacetylated pattern similar to that of wild-type virus. The hyperacetylation defect of hr-t mutant NG59 was partially complemented by growth of the mutant in cells expressing wild-type small T. In contrast to the hypoacetylated state of NG59, NG59 minichromosomes were hypermethylated at specific lysines in H3 and also showed a higher level of phosphorylation at H3ser10, a modification associated with the late G(2) and M phases of the cell cycle. Comparisons of virus growth kinetics and cell cycle progression in wild-type- and NG59-infected cells showed a correlation between the phase of the cell cycle at which virus assembly occurred and histone modifications in the progeny virus. Replication and assembly of wild-type virus were completed largely during S phase. Growth of NG59 was delayed by about 12 h with assembly occurring predominantly in G(2). These results suggest that small T affects modifications of viral chromatin by altering the temporal coordination of virus growth and the cell cycle.

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.

Signaling from polyomavirus middle T and small T defines different roles for protein phosphatase 2A

Polyomavirus causes a broad spectrum of tumors as the result of the action of its early proteins. This work compares signaling from middle T antigen (MT), the major transforming protein, to that from small T antigen (ST). The abilities of MT mutants to promote cell cycle progression in serum-starved NIH 3T3 cells were compared. Transformation-defective mutants lacking association with SHC or with phosphatidylinositol 3-kinase (PI3-K) retained the ability to induce DNA synthesis as measured by bromodeoxyuridine incorporation. Only when both interactions were lost in the Y250F/Y315F double mutant was MT inactive. ST promoted cell cycle progression in a manner dependent on its binding of protein phosphatase 2A (PP2A). Since the Y250F/Y315F MT mutant was wild type for PP2A binding yet unable to promote cell cycle progression, while ST was capable of promoting cell cycle progression, these experiments revealed a functional difference in MT and ST signaling via PP2A. Assays testing the abilities of MT and ST to induce the c-fos promoter and to activate c-jun kinase led to the same conclusion. ST, but not Y250F/Y315F MT, was able to activate the c-fos promoter through its interaction with PP2A. In contrast, MT, but not ST, was able to activate c-jun kinase by virtue of its interaction with PP2A.

Serine 257 phosphorylation regulates association of polyomavirus middle T antigen with 14-3-3 proteins

Polyomavirus middle T antigen (MT) is phosphorylated on serine residues. Partial proteolytic mapping and Edman degradation identified serine 257 as a major site of phosphorylation. This was confirmed by site-directed mutagenesis. Isoelectric focusing of immunoprecipitated MT from transfected 293T cells showed that phosphorylation on wild-type MT occurred at near molar stoichiometry at S257. MT was previously shown to be associated with 14-3-3 proteins, which have been connected to cell cycle regulation and signaling. The association of 14-3-3 proteins with MT depended on the serine 257 phosphorylation site. This has been demonstrated by comparing wild-type and S257A mutant MTs expressed with transfected 293T cells or with Sf9 cells infected with recombinant baculoviruses. The 257 site is not critical for transformation of fibroblasts in vitro, since S257A and S257C mutant MTs retained the ability to form foci or colonies in agar. The tumor profile of a virus expressing S257C MT showed a striking deficiency in the induction of salivary gland tumors. The basis for this defect is uncertain. However, differences in activity for the wild type and mutant MT lacking the 14-3-3 binding site have been observed in transient reporter assays.

Transformation and tumorigenic properties of a mutant polyomavirus containing a middle T antigen defective in Shc binding

Polyomavirus middle T antigen is phosphorylated on several tyrosine residues which act as binding sites for cellular proteins, including phosphatidylinositol 3-kinase, Shc, and phospholipase C-gamma. In this report we describe the transforming properties and tumor-inducing ability of a polyomavirus that contains a single-site mutation in middle T antigen which changes a tyrosine residue at amino acid position 250 to serine. This mutation disrupts the association of middle T with the transforming protein Shc. The mutant virus is weakly transforming, inducing foci which are smaller and of different morphology than those of the wild type. Although the virus induced tumors in close to 100% of inoculated mice, the spectrum of tumors and their morphology were altered compared to those of wild-type virus. The mutant virus induced a reduced frequency of kidney and thymic tumors. Both the mammary gland and the thymic tumors that were induced were histologically distinct from those induced by wild-type polyomavirus. These results demonstrate that the signal transduction pathway that is deregulated by the middle T-Shc association is important for full transformation of cells in culture and for tumor induction in some target tissues in the mouse-polyomavirus system.

Studies of partially transforming polyomavirus mutants establish a role for phosphatidylinositol 3-kinase in activation of pp70 S6 kinase

Infection of mouse fibroblasts by wild-type polyomavirus results in increased phosphorylation of ribosomal protein S6 (D.A. Talmage, J. Blenis, and T.L. Benjamin, Mol. Cell. Biol. 8:2309-2315, 1988). Here we identify pp70 S6 kinase (pp70S6K) as a target for signal transduction events leading from polyomavirus middle tumor antigen (mT). Two partially transforming virus mutants altered in different mT signalling pathways have been studied to elucidate the pathway leading to S6 phosphorylation. An upstream role for mT-phosphatidylinositol 3-kinase (PI3K) complexes in pp70S6K activation is implicated by the failure of 315YF, a mutant unable to promote PI3K binding, to elicit a response. This conclusion is supported by studies using wortmannin, a known inhibitor of PI3K. In contrast, stable interaction of mT with Shc, a protein thought to be involved upstream of Ras, is dispensable for pp70S6K activation. 250YS, a mutant mT which retains a binding site for PI3K but lacks one for Shc, stimulates pp70S6K to wild-type levels. Mutants 315YF and 250YS induce partial transformation of rats fibroblasts with distinct phenotypes, as judged from morphological and growth criteria. Neither mutant induces growth in soft agar, indicating that an increase in S6 phosphorylation, while necessary for cell cycle progression in normal mitogenesis, is not sufficient for anchorage-independent cell growth. In the polyomavirus systems, the latter requires integration of signals from mT involving both Shc and PI3K.

Phosphatidylinositol 3-kinase binding to polyoma virus middle tumor antigen mediates elevation of glucose transport by increasing translocation of the GLUT1 transporter

Elevation in the rate of glucose transport in polyoma virus-infected mouse fibroblasts was dependent upon phosphatidylinositol 3-kinase (PI 3-kinase; EC 2.7.1.137) binding to complexes of middle tumor antigen (middle T) and pp60c-src. Wild-type polyoma virus infection led to a 3-fold increase in the rate of 2-deoxyglucose (2DG) uptake, whereas a weakly transforming polyoma virus mutant that encodes a middle T capable of activating pp60c-src but unable to promote binding of PI 3-kinase induced little or no change in the rate of 2DG transport. Another transformation-defective mutant encoding a middle T that retains functional binding of both pp60c-src and PI 3-kinase but is incapable of binding Shc (a protein involved in activation of Ras) induced 2DG transport to wild-type levels. Wortmannin (< or = 100 nM), a known inhibitor of PI 3-kinase, blocked elevation of glucose transport in wild-type virus-infected cells. In contrast to serum stimulation, which led to increased levels of glucose transporter 1 (GLUT1) RNA and protein, wild-type virus infection induced no significant change in levels of either GLUT1 RNA or protein. Nevertheless, virus-infected cells did show increases in GLUT1 protein in plasma membranes. These results point to a posttranslational mechanism in the elevation of glucose transport by polyoma virus middle T involving activation of PI 3-kinase and translocation of GLUT1.

Enhancer-mediated role for polyomavirus middle T/small T in DNA replication

A major role for polyomavirus middle T/small T antigens in viral DNA synthesis was uncovered by examining the replication of middle T/small T-deficient mutants (hr-t mutants). hr-t mutants in the A2 genetic background showed a 16- to 100-fold defect in genome accumulation relative to the wild type when infections were carried out in exponentially growing NIH 3T3 cells in medium supplemented with low levels of serum (< 2.0%). A proportional decrease in the level of viral early transcripts was also seen. The replication defect of the hr-t mutants was partially overcome in the presence of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. The defect was also alleviated by a duplication encompassing the alpha core enhancer domain that contains binding sites for the transcriptional activators PEA1/AP-1 and PEA3/c-ets. Such a duplication is present in all naturally occurring hr-t mutants and absent in the A2 strain. The effects of 12-O-tetradecanoylphorbol-13-acetate and alpha core duplication were additive but did not fully complement the absence of middle T/small T. In mixed infection competition experiments with two hr-t mutants, a genome that carried an alpha core duplication had a replication advantage (up to 17-fold) over a genome without duplication. This result demonstrates that one effect of the duplication is exerted directly at the level of DNA replication. The advantage of the duplication-bearing genome was established during the earliest stages of replication and was not further amplified in later rounds of replication. In the presence of middle T/small T, both genomes replicated to high levels and the advantage of the duplication-bearing genome was eliminated. On the basis of these results, we propose that factors that bind the alpha core domain (presumably PEA1 and PEA3) are present in limiting amounts in exponentially growing NIH 3T3 cells and play a crucial role in polyomavirus DNA replication. We further suggest that middle T and/or small T stimulates viral DNA replication by activating these factors. The fact that all middle T-/small T-defective hr-t mutants have evolved to contain enhancer duplications that encompass the PEA1 and PEA3 binding sites in the alpha core domain and partially restore their replication defect (A. Amalfitano, M. C. Chen, and M. Fluck, unpublished data) provides an adequate explanation for the fact that the importance of the role of the middle T and/or small T function in DNA replication has not been recognized previously. Much evidence is available in support of separate elements of this model.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of the threonine phosphorylation sites on the polyomavirus major capsid protein VP1: relationship to the activity of middle T antigen

Phosphorylation of the polyomavirus major capsid protein VP1 was examined after in vivo 32P labeling of virus-infected cells. Two phosphorylated peptide fragments of VP1 were identified by protease digestion, high-performance liquid chromatography purification, mass spectrometry, and N-terminal sequencing. The peptides from residues 58 to 78 and residues 153 to 173 were phosphorylated on threonine. Site-directed mutations were introduced at these threonine sites, and mutant viruses were reconstructed. A threonine-to-glycine change at residue 63 (mutant G63) and a threonine-to-alanine change at residue 156 (mutant A156) resulted in viruses defective in phosphorylation of the respective peptides after in vivo labeling. Growth of the mutant G63 virus was similar to that of the wild-type virus, but the mutant A156 was inefficient in assembly of 240S viral particles. Polyomavirus nontransforming host range (hr-t) mutants are defective in VP1 threonine phosphorylation when grown in nonpermissive cells (R. L. Garcea, K. Ballmer-Hofer, and T. L. Benjamin, J. Virol. 54:311-316, 1985). Proteolytic mapping of VP1 peptides after in vivo labeling from hr-t mutant virus infections demonstrated that both residues T-63 and T-156 were affected. These results suggest that the block in virion assembly in hr-t mutant viruses is associated with a defect in phosphorylation of threonine 156.

Functional asymmetry of the regions juxtaposed to the membrane-binding sequence of polyomavirus middle T antigen

The functional importance of the two clusters of positively charged amino acids which flank the hydrophobic membrane-anchoring sequence of polyomavirus middle T (mT) protein has been investigated by using site-directed mutagenesis. A clear asymmetry was apparent. No effect on transformation was seen following multiple alterations or complete removal of the cluster at the carboxyl end of the protein. In contrast, a single substitution replacing the first arginine amino terminal to the hydrophobic stretch with glutamic acid, but not with lysine, histidine, or methionine, produced a partially transformation-defective mutant with a novel phenotype. This mutant failed to confer anchorage-independent growth on F111 established rat embryo fibroblasts but induced foci with altered morphology compared with wild-type mT. Biochemical studies on this mutant revealed that F111 clones expressing levels of mutant mT equivalent to those of wild-type controls showed a 65% reduction in pp60c-src activation and an 87% reduction in mT-associated phosphatidylinositol 3-kinase activity. However, F111 clones expressing seven times more mutant mT than did wild-type controls showed equal or greater levels of kinase activities yet remained incompletely transformed. Possible mechanisms involving this transformation-sensitive region of mT are discussed.

Functional asymmetry of the regions juxtaposed to the membrane-binding sequence of polyomavirus middle T antigen

The functional importance of the two clusters of positively charged amino acids which flank the hydrophobic membrane-anchoring sequence of polyomavirus middle T (mT) protein has been investigated by using site-directed mutagenesis. A clear asymmetry was apparent. No effect on transformation was seen following multiple alterations or complete removal of the cluster at the carboxyl end of the protein. In contrast, a single substitution replacing the first arginine amino terminal to the hydrophobic stretch with glutamic acid, but not with lysine, histidine, or methionine, produced a partially transformation-defective mutant with a novel phenotype. This mutant failed to confer anchorage-independent growth on F111 established rat embryo fibroblasts but induced foci with altered morphology compared with wild-type mT. Biochemical studies on this mutant revealed that F111 clones expressing levels of mutant mT equivalent to those of wild-type controls showed a 65% reduction in pp60c-src activation and an 87% reduction in mT-associated phosphatidylinositol 3-kinase activity. However, F111 clones expressing seven times more mutant mT than did wild-type controls showed equal or greater levels of kinase activities yet remained incompletely transformed. Possible mechanisms involving this transformation-sensitive region of mT are discussed.

Interactions of polyomavirus middle T with the SH2 domains of the pp85 subunit of phosphatidylinositol-3-kinase

The binding of phosphatidylinositol-3-kinase to the polyomavirus middle T antigen is facilitated by tyrosine phosphorylation of middle T on residue 315. The pp85 subunit of phosphatidylinositol-3-kinase contains two SH2 domains, one in the middle of the molecule and one at the C terminus. When assayed by blotting with phosphorylated middle T, the more N-terminal SH2 domain is responsible for binding to middle T. When assayed in solution with glutathione S transferase fusions, both SH2s are capable of binding phosphorylated middle T. While both SH2 fusions can compete with intact pp85 for binding to middle T, the C-terminal SH2 is the more efficient of the two. Interaction between pp85 or its SH2 domains and middle T can be blocked by a synthetic peptide comprising the tyrosine phosphorylation sequence around middle T residue 315. Despite the fact that middle T can interact with both SH2s, these domains are not equivalent. Only the C-terminal SH2-middle T interaction was blocked by anti-SH2 antibody; the two SH2 fusions also interact with different cellular proteins.

Transformation by hamster polyomavirus: identification and functional analysis of the early genes

A strategy involving polymerase chain reaction amplification of cDNAs was designed to study the expression of the hamster polyomavirus (HaPV) early region in HaPV-transformed rat fibroblasts, productively HaPV-infected cells, and HaPV-induced lymphoma. We identified three mRNAs resulting from alternative splicing of open reading frames leading to coding capacities for three polypeptides with molecular weights similar to those of the murine polyomavirus large T, middle T (MT), and small T (ST) antigens. The corresponding intronless cDNAs direct the in vitro synthesis of polypeptides with the expected electrophoretic mobilities. The biological activities carried by the HaPV early genes were assayed by transfection of appropriate cell systems. The fragment of genomic viral DNA that encodes the three early antigens contains all of the genetic information necessary for immortalization of primary rat embryo fibroblasts and transformation of F111 rat cells. The large T antigen is sufficient for immortalization, although the MT and ST antigens stimulate the growth and modify the phenotype of immortal cell lines. A stringent cooperative effect was observed in the transformation of F111 cells, which requires the simultaneous presence of the MT and ST antigens, as opposed to the transformation by murine polyomavirus, which can be carried out by the MT antigen alone.

A completely transformation-defective point mutant of polyomavirus middle T antigen which retains full associated phosphatidylinositol kinase activity

By using a random mutagenesis procedure combined with a recombinant retrovirus vector, mutants of polyomavirus middle T antigen (MTAg) were generated. Three new MTAg mutants with various degrees of transformation competence were more thoroughly characterized. All of the mutants produced a stable MTAg, as assessed by metabolic labeling or immunoblotting, and each mutant possessed wild-type levels of associated tyrosine kinase activity and associated phosphatidylinositol-3 (PI-3) kinase activity. One of these mutants, with a substitution of leucine for proline at amino acid 248 of MTAg (248m) was completely transformation defective, as measured in a focus-forming assay. Furthermore, the pattern of phosphorylation of 248m in vivo was identical to that of wild-type MTAg, and the kinetics of association of MTAg with an 85-kilodalton protein, the putative PI kinase, was not altered. Similarly, the pattern of PI derivatives obtained in an in vitro kinase assay was not altered by the substitution at amino acid 248. Since the single base pair mutation at amino acid 248 resulted in an MTAg that was completely transformation defective despite possessing wild-type levels of kinase activities, this suggests that neither tyrosine kinase nor PI-3 kinase activity nor the combination of both are sufficient for transformation by MTAg.

Characterization of pp85, a target of oncogenes and growth factor receptors

An 85,000-molecular-weight polypeptide (85K polypeptide) has previously been identified as a common substrate for tyrosine phosphorylation upon polyomavirus middle T transformation or upon platelet-derived growth factor stimulation of 3T3 cells. In each case, pp85 has an associated phosphatidylinositol kinase activity. The tissue distribution of pp85 was determined by middle T blotting experiments; the highest levels were found in brain, lung, and spleen tissues. High-resolution examination of 85K by isoelectric focusing demonstrated that there are at least 10 different forms. These were resolved into two families, 85K and 86K; the ratio of the two families changed in different cells. Similar forms were found for pp85 associated with pp60v-src. Individual species within each family differed by phosphorylation. Analysis of pp85 and pp86 by immunoprecipitation with anti-phosphotyrosine antibody showed increasing phosphorylation in response to middle T or pp60v-src transformation. The association of middle T with pp85 and pp60c-src was examined in pulse-chase experiments. Association of middle T with pp60c-src was slow and was accompanied by progressive modification of middle T. pp85 formed a dissociable complex with middle T within 2.5 min.

Characterization of pp85, a target of oncogenes and growth factor receptors

An 85,000-molecular-weight polypeptide (85K polypeptide) has previously been identified as a common substrate for tyrosine phosphorylation upon polyomavirus middle T transformation or upon platelet-derived growth factor stimulation of 3T3 cells. In each case, pp85 has an associated phosphatidylinositol kinase activity. The tissue distribution of pp85 was determined by middle T blotting experiments; the highest levels were found in brain, lung, and spleen tissues. High-resolution examination of 85K by isoelectric focusing demonstrated that there are at least 10 different forms. These were resolved into two families, 85K and 86K; the ratio of the two families changed in different cells. Similar forms were found for pp85 associated with pp60v-src. Individual species within each family differed by phosphorylation. Analysis of pp85 and pp86 by immunoprecipitation with anti-phosphotyrosine antibody showed increasing phosphorylation in response to middle T or pp60v-src transformation. The association of middle T with pp85 and pp60c-src was examined in pulse-chase experiments. Association of middle T with pp60c-src was slow and was accompanied by progressive modification of middle T. pp85 formed a dissociable complex with middle T within 2.5 min.

Polyoma virus small tumor antigen pre-mRNA splicing requires cooperation between two 3' splice sites

We have studied splicing of the polyoma virus early region pre-mRNA in vitro. This RNA is alternatively spliced in vivo to produce mRNA encoding the large, middle-sized (MTAg), and small (StAg) tumor antigens. Our primary interest was to learn how the 48-nucleotide StAg intron is excised, because the length of this intron is significantly less than the apparent minimum established for mammalian introns. Although the products of all three splices are detected in vitro, characterization of the pathway and sequence requirements of StAg splicing suggests that splicing factors interact with the precursor RNA in an unexpected way to catalyze removal of this intron. Specifically, StAg splicing uses either of two lariat branch points, one of which is located only 4 nucleotides from the 3' splice site. Furthermore, the StAg splice absolutely requires that the alternative MTAg 3' splice site, located 14 nucleotides downstream of the StAg 3' splice site, be intact. Insertion mutations that increase or decrease the quality of the MTAg pyrimidine stretch enhance or repress StAg as well as MTAg splicing, and a single-base change in the MTAg AG splice acceptor totally blocks both splices. These results demonstrate the ability of two 3' splice sites to cooperate with each other to bring about removal of a single intron.

Mutational analysis of polyomavirus small-T-antigen functions in productive infection and in transformation

The function of polyomavirus small T antigen in productive infection and in transformation was studied. Transfection of permissive mouse cells with mixtures of mutants that express only one type of T antigen showed that small T antigen increased large-T-antigen-dependent viral DNA synthesis approximately 10-fold. Under the same conditions, small T antigen was also essential for the formation of infectious virus particles. To analyze these activities of small T antigen, mutants producing protein with single amino acid replacements were constructed. Two mutants, bc1073 and bc1075, were characterized. Although both mutations led to the substitution of amino acid residues of more than one T antigen, the phenotype of both mutants was associated with alterations of the small T antigen. Both mutant proteins had lost their activity in the maturation of infectious virus particles. The bc1075 but not the bc1073 small T antigen had also lost its ability to stimulate viral DNA synthesis in mouse 3T6 cells. Finally, both mutants retained a third activity of small T antigen: to confer on rat cells also expressing middle T antigen the ability to grow efficiently in semisolid medium. The phenotypes of the mutants in these three assays suggest that small T antigen has at least three separate functions.

Polyomavirus middle T antigen induces ribosomal protein S6 phosphorylation through pp60c-src-dependent and -independent pathways

Phosphorylation of ribosomal protein S6 is elevated in polyomavirus-infected cells. This elevation results only in part from activation of S6 kinase activity. These effects appear to reflect independent activities of wild-type middle T antigen. Hr-t mutant NG59, encoding a defective middle T protein, and mutant Py808A, encoding no middle T protein, were unable to induce S6 kinase activity or elevate S6 phosphorylation. Two other site-directed mutants encoding altered middle T proteins did elevate S6 phosphorylation while only weakly stimulating S6 kinase activity. These results suggest at least two independent pathways leading to elevation of S6 phosphorylation. One pathway leads to induction of S6 kinase activity following activation of pp60c-src by transformation-competent middle T antigen. Another pathway operates independently of S6 kinase induction and can be regulated by transformation-defective middle T mutants such as Py1387T. This mutant, encoding a truncated middle T protein that failed to associate with the plasma membrane and to activate pp60c-src, caused increased levels of S6 phosphorylation without detectably increasing S6 kinase activity. The ability of mutants such as Py1387T to induce S6 phosphorylation correlated with their ability to increase phosphorylation of VP1, an event linked to maturation of infectious virions.

Polyomavirus middle T antigen induces ribosomal protein S6 phosphorylation through pp60c-src-dependent and -independent pathways

Phosphorylation of ribosomal protein S6 is elevated in polyomavirus-infected cells. This elevation results only in part from activation of S6 kinase activity. These effects appear to reflect independent activities of wild-type middle T antigen. Hr-t mutant NG59, encoding a defective middle T protein, and mutant Py808A, encoding no middle T protein, were unable to induce S6 kinase activity or elevate S6 phosphorylation. Two other site-directed mutants encoding altered middle T proteins did elevate S6 phosphorylation while only weakly stimulating S6 kinase activity. These results suggest at least two independent pathways leading to elevation of S6 phosphorylation. One pathway leads to induction of S6 kinase activity following activation of pp60c-src by transformation-competent middle T antigen. Another pathway operates independently of S6 kinase induction and can be regulated by transformation-defective middle T mutants such as Py1387T. This mutant, encoding a truncated middle T protein that failed to associate with the plasma membrane and to activate pp60c-src, caused increased levels of S6 phosphorylation without detectably increasing S6 kinase activity. The ability of mutants such as Py1387T to induce S6 phosphorylation correlated with their ability to increase phosphorylation of VP1, an event linked to maturation of infectious virions.

Failure of simian virus 40 small t antigen to disorganize actin cables in nonpermissive cell lines

Mouse C3H 10T1/2 cell lines expressing the simian virus 40 (SV40) small t antigen were obtained by cotransfection of pSV2neo and plasmids which encode small t. Cell lines derived from two plasmids which encode small t in the absence of stable deletion fragments of the large T antigen were morphologically normal and grew to slightly higher saturation densities in low serum than control cell lines. Unexpectedly, the clones had highly organized actin cables, as did parental 10T1/2 cells infected with wild-type SV40. These observations and comparisons of rat F111 cells infected with either polyomavirus or SV40 suggest that the SV40 small t antigen does not directly affect cytoskeletal organization.

Phosphorylation of polyomavirus large T antigen: effects of viral mutations and cell growth state

Phosphorylation is responsible for the shift in electrophoretic mobility of polyomavirus large T antigen observed in pulse-chase or continuous-labeling experiments. Phosphorylated forms migrated more slowly than newly synthesized [35S]methionine large T antigen, and alkaline phosphatase treatment reversed the mobility shift. Analysis of phosphopeptides with Staphylococcus aureus V8 protease showed that large T antigen forms of intermediate mobility were enriched in peptides 1 to 4, 8, and 9, while the slower migrating species had all nine phosphopeptides, including peptides 5 and 7. The phosphorylations represented by phosphopeptides 5 and 7 were of particular interest. These phosphopeptides were entirely lacking in large T antigen from tsa mutants such as ts616 labeled at the nonpermissive temperature. Also, the phosphorylation of peptides 5 and 7 depends on the growth state of the cell. Early in infection of quiescent cells intermediate mobility forms of large T antigen with little or no phosphorylation, particularly of peptides 5 and 7, were seen, whereas peptides 5 and 7 were well represented at the same time in patterns from growing cells. Later in infection of growth-arrested cells, these phosphorylations were observed, suggesting that infection stimulates the relevant kinase. Because large T antigen of hrt mutants, which lack middle and small T antigens, showed phosphorylation of peptides 5 and 7, large T antigen was apparently responsible for the stimulation. Because some differences in the distribution of phosphopeptides were noted between hrt mutants and the wild type, middle T antigen, small T antigen, or both may play a modulating role in large T antigen phosphorylation.

Regulation of c-myc and c-fos mRNA levels by polyomavirus: distinct roles for the capsid protein VP1 and the viral early proteins

The levels of c-myc, c-fos, and JE mRNAs accumulate in a biphasic pattern following infection of quiescent BALB/c 3T3 mouse cells with polyomavirus. Maximal levels of c-myc and c-fos mRNAs were seen within 1 hr and were nearly undetectable at 6 hr after infection. At 12 hr after infection mRNA levels were again maximal and remained elevated thereafter. Empty virions (capsids) and recombinant VP1 protein, purified from Escherichia coli, induced the early but not the late phase of mRNA accumulation. Virions, capsids, and recombinant VP1 protein stimulated [3H]thymidine nuclear labeling and c-myc mRNA accumulation in a dose-responsive manner paralleling their affinity for the cell receptor for polyoma. The second phase of mRNA accumulation is regulated by the viral early gene products, as shown by polyomavirus early gene mutants and by a transfected cell line (336a) expressing middle tumor antigen upon glucocorticoid addition. These results suggest that polyomavirus interacts with the cell membrane at the onset of infection to increase the levels of mRNA for cellular genes associated with cell competence for DNA replication, and subsequently these levels are maintained by the action of the early viral proteins.

Isolation and characterization of NIH 3T3 cells expressing polyomavirus small T antigen

The polyomavirus small T-antigen gene, together with the polyomavirus promoter, was inserted into a retrovirus vector pGV16 which contains the Moloney sarcoma virus long terminal repeat and neomycin resistance gene driven by the simian virus 40 promoter. This expression vector, pGVST, was packaged into retrovirus particles by transfection of psi 2 cells which harbor packaging-defective murine retrovirus genome. NIH 3T3 cells were infected by this replication-defective retrovirus containing pGVST. Of the 15 G418-resistant cell clones, 8 express small T antigen at various levels as revealed by immunoprecipitation. A cellular protein with an apparent molecular weight of about 32,000 coprecipitates with small T antigen. Immunofluorescent staining shows that small T antigen is mainly present in the nuclei. Morphologically, cells expressing small T antigen are indistinguishable from parental NIH 3T3 cells and have a microfilament pattern similar to that in parental NIH 3T3 cells. Cells expressing small T antigen form a flat monolayer but continue to grow beyond the saturation density observed for parental NIH 3T3 cells and eventually come off the culture plate as a result of overconfluency. There is some correlation between the level of expression of small T antigen and the growth rate of the cells. Small T-antigen-expressing cells form small colonies in soft agar. However, the proportion of cells which form these small colonies is rather small. A clone of these cells tested did not form tumors in nude mice within 3 months after inoculation of 10(6) cells per animal. Thus, present studies establish that the small T antigen of polyomavirus is a second nucleus-localized transforming gene product of the virus (the first one being large T antigen) and by itself has a function which is to stimulate the growth of NIH 3T3 cells beyond their saturation density in monolayer culture.

Regulation of cellular phenotype and expression of polyomavirus middle T antigen in rat fibroblasts

Polyoma middle T antigen (mT) was expressed in rat F-111 cells under control of the dexamethasone-regulatable mouse mammary tumor virus promoter. Graded phenotypic responses to levels of mT induction by the hormone were seen, with morphological transformation, focus formation, and anchorage-independent growth requiring increasing levels of mT expression. The ability of different clones to form tumors reflected their maximum level of induction of mT-associated kinase and their ability to grow in soft agar. Expression of transformation parameters and tumorigenicity correlates with the level of mT phosphorylated by pp60c-src in immune complexes and not with the total amount of mT determined by metabolic labeling. We suggest that cellular factors regulate mT activity by forming a kinase-active fraction of mT molecules that controls the transformed state.

Recombinant retroviruses that transduce individual polyoma tumor antigens: effects on growth and differentiation

We have constructed infectious retroviral vectors, derived from Moloney murine leukemia virus, that efficiently transduce the polyoma virus tumor (T) antigens individually. The parental vector we have chosen [pZIP-NeoSV(X)1] expresses a dominant selectable marker for neomycin resistance and is a shuttle vector capable of propagation in both eukaryotic and prokaryotic cells, thus facilitating its use in structure-function studies. To address the relationship between polyoma T-antigen tumorigenesis and the effects of individual T antigens on growth control and differentiation, we used these vectors to introduce and stably express large, middle-sized, or small T antigens into mouse fibroblasts and preadipocytes. All cDNAs introduced into the vector are expressed stably even in the absence of selective pressure. The stable expression of small T antigen is noted particularly because cell lines expressing small T antigen have not been readily available prior to the use of retroviral vectors. Small T antigen-induced increase in saturation density of NIH 3T3 cells can be scored on the basis of the morphology of drug-resistant colonies. Middle-sized T antigen eliminates the growth requirement of NIH 3T3 cells for epidermal growth factor in a defined medium and permits growth in platelet-poor plasma, indicating elimination of the platelet-derived growth factor requirement as well. Large T antigen suppresses mouse preadipocyte (3T3-F442A) differentiation. These vectors and these functional assays of T-antigen activity permit genetic analysis of the relationship between tumorigenesis by T antigens and the alteration of cellular growth and differentiation.

Truncated forms of the polyomavirus middle T antigen can substitute for the small T antigen in lytic infection

Cloned polyomavirus genomes encoding the small T antigen or truncated forms of the middle T antigen facilitated the growth of genomes encoding only the large T antigen in mouse 3T6 cells. We conclude that an N-terminal domain of the middle T antigen, in the appropriate cellular location, can substitute for the small T antigen during lytic infection.

Regulation of cellular phenotype and expression of polyomavirus middle T antigen in rat fibroblasts

Polyoma middle T antigen (mT) was expressed in rat F-111 cells under control of the dexamethasone-regulatable mouse mammary tumor virus promoter. Graded phenotypic responses to levels of mT induction by the hormone were seen, with morphological transformation, focus formation, and anchorage-independent growth requiring increasing levels of mT expression. The ability of different clones to form tumors reflected their maximum level of induction of mT-associated kinase and their ability to grow in soft agar. Expression of transformation parameters and tumorigenicity correlates with the level of mT phosphorylated by pp60c-src in immune complexes and not with the total amount of mT determined by metabolic labeling. We suggest that cellular factors regulate mT activity by forming a kinase-active fraction of mT molecules that controls the transformed state.