Mapping of the amino-terminal half of polyomavirus middle-T antigen indicates that this region is the binding domain for pp60c-src

Polyomavirus middle T-antigen is a transmembrane protein that binds signaling proteins in discrete subcellular membrane sites

Murine polyomavirus middle T-antigen (MT) induces tumors by mimicking an activated growth factor receptor. An essential component of this action is a 22-amino-acid hydrophobic region close to the C terminus which locates MT to cell membranes. Here, we demonstrate that this sequence is a transmembrane domain (TMD) by showing that a hemagglutinin (HA) tag added to the MT C terminus is exposed on the outside of the cells, with the N terminus inside. To determine whether this MT TMD is inserted into the endoplasmic reticulum (ER) membrane, we added the ER retention signal KDEL to the MT C terminus (MTKDEL). This mutant protein locates only in the ER, demonstrating that MT does insert into membranes solely at this location. In addition, this ER-located MT failed to transform. Examination of the binding proteins associated with the MTKDEL protein demonstrated that it associates with PP2A and c-Src but fails to interact with ShcA, phosphatidylinositol 3-kinase (PI3K), and phospholipase C-γ1 (PLC-γ1), despite being tyrosine phosphorylated. Additional mutant and antibody studies show that MT binding to PP2A is probably required for MT to efficiently exit the ER and migrate to the plasma membrane though the TMD also plays a role in this relocation. Overall, these data, together with previous publications, illustrate that MT associates with signaling proteins at different sites in its maturation pathway. MT binds to PP2A in the cytoplasm, to c-Src at the endoplasmic reticulum, and to ShcA, PI3K, and PLC-γ1 at subsequent locations en route to the plasma membrane.

The N terminus of hamster polyomavirus middle T antigen carries a determinant for specific activation of p59c-Fyn

Transformation by rodent polyomaviruses is mediated primarily by middle T antigen, a membrane-bound protein that does not carry an intrinsic enzymatic activity but interacts and subverts the activity of cellular regulators of proliferation. The multiple protein partners of murine polyomavirus (Py) middle T antigen include the tyrosine kinases c-Src and, to a lesser extent, c-Fyn and c-Yes. By contrast, the hamster polyomavirus (HaPV) middle T antigen selectively activates the c-Fyn gene product. This difference may account for the contrasting tumor patterns induced by the two viruses. The sequences of the respective N-terminal and C-terminal functional domains of murine Py and HaPV middle T antigens are highly conserved whereas the intervening stretches are clearly divergent, leading to the speculation that this divergence may direct the specificity for tyrosine kinase activation. We have addressed this issue by constructing a chimera middle T antigen molecule carrying the N-terminal domain from HaPV (exon 1) in phase with the other two domains from murine Py (exon 2). The biological properties of this chimera molecule are indistinguishable from those of HaPV middle T antigen; it specifically activates p59c-Fyn and carries the transforming phenotype of the HaPV middle T antigen on rat fibroblasts.

Functional interaction between the SH2 domain of Fyn and tyrosine 324 of hamster polyomavirus middle-T antigen

Middle-T antigen of mouse polyomavirus (MomT) associates with the cellular tyrosine kinases c-Src, c-Yes, and Fyn, while middle-T antigen of hamster polyomavirus (HamT) exclusively binds Fyn. This interaction is essential for polyomavirus-mediated transformation of cells in culture and tumor formation in animals. Here we show that the kinase domain of Fyn is sufficient for association with MomT but not for binding of HamT. We further demonstrate that a Fyn mutant lacking the SH2 domain is able to bind MomT but fails to associate with HamT, indicating that the SH2 domain of Fyn is essential for stable association with HamT. HamT, but not MomT, contains a tyrosine residue, Tyr-324, in the sequence context YEEI. Mutation of Tyr-324 to phenylalanine led to a drastic reduction of associated Fyn and abolished the oncogenicity of HamT. This suggests that Tyr-324 is the major phosphotyrosine residue mediating the binding of HamT to the SH2 domain of Fyn. These findings show that mouse and hamster polyomaviruses use different strategies to target Src-related tyrosine kinases.

Identification of regions in polyomavirus middle T and small t antigens important for association with protein phosphatase 2A

Two subunits of protein phosphatase 2A (PP2A) have been shown previously to bind to the small t and middle T antigens (ST and MT, respectively) of polyomavirus. To determine sequences important for binding of PP2A to ST and MT, we first constructed a series of ST mutants in regions known to be important for biological activity of ST and MT. Several mutations in two small regions just amino terminal to the Cys-X-Cys-X-X-Cys motifs of ST and MT abolished PP2A binding to ST in vitro. Parallel mutations were constructed in MT to investigate the role of PP2A binding in the function of polyomavirus MT. Wild-type and mutant MT proteins were stably expressed in NIH 3T3 cells and analyzed (i) for their ability to induce transformation and (ii) for associated cellular proteins and corresponding enzymatic activities previously described as associating with wild-type MT. A number of the mutant MTs were found to be defective in binding of PP2A as assayed by coimmunoprecipitation. In contrast, a deletion of the highly conserved stretch of amino acids 42 to 47 (His-Pro-Asp-Lys-Gly-Gly) in the ST-MT-large T antigen common region did not affect PP2A binding to MT. MT mutants defective for PP2A binding were also defective in transformation, providing further evidence that association with PP2A is important for the ability of MT to transform cells. All mutants which were impaired for PP2A binding were similarly or more dramatically impaired for associated protein and lipid kinase activities, supporting the possibility that PP2A binding is necessary for the formation and/or stability of an MT-pp60c-src complex.

Zinc-binding and protein-protein interactions mediated by the polyomavirus large T antigen zinc finger

Polyomavirus large tumor antigen (LT) contains a potential C2H2 zinc binding element between residues 452 and 472. LT also contains a third histidine in this region, conserved among the polyomavirus LTs. Synthetic peptides of this region bound a single atom of zinc, as determined by spectroscopic analysis. Blotting experiments also showed that fusion proteins containing the element, as well as full-length LT, bound 65Zn. Polyomavirus middle T and small T antigens also bound zinc in the blotting assay. Site-directed mutagenesis showed the importance of this element in LT. Point mutations in four of the conserved residues (C-452, C-455, H-465, and H-469) blocked the ability of LT to function in viral DNA replication, while mutation of H-472-->L decreased replication to 1/30th that of the wild type. Point mutations in intervening residues tested had little effect on replication. Mutants resulting from mutations in the conserved cysteine or histidine residues retained the ability to bind origin DNA. However, they did show a defect in self-association. Because double-hexamer formation is involved in DNA replication, this deficiency is sufficient to explain the defect in replication. Mutants created by point mutations of the coordinating residues were also deficient in replication-associated phosphorylations.

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)

Characterization of the interaction of polyomavirus middle T antigen with type 2A protein phosphatase

Two cellular proteins of 36 and 63 kDa which bind the small T and middle T antigens of polyomavirus recently have been identified as the catalytic and regulatory subunits of the phosphoserine/threonine-specific type 2A protein phosphatase (PP2A). We report here the presence of phosphoseryl phosphatase activity associated with polyomavirus small T and middle T antigens in immunoprecipitates prepared from virus-infected and transformed cells. Phosphatase activity was also found associated with middle T-antigen mutants, some of which had been defined previously to associate with 36- and 63-kDa cellular proteins. Middle T-antigen-associated phosphatase activity was sensitive to okadaic acid and microcystin-LR, inhibitors of PP2A, and insensitive to inhibitor 1 or 2, orthovanadate, or EDTA. Using antiserum specific for the catalytic subunit of PP2A, we found that unlike the majority of PP2A, middle T-antigen-bound PP2A was membrane associated. However, no gross change in the amount, activity, or localization of PP2A could be attributed to middle T-antigen expression in transformed cells. Anti-PP2A antibodies coprecipitated a 63-kDa protein from normal cells and in addition coprecipitated middle T antigen, 60- and 61-kDa proteins (identified as src family members), and an 81-kDa protein from middle T-antigen-transformed cells. Furthermore, we detected protein kinase activity in PP2A immunoprecipitates and protein phosphatase activity in src immune complexes from extracts of middle T-antigen-transformed, but not normal, cells. These results reinforce the notion that at least a portion of middle T antigen bridges a protein kinase with a protein phosphatase.

Identification and characterization of the hamster polyomavirus middle T antigen

Hamster polyomavirus (HaPV) is associated with lymphoid and hair follicle tumors in Syrian hamsters. The early region of HaPV has the potential to encode three polypeptides (which are related to the mouse polyomavirus early proteins) and can transform fibroblasts in vitro. We identified the HaPV middle T antigen (HamT) as a 45-kDa protein. Like its murine counterpart, HamT was associated with serine/threonine phosphatase, phosphatidylinositol-3 kinase, and protein tyrosine kinase activities. However, whereas mouse middle T antigen associates predominantly with pp60c-src and pp62c-yes, HamT was associated with a different tyrosine kinase, p59fyn. The ability of HaPV to cause lymphoid tumors may therefore reside in its ability to associate with p59fyn, a potentially important tyrosine kinase in lymphocytes.

The divergence between two oncogenic Herpesvirus saimiri strains in a genomic region related to the transforming phenotype

Herpesvirus saimiri strains can be divided into at least three subgroups (A, B, C) based on sequence divergence at the left end of viral unique sequence DNA. Strains of subgroups A and C are highly oncogenic and readily transform simian T-lymphocytes in vitro to interleukin-2 independent growth, while subgroup B strains do not. A left terminal reading frame of a H. saimiri subgroup A strain was shown previously to correlate with the oncogenic phenotype and in vitro transforming potential; the deduced polypeptide was termed STP-A. Furthermore, this same region contains an open reading frame (ORF) for dihydrofolate reductase (DHFR) and genes for five virus-specific U RNAs (HSURs). We now show by sequence analysis of the corresponding region in a subgroup C strain that DHFR and HSUR genes are present in both virus subgroups; however, no sequence homologous to the STP-A reading frame was found in this subgroup C virus. At a position and orientation similar to STP-A, two ORFs were found for peptides sharing a putative transmembrane domain. One of them encodes a peptide with collagen-like repetitions. In addition to the lack of similarity to STP-A, these two reading frames also did not show any similarity to known oncogenes. The organization of sequences at the left junction of unique L- and repetitive H-DNA of H. saimiri suggests frequent recombinational events, possibly accelerating the uptake of foreign genes by the virus.

The amino terminus of polyomavirus middle T antigen is required for transformation

In polyomavirus-transformed cells, pp60c-src is activated by association with polyomavirus middle T antigen. These complexes have a higher tyrosine kinase activity compared with that of unassociated pp60c-src. Genetic analyses have revealed that the carboxy-terminal 15 amino acids of pp60c-src and the amino-terminal half of middle T antigen are required for this association and consequent activation of the tyrosine kinase. To define in greater detail the borders of the domain in middle T antigen required for activation of pp60c-src, we constructed a set of unidirectional amino-terminal deletion mutants of middle T antigen. Analysis of these mutants revealed that the first six amino acids of middle T antigen are required for it to activate the kinase activity of pp60c-src and to transform Rat-1 fibroblasts. Analysis of a series of insertion and substitution mutants confirmed these observations and further revealed that mutations affecting the first four amino acids of middle T antigen reduced or abolished its capacity to activate the kinase activity of pp60c-src and to transform Rat-1 cells in culture. Our results suggest that the first four amino acids of middle T antigen constitute part of a domain required for activation of the pp60c-src tyrosyl kinase activity and for consequent cellular transformation.

Association of p60fyn with middle tumor antigen in murine polyomavirus-transformed rat cells

Rabbit antisera raised against human FYN-specific peptides were used to evaluate the expression of the fyn gene product in normal and murine polyomavirus middle tumor antigen (MTAg)-transformed rat cells. The antisera were capable of detecting p60fyn in both normal and MTAg-transformed cells. Two different antisera directed against unique p60fyn sequences were found to detect p60fyn-MTAg complexes in cell lysates from the MTAg-transformed cells. The MTAg molecules immunoprecipitated by FYN antisera were phosphorylated on tyrosine during immune-complex kinase reactions at sites similar to those found on MTAg in complexes with pp60c-src. Whereas the abundance of p60fyn was estimated to be less in the MTAg-transformed cells than in their normal counterparts, the specific activities of p60fyn molecules in the normal and transformed cells were similar.

Mechanisms of transformation by polyoma virus middle T antigen

This review addresses a fundamental question of polyoma virus biology: What is the molecular mechanism by which the polyoma virus middle T antigen (MTAg) transforms cells in culture? Since MTAg has no known intrinsic biochemical activity, it is believed to act by modulating the properties of the host cell's proteins (see review by Courtneidge [26]). Experiments to date have largely focused on the interaction of MTAg with the cellular tyrosine kinase, pp60c-src. However, recent data from a number of laboratories have demonstrated the importance of other MTAg-associating cellular proteins in MTAg-mediated transformation, including pp62c-yes and a phosphatidylinositol kinase. In this review, we will summarize what is presently known about the proteins interacting with MTAg. The extent to which the currently known details of the biochemistry of MTAg and its associated proteins can explain the transforming properties of the various mutant alleles of MTAg will be assessed.

p56lck protein-tyrosine kinase is cytoskeletal and does not bind to polyomavirus middle T antigen

p56lck and p60c-src are closely related protein-tyrosine kinases that are activated by similar oncogenic mutations. We have used fibroblast cell lines that express p56lck from introduced DNA molecules to compare the subcellular localizations of p60c-src and p56lck and their abilities to bind polyomavirus middle T antigen (mT). p56lck is associated with the detergent-insoluble matrix, as defined by extraction with solutions containing nonionic detergents, whereas p60c-src is soluble under these conditions. p56lck is also associated with detergent-insoluble structures in a lymphoid cell line, LSTRA. p60c-src binds to mT, but p56lck does not bind detectably. In terms of both solubility and mT interactions, the nononcogenic p56lck more closely resembles oncogenically activated p60c-src mutants than it resembles p60c-src. Because published results show that an intact carboxy terminus is required for p60c-src to bind mT and be soluble, we tested whether the different localization and mT binding properties of p56lck and p60c-src were dictated by their different carboxy termini. A protein consisting largely of p60c-src sequences but carrying a p56lck carboxy terminus was soluble and bound to mT. We suggest that both the solubility and mT-binding properties of p60c-src not only require sequences common to the carboxy termini of p60c-src and p56lck, but also require sequences unique to the body of p60c-src.

Mutants of polyomavirus middle-T antigen

Polyomavirus middle-T antigen induces the transformation of established cell lines in culture and is known to interact with and/or modulate the activity of several enzymes (pp60c.src, protein kinase C and phosphatidylinositol kinase) in vitro. This review is a compilation of the reported mutants of middle-T antigen and their biochemical and biological properties as they relate to the transformation event. The mutants of polyomavirus middle-T antigen have been previously classified phenotypically. Given the now large number of mutants, the classification presented here is based upon the position within the molecule. A model of middle-T is presented in which the protein is considered as consisting of three domains: a hydrophobic domain (the putative membrane-binding domain), the amino-terminal half of the molecule (the putative pp60c.src-binding domain) and the intervening amino acids (the putative modulatory domain). A current model for the induction of transformation by polyomavirus middle-T is presented.

Analysis of middle tumor antigen and pp60c-src interactions in polyomavirus-transformed rat cells

The relative abundance of pp60c-src molecules associated with polyomavirus (Py) middle tumor antigen (MTAg) and the relative abundance of MTAg associated with pp60c-src in a variety of Py-transformed rat cells was determined by quantitative immunoblot analyses which detect pp60c-src or Py MTAg. The results demonstrate that approximately 5 to 10% of the total immunoprecipitable pp60c-src molecules in Py-transformed rat cells are stably associated with MTAg and have elevated protein kinase activities. In these same cells, it was found that approximately 10 to 15% of the detectable MTAg molecules are stably associated with pp60c-src. Other results presented in this report demonstrate that approximately 50 to 75% of the total MTAg-associated cellular tyrosine kinase activity potentially represents the enzymatic activity of pp60c-src, while the remaining 25 to 50% represents the activity of other cellular tyrosine kinases. Our results also show that most pp60c-src molecules associated with Py MTAg do not possess electrophoretic mobilities that are altered from those of pp60c-src molecules not associated with MTAg or pp60c-src molecules obtained from normal rodent cells.