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

Lessons in signaling and tumorigenesis from polyomavirus middle T antigen

The small DNA tumor viruses have provided a very long-lived source of insights into many aspects of the life cycle of eukaryotic cells. In recent years, the emphasis has been on cancer-related signaling. Here we review murine polyomavirus middle T antigen, its mechanisms, and its downstream pathways of transformation. We concentrate on the MMTV-PyMT transgenic mouse, one of the most studied models of breast cancer, which permits the examination of in situ tumor progression from hyperplasia to metastasis.

Lessons from polyoma middle T antigen on signaling and transformation: A DNA tumor virus contribution to the war on cancer

Middle T antigen (MT) is the principal oncogene of murine polyomavirus. Its study has led to the discovery of the roles of tyrosine kinase and phosphoinositide 3-kinase (PI3K) signaling in mammalian growth control and transformation. MT is necessary for viral transformation in tissue culture cells and tumorigenesis in animals. When expressed alone as a transgene, MT causes tumors in a wide variety of tissues. It has no known catalytic activity, but rather acts by assembling cellular signal transduction molecules. Protein phosphatase 2A, protein tyrosine kinases of the src family, PI3K, phospholipase Cgamma1 as well as the Shc/Grb2 adaptors are all assembled on MT. Their activation sets off a series of signaling cascades. Analyses of virus mutants as well as transgenic animals have demonstrated that the effects of a given signal depend not only tissue type, but on the genetic background of the host animal. There remain many opportunities as we seek a full molecular understanding of MT and apply some of its lessons to human cancer.

Polyomavirus middle T antigen induces the transcription of osteopontin, a gene important for the migration of transformed cells

Middle T antigen (MT) is the principal oncoprotein of murine polyomavirus. Experiments on the acute immediate effects of MT expression on cellular RNA levels showed that expression of osteopontin (OPN) was strongly induced by MT expression. Osteopontin is a protein known to be associated with cancer. It has a role in tumor progression and invasion. Protein analysis confirmed that MT induced the secretion of OPN into the extracellular medium. Expression of antisense OPN RNA had no effect on the growth of MT-transformed cells. However, it had a strong effect on the ability of MT transformants to migrate or to fill a wound. Analysis of MT mutants implicated both the SHC and phosphatidylinositol 3-kinase pathways in OPN induction. Reporter assays showed that MT regulated the OPN promoter through two of its PEA3 (polyoma enhancer activator 3) sites. As critical PEA3 sites are also part of the polyomavirus enhancer, the same signaling important for viral replication also contributes to virally induced metastatic potential.

The p110alpha isoform of phosphatidylinositol 3-kinase is essential for polyomavirus middle T antigen-mediated transformation

Middle T antigen (MT) of polyomavirus is known to play an important role in virus-mediated cellular transformation. While MT has been extensively examined in spontaneously immortalized rodent fibroblasts, its interactions with cells of other types and species are less well understood. We have undertaken a cross-species and cross-cell-type comparison of MT-induced transformation in cells with genetically defined backgrounds. We tested the transforming abilities of a panel of MT mutants, Y250F, Y315F, and Y322F, that are selectively mutated in the binding sites for the principal effectors of MT--Src homology 2 domain-containing transforming protein, phosphatidylinositol 3-kinase (PI3K), and phospholipase C-gamma, respectively--in fibroblasts and epithelial cells of murine or human origin. We found that the Y315F mutation disabled the ability of MT to induce transformation in all cell types and species tested. While Y315F also failed to activate the PI3K pathway in these cells, genetic evidence has indicated Y315 may make other contributions to transformation. To confirm the role of PI3K, the PIK3CA gene, encoding p110alpha, the prime effector of PI3K signaling downstream from activated growth factor receptors, was genetically ablated. This abolished the transforming activity of MT, demonstrating the essential role for this PI3K isoform in MT-mediated transformation. The Y250F mutant was able to transform the human, but not the murine, cells that were examined. Interestingly, this mutant fully activates the PI3K pathway in human cells but activated PI3K signaling poorly in the murine cells used in the study. This again points to the importance of PI3K activation for transformation and suggests that the mechanism by which MT activates the PI3K pathway differs in different species.

Independent contributions of polyomavirus middle T and small T to the regulation of early and late gene expression and DNA replication

We previously showed that murine polyomavirus mutants that lack both middle T (MT) and small T (ST) functions have a severe pleiotropic defect in early and late viral gene expression as well as genome amplification. The respective contribution of MT and ST to this phenotype was unclear. This work separates the roles of MT and ST in both permissive mouse cells and nonpermissive rat cells. It demonstrates for the first time a role for both proteins. To gain insight into the signaling pathways that might be required, we focused on MT and its mutants. The results show that each of the major MT signaling connections, Shc, phosphatidylinositol 3'-kinase, and phospholipase C gamma1, could contribute in an additive way. Unexpectedly, a mutant lacking all these connections because the three major tyrosines had been converted to phenylalanine retained some activity. A mutant in which all six MT C-terminal tyrosines had been mutated was inactive. This suggests a novel signaling pathway for MT that uses the minor tyrosines. What is common to ST and the individual MT signaling pathways is the ability to signal to the polyomavirus enhancer, in particular to the crucial AP-1 and PEA3/ets binding sites. This connection explains the pleiotropy of MT and ST effects on transcription and DNA replication.

Suppression of AP-1 constitutive activity interferes with polyomavirus MT antigen transformation ability

Polyomavirus (Py) encodes a potent oncogene, the middle T antigen (MT), that induces cell transformation by binding to and activating several cytoplasmic proteins which take part in transduction of growth factors-induced mitogenic signal to the nucleus. We have previously reported that the AP-1 transcriptional complex is a target for MT during cell transformation although, its activation was not sufficient for establishment of the transformed phenotype. Here we show that expression of a dominant-negative cJun mutant in MT transformed cell lines inhibits its transformation ability, indicating that constitutive AP-1 activity is necessary for cell transformation mediated by MT. Evidences also suggest that proliferation of MT transformed cells in low serum concentrations and their ability to form colonies in agarose are controlled by distinct mechanisms.

A new class of mutations reveals a novel function for the original phosphatidylinositol 3-kinase binding site

Previous studies have demonstrated that the specificity of Src homology 2 (SH2) and phosphotyrosine-binding domain interactions are mediated by phosphorylated tyrosines and their neighboring amino acids. Two of the first phosphotyrosine-based binding sites were found on middle T antigen of polyoma virus. Tyr-250 acts as a binding site for ShcA, whereas Tyr-315 forms a binding site for the SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase. However, genetic analysis of a given phosphotyrosine's role in signaling can be complicated when it serves as a binding site for multiple proteins. The situation is particularly difficult when the phosphotyrosine serves as a secondary binding site for a protein with primary binding determinates elsewhere. Mutation of a tyrosine residue to phenylalanine blocks association of all bound proteins. Here we show that the mutation of the amino acids following the phosphorylated tyrosine to alanine can reveal phosphotyrosine function as a secondary binding site, while abrogating the phosphotyrosine motif's role as a primary binding site for SH2 domains. We tested this methodology by using middle T antigen. Our results suggest that Tyr-250 is a secondary binding site for phosphatidylinositol 3-kinase, whereas Tyr-315 is a secondary binding site for a yet-to-be-identified protein, which is critical for transformation.

Polyoma virus middle T antigen and its role in identifying cancer-related molecules

Most cancer researchers take for granted some of the basic concepts about the molecular changes that underlie tumorigenesis. These include the principles that tyrosine kinases and the phosphorylation of phosphatidylinositol by phosphatidylinositol 3-kinases are important in the signalling pathways that control proliferation and apoptosis, and hence cancer formation. However, how many know that a small DNA mouse virus was crucial in establishing both of these tenets?

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.

Mechanisms of cell transformation induced by polyomavirus

Polyomavirus is a DNA tumor virus that induces a variety of tumors in mice. Its genome encodes three proteins, namely large T (LT), middle T (MT), and small T (ST) antigens, that have been implicated in cell transformation and tumorigenesis. LT is associated with cell immortalization, whereas MT plays an essential role in cell transformation by binding to and activating several cytoplasmic proteins that participate in growth factor-induced mitogenic signal transduction to the nucleus. The use of different MT mutants has led to the identification of MT-binding proteins as well as analysis of their importance during cell transformation. Studying the molecular mechanisms of cell transformation by MT has contributed to a better understanding of cell cycle regulation and growth control.

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.

Phosphoinositide kinases

Phosphatidylinositol, a component of eukaryotic cell membranes, is unique among phospholipids in that its head group can be phosphorylated at multiple free hydroxyls. Several phosphorylated derivatives of phosphatidylinositol, collectively termed phosphoinositides, have been identified in eukaryotic cells from yeast to mammals. Phosphoinositides are involved in the regulation of diverse cellular processes, including proliferation, survival, cytoskeletal organization, vesicle trafficking, glucose transport, and platelet function. The enzymes that phosphorylate phosphatidylinositol and its derivatives are termed phosphoinositide kinases. Recent advances have challenged previous hypotheses about the substrate selectivity of different phosphoinositide kinase families. Here we re-examine the pathways of phosphoinositide synthesis and the enzymes involved.

Deletion of proline-rich domain in polyomavirus T antigens results in virus partially defective in transformation and tumorigenesis

Polyomavirus productively infects mouse cells, transforms rat fibroblasts in culture, and induces a broad spectrum of tumors when inoculated into newborn mice. The expression of large, middle, and small T antigen are necessary for virus growth and oncogenic transformation. We have generated a small deletion in a region common to both large and middle T antigen that encodes three consecutive prolines. In this report we characterize this mutant virus in terms of its ability to replicate in mouse cells, transform rat fibroblasts, and induce tumors in the mouse. We find that the virus immortalizes primary cells and that viral DNA replication is not impaired, indicating that these functions of large T antigen are not altered. However, the ability of the virus to transform rat fibroblasts is defective. The mutant virus makes fewer foci and the foci are weaker in appearance. The mutant middle T still associates with PI 3-kinase and shc, suggesting that the overall structure of the protein has not been disrupted. When inoculated into newborn C3H mice, the mutant virus induces fewer overall tumors with a longer latency than wild-type virus. These results indicate that this proline-rich domain in middle T antigen is important for oncogenesis in a wide variety of tissues and cell types.

Polyoma middle T antigen in HL-60 cells accelerates hematopoietic myeloid and monocytic cell differentiation

Expression of the polyoma virus middle T antigen in HL-60 cells accelerates their differentiation in response to both monocytic and granulocytic differentiation-inducing agents. Middle T-expressing cells treated with the granulocytic inducer retinoic acid or the monocytic inducer 1,25-dihydroxy vitamin D3 differentiated 24 h earlier than parental, mock-electroporated, or vector control cell lines. The rapid onset of differentiation correlated with an increase in the cellular level of the middle T protein as well as two known retinoic-acid-inducible markers in HL-60 cells: the paxillin and transglutaminase gene products. The accelerated functional differentiation response and expression of retinoic-acid-inducible markers indicate that middle T played a causal role in differentiation. Thus, expression of the polyoma middle T antigen in HL-60 cells enhanced a variety of molecular changes associated with cellular differentiation.

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.

The role of the Shc phosphotyrosine interaction/phosphotyrosine binding domain and tyrosine phosphorylation sites in polyoma middle T antigen-mediated cell transformation

The phosphotyrosine interaction (PI)/phosphotyrosine binding (PTB) domain of Shc binds specific tyrosine-phosphorylated motifs found on activated growth factor receptors and proteins such as polyoma virus middle T antigen (MT). Phenylalanine 198 (Phe198) has been identified as a crucial residue involved in the interaction of the Shc PI/PTB with phosphopeptides. In NIH 3T3 cells expressing MT, p52 Shc carrying the F198V mutation is weakly phosphorylated and does not bind MT or Grb2. Overexpression of the PI/PTB domain alone as Shc amino acids 1-238 acted in a dominant interfering fashion blocking MT-induced transformation. However, expression of a slightly longer construct, Shc 1-260, which encompasses Tyr239/Tyr240, a novel Shc tyrosine phosphorylation site, did not block transformation. This was found to be due to the ability of Shc 1-260 to become tyrosine-phosphorylated and bind Grb2. Furthermore, full-length Shc in which Tyr239/Tyr240 had been mutated to phenylalanine did not become tyrosine-phosphorylated or bind Grb2 but did inhibit colony formation in soft agar. Conversely, p52 Shc carrying a mutation in the other tyrosine phosphorylation site, Tyr317, became heavily tyrosine-phosphorylated, bound Grb2, and gave rise to colonies in soft agar.

Tumor induction by a transformation-defective polyoma virus mutant blocked in signaling through Shc

Transformation of cells in culture by polyoma virus requires integration of signals downstream of middle T-Shc and middle T-phosphatidylinositol 3-kinase interactions, but the same is not true for induction of tumors in the mouse. Thus, a middle T mutant defective in transformation and blocked in binding Shc is able to induce a broad spectrum of tumors after inoculation into newborn mice. The "tumor profile" induced by the mutant shows enhancement of tumors at some sites and reductions at others but otherwise resembles that induced by the wild-type virus. A nontransforming double-mutant blocked in binding phosphatidylinositol 3-kinase as well as Shc is severely affected but still induces some tumors. These results show that pathways that must cooperate to induce full transformation of cells in vitro can act independently and are to a large extent redundant in tumor induction.

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.

Polyomavirus middle-sized tumor antigen modulates c-Jun phosphorylation and transcriptional activity

Polyomavirus middle-sized tumor antigen (MT) increases the expression of c-jun through a phorbol 12-O-tetradecanoate 13-acetate response element in the c-jun promoter. To investigate the cellular signaling pathways affected by MT, we studied the role of the c-Ras and Raf-1 proteins in MT-induced transactivation of c-jun and cell transformation. There was an increase in GTP complexed to Ras in MT-expressing cells, indicating an increase in Ras activity. Coexpression of dominant inhibitory mutants of Ha-ras and raf-1 with MT inhibited MT-mediated transactivation and focus formation. Studies of the phosphorylation of c-Jun showed that MT expression increased the phosphorylation of Ser-63 and Ser-73 in the transactivation domain and decreased the phosphorylation of a peptide containing Ser-243, Ser-249, and Thr-231 in the DNA binding domain. MT increased the transcriptional activating ability of c-Jun but failed to increase the transcriptional activating ability of c-Jun mutants with Ser-63 and Ser-73 changed to nonphosphorylatable Ala, indicating that MT modulates c-Jun activity through phosphorylation. The dominant inhibitory mutants of Ha-ras and raf-1 interfered with the ability of MT to activate c-Jun. The results indicate that MT induces a phosphorylation cascade through the activation of c-Ras and Raf-1 and that c-Jun is one of the downstream targets that may cause changes in gene expression leading to cell transformation.

Polyoma middle tumor antigen interacts with SHC protein via the NPTY (Asn-Pro-Thr-Tyr) motif in middle tumor antigen

Polyomavirus middle tumor antigen (MT) transforms a large number of cell types by binding to and modulating the activities of cellular proteins. Previous genetic analysis defined in MT an independent motif, NPTY (Asn-Pro-Thr-Tyr), required for transformation. This report demonstrates that NPTY is required for interaction between MT and SHC protein, a Src homology 2 (SH2)-containing protooncogene product implicated in activating Ras via association with GRB2 protein. SHC is phosphorylated on tyrosine and associates with GRB2 in MT-transformed cells. These effects require an intact NPTY motif in MT. SHC immunoprecipitates from MT-transformed cells possess kinase activity that phosphorylates not only SHC and MT but also the 85-kDa subunit of phosphatidylinositol 3-kinase. This result suggests that a complex exists that contains, at a minimum, MT, Src family tyrosine kinases, phosphatidylinositol 3-kinase, and SHC.

Specific motifs recognized by the SH2 domains of Csk, 3BP2, fps/fes, GRB-2, HCP, SHC, Syk, and Vav

Src homology 2 (SH2) domains provide specificity to intracellular signaling by binding to specific phosphotyrosine (phospho-Tyr)-containing sequences. We recently developed a technique using a degenerate phosphopeptide library to predict the specificity of individual SH2 domains (src family members, Abl, Nck, Sem5, phospholipase C-gamma, p85 subunit of phosphatidylinositol-3-kinase, and SHPTP2 (Z. Songyang, S. E. Shoelson, M. Chaudhuri, G. Gish, T. Pawson, W. G. Haser, F. King, T. Roberts, S. Ratnofsky, R. J. Lechleider, B. G. Neel, R. B. Birge, J. E. Fajardo, M. M. Chou, H. Hanafusa, B. Schaffhausen, and L. C. Cantley, Cell 72:767-778, 1993). We report here the optimal recognition motifs for SH2 domains from GRB-2, Drk, Csk, Vav, fps/fes, SHC, Syk (carboxy-terminal SH2), 3BP2, and HCP (amino-terminal SH2 domain, also called PTP1C and SHPTP1). As predicted, SH2 domains from proteins that fall into group I on the basis of a Phe or Tyr at the beta D5 position (GRB-2, 3BP2, Csk, fps/fes, Syk C-terminal SH2) select phosphopeptides with the general motif phospho-Tyr-hydrophilic (residue)-hydrophilic (residue)-hydrophobic (residue). The SH2 domains of SHC and HCP (group III proteins with Ile, Leu, of Cys at the beta D5 position) selected the general motif phospho-Tyr-hydrophobic-Xxx-hydrophobic, also as predicted. Vav, which has a Thr at the beta D5 position, selected phospho-Tyr-Met-Glu-Pro as the optimal motif. Each SH2 domain selected a unique optimal motif distinct from motifs previously determined for other SH2 domains. These motifs are used to predict potential sites in signaling proteins for interaction with specific SH2 domain-containing proteins. The Syk SH2 domain is predicted to bind to Tyr-hydrophilic-hydrophilic-Leu/Ile motifs like those repeated at 10-residue intervals in T- and B-cell receptor-associated proteins. SHC is predicted to bind to a subgroup og these same motifs. A structural basis for the association of Csk with Src family members is also suggested from these studies.

Specific motifs recognized by the SH2 domains of Csk, 3BP2, fps/fes, GRB-2, HCP, SHC, Syk, and Vav

Src homology 2 (SH2) domains provide specificity to intracellular signaling by binding to specific phosphotyrosine (phospho-Tyr)-containing sequences. We recently developed a technique using a degenerate phosphopeptide library to predict the specificity of individual SH2 domains (src family members, Abl, Nck, Sem5, phospholipase C-gamma, p85 subunit of phosphatidylinositol-3-kinase, and SHPTP2 (Z. Songyang, S. E. Shoelson, M. Chaudhuri, G. Gish, T. Pawson, W. G. Haser, F. King, T. Roberts, S. Ratnofsky, R. J. Lechleider, B. G. Neel, R. B. Birge, J. E. Fajardo, M. M. Chou, H. Hanafusa, B. Schaffhausen, and L. C. Cantley, Cell 72:767-778, 1993). We report here the optimal recognition motifs for SH2 domains from GRB-2, Drk, Csk, Vav, fps/fes, SHC, Syk (carboxy-terminal SH2), 3BP2, and HCP (amino-terminal SH2 domain, also called PTP1C and SHPTP1). As predicted, SH2 domains from proteins that fall into group I on the basis of a Phe or Tyr at the beta D5 position (GRB-2, 3BP2, Csk, fps/fes, Syk C-terminal SH2) select phosphopeptides with the general motif phospho-Tyr-hydrophilic (residue)-hydrophilic (residue)-hydrophobic (residue). The SH2 domains of SHC and HCP (group III proteins with Ile, Leu, of Cys at the beta D5 position) selected the general motif phospho-Tyr-hydrophobic-Xxx-hydrophobic, also as predicted. Vav, which has a Thr at the beta D5 position, selected phospho-Tyr-Met-Glu-Pro as the optimal motif. Each SH2 domain selected a unique optimal motif distinct from motifs previously determined for other SH2 domains. These motifs are used to predict potential sites in signaling proteins for interaction with specific SH2 domain-containing proteins. The Syk SH2 domain is predicted to bind to Tyr-hydrophilic-hydrophilic-Leu/Ile motifs like those repeated at 10-residue intervals in T- and B-cell receptor-associated proteins. SHC is predicted to bind to a subgroup og these same motifs. A structural basis for the association of Csk with Src family members is also suggested from these studies.

Transformation by polyoma virus middle T-antigen involves the binding and tyrosine phosphorylation of Shc

Polyoma virus middle T-antigen converts normal fibroblasts to a fully transformed, tumorigenic phenotype. It achieves this, at least in part, by binding and activating one of the non-receptor tyrosine kinases, pp60c-src, pp62c-yes or pp59c-fyn (reviewed in refs 2 and 3). As a result, middle T-antigen itself is phosphorylated on tyrosine residues, one of which (Tyr 315) acts as a binding site for the SH2 domains of phosphatidylinositol-3'OH kinase 85K subunit. Here we show that another tyrosine phosphorylation site in middle T-antigen (Tyr 250; refs 4, 5) acts as a binding region for the SH2 domain of the transforming protein Shc. This results in Shc also becoming tyrosine-phosphorylated and binding to the SH2 domain of Grb2 (ref. 10). This probably stimulates p21ras activity through the mammalian homologue of the Drosophila guanine-nucleotide-exchange factor Sos (reviewed in ref. 11). We suggest that middle T-antigen transforms cells by acting as a functional homologue of an activated tyrosine kinase-associated growth-factor receptor.

Platelet-derived growth factor induces phosphatidylinositol 3-kinase release from the middle T-pp60c-src complex and association with the platelet-derived growth factor receptor

Both platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) induce mitogenesis in normal rat kidney (NRK) fibroblasts transformed by the polyoma virus middle T (pmt) oncogene. In unstimulated pmt-NRK cells phosphatidylinositol 3-kinase forms a complex with the middle T protein and pp60c-src. PDGF treatment causes a release of phosphatidylinositol 3-kinase activity from the complex and a simultaneous increase in activity associated with the PDGF receptor. In contrast after treatment with EGF the majority of phosphatidylinositol 3-kinase activity remains associated with the middle T-pp60c-src complex. Proliferation of NRK fibroblasts transformed by the v-src oncogene is already maximal, and no further stimulation is observed with either PDGF or EGF. Neither growth factor induces dissociation of the complex between phosphatidylinositol 3-kinase and pp60v-src. These observations suggest that the complex between phosphatidylinositol 3-kinase, the middle T protein and pp60c-src is dissociable, and that phosphatidylinositol 3-kinase plays different roles in mitogenic signal transduction by the PDGF and EGF receptors.

Mitosis-specific phosphorylation of polyomavirus middle-sized tumor antigen and its role during cell transformation

Transformation of cells in culture by polyomavirus is mediated by one of its early gene products, middle-sized tumor antigen (MTAg). This protein forms multiple complexes with cellular enzymes such as tyrosine kinases (pp60c-src), a phosphatidylinositol 3-kinase, and phosphatase 2A. Association with MTAg leads to the activation of pp60c-src through interference with phosphorylation at Tyr-527, a site negatively regulating src kinase activity. MTAg abrogates mitosis-specific activation of pp60c-src, resulting in constitutive high kinase activity of the enzyme throughout all phases of the cell cycle. Here we report that MTAg is transiently modified during mitosis, resulting in an increase in its apparent molecular size on SDS/acrylamide gels. Similarly, MTAg isolated from interphase cells and phosphorylated by the cell cycle-regulated serine/threonine kinase p34cdc2 in vitro has increased molecular mass. The large molecular mass form of the protein can be converted to the authentic 56-kDa form upon dephosphorylation by potato acid phosphatase. Two putative phosphorylation sites for a cdc2-like kinase were identified as Thr-160 and -291, respectively. Conversion of Thr-160 to Ala resulted in a transformation-defective mutant protein that was still capable of associating with pp60c-src, phosphatidylinositol 3-kinase, and phosphatase 2A, while the corresponding mutant in position 291 was wild type with respect to all parameters measured so far. These data suggest that phosphorylation by p34cdc2 or a related cell cycle-regulated kinase modulates the interaction of MTAg with cellular targets that are crucial for cell transformation.

Mutation of a cysteine residue in polyomavirus middle T antigen abolishes interactions with protein phosphatase 2A, pp60c-src, and phosphatidylinositol-3 kinase, activation of c-fos expression, and cellular transformation

Polyomavirus middle T antigen (MT) interacts with several cellular proteins involved in cell proliferation. MT forms complexes with protein phosphatase 2A (PP2A), pp60c-src (and the related kinases c-fyn and c-yes), and phosphatidylinositol-3 kinase. We made a single point mutation in MT, changing a conserved cysteine residue at position 120 to tryptophan, and characterized the biochemical and biological properties of the mutant (C120W) protein. The mutant MT protein does not associate with PP2A, pp60c-src, or phosphatidylinositol-3 kinase as judged by coimmunoprecipitation and associated phosphatase or kinase activity. The C120W mutant is defective in activation of c-fos expression and in morphological transformation of NIH 3T3 cells.

Novel monoclonal antibodies that differentiate between the binding of pp60c-src or protein phosphatase 2A by polyomavirus middle T antigen

Fourteen pGEX plasmids that express defined regions of polyomavirus middle T antigen in bacteria have been constructed. These polypeptides have been used to generate 18 new monoclonal antibodies directed against the unique portion of middle T and to map the approximate position of the antibody recognition sites onto the protein sequence. All of the antibodies effectively immunoprecipitate middle T and the associated 60- and 35-kDa components of protein phosphatase 2A. Four of the antibodies, however, do not react with middle T when it is bound to pp60c-src. These four probably bind to amino acids 203 to 218 of the middle T protein sequence, which are encoded by the mRNA immediately 3' to the splice junction that creates the C-terminal unique region. This suggests that additional middle T sequences are required for middle T's interaction with pp60c-src than are needed for its binding to protein phosphatase 2A. The antibodies localize this extra region and provide a means of distinguishing between these two associations.

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.

Polyomavirus middle T-antigen NPTY mutants

A polyomavirus middle T-antigen (MTAg) mutant containing a substitution of Leu for Pro at amino acid 248 has previously been described as completely transformation defective (B. J. Druker, L. Ling, B. Cohen, T. M. Roberts, and B. S. Schaffhausen, J. Virol. 64:4454-4461, 1990). This mutant had no alterations in associated proteins or associated kinase activities compared with wild-type MTAg. Pro-248 lies in a tetrameric sequence, NPTY, which is reminiscent of the so-called NPXY sequence in the low-density-lipoprotein receptor. In the low-density-lipoprotein receptor, mutations in the NPXY motif but not in the surrounding amino acids abolish receptor function, apparently by decreasing receptor internalization (W. Chen, J. L. Goldstein, and M. S. Brown, J. Biol. Chem. 265:3116-3123, 1990). To determine whether this sequence represents a functional motif in MTAg as well, a series of single amino acid substitutions was constructed in this region of MTAg. All of the mutations of N, P, T, or Y, including the relatively conservative substitution of Ser for Thr at amino acid 249, resulted in a transformation-defective MTAg, whereas mutations outside of this sequence allowed mutants to retain near-wild-type transformation capabilities. Transformation-defective mutants with mutations in the NPTY region behaved similarly to the mutant with the original Pro-248-to-Leu-248 mutation when assayed for associated proteins and activities in vitro; that is, they retained a full complement of wild-type activities and associated proteins. Further, insertion of the tetrameric sequence NPTY downstream of the mutated motif restored transforming abilities to these mutants. Thus, the tetrameric sequence NPTY in MTAg appears to represent a well-defined functional motif of MTAg.

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.

Induction of c-jun protooncogene expression and transcription factor AP-1 activity by the polyoma virus middle-sized tumor antigen

Polyoma virus middle-sized tumor (PymT) antigen is required for neoplastic cell transformation by polyoma virus. We studied changes in gene expression accompanying expression of PymT in murine fibroblasts. These experiments showed that PymT differentially affects several growth-related genes. c-jun protooncogene expression was highly increased, whereas the expression of two growth arrest-specific genes (gas) was reduced, in cells transformed by PymT. Cotransfection experiments showed that the increase in c-jun expression resulted from elevated activity of the transcription factor AP-1 and was mediated through the phorbol 12-tetradecanoate 13-acetate response element in the c-jun promoter. The degree of c-Jun/AP-1 activation by different PymT mutants correlated with their transforming capability, suggesting that regulation of c-Jun/AP-1 activity may play a role in cell transformation by polyoma virus.

Transformation-defective mutants of polyomavirus middle T antigen associate with phosphatidylinositol 3-kinase (PI 3-kinase) but are unable to maintain wild-type levels of PI 3-kinase products in intact cells

Middle T antigen (MT) of polyomavirus causes transformation by associating with a number of cellular proteins. The association with and activation of two such proteins, phosphatidylinositol 3-kinase (PI 3-kinase) and pp60c-src, appears to be necessary for transformation by MT. The tyrosine kinase activity of MT-associated pp60c-src is significantly increased when assayed in vitro, and levels of phosphotyrosine-containing proteins are elevated in vivo. Similarly, levels of the PI 3-kinase products phosphatidylinositol-3,4-bisphosphate [PI(3,4)P2] and phosphatiylinositol-3,4,5-trisphosphate [PI(3,4,5)P3] are constitutively elevated in MT-transformed cells. However, the formation of a complete MT/cellular protein complex and the activation of tyrosine kinase are not sufficient to cause transformation, since the transformation-defective mutants 248m and dl1015 associate with all wild-type MT-associated proteins, including PI 3-kinase and pp60c-src, and neither mutant appears to be defective in MT-associated tyrosine kinase activity. Studies presented here compared (i) the amount of PI 3-kinase activity associated with the MT complex and (ii) levels of [3H]inositol incorporation into PI 3-kinase products in cells expressing mutant or wild-type MT. The results show that dl1015 is defective in both assays, whereas 248m is defective only for incorporation of [3H]inositol into PI(3,4,5)P2 and PI(3,4)P3. These findings identify a biochemical defect in the 248m mutant and corroborate previous results correlating transformation and elevated levels of PI 3-kinase products in vivo. In addition, they indicate that PI 3-kinase product levels are affected by factors other than simply the amount of PI 3-kinase activity associated with the MT complex.

Generation of a large library of point mutations in polyoma middle T antigen

Polyoma middle T antigen (MTAg) transforms cells by associating with and activating a variety of intracellular proteins, including src family members and a phosphatidylinositol-3 kinase. In order to assist in the study of the relative importance of the various associated biochemical activities for transformation by polyomavirus MTAg, a library of MTAg mutants was constructed. Chemically mutagenized MTAg DNA was purified from wild-type DNA by separation on denaturing gradient gels and placed into a recombinant retrovirus vector. Utilizing the resultant library of MTAg-expressing retroviruses, fibroblast cell lines expressing retroviruses, fibroblast cell lines expressing individual MTAg mutants were generated and screened for a non-transformed morphology. Of the first seven non-transformed clones tested, all express the MTAg protein. We estimate that approximately 24% of the G418-resistant colonies contain a transformation-defective MTAg mutant. A more thorough evaluation of one such clone revealed four single base-pair changes as compared to wild-type. Further genetic dissection of this mutant reveals that substituting leucine for proline at amino acid 248 results in a completely transformation defective MTAg. The utility of this mutagenesis and screening procedure as well as the description of several new MTAg mutants is described. This library of mutations should be of general interest for studying the transforming ability of MTAg.

Presence of phospholipase C in coated vesicles from bovine brain. Dual regulatory effects of GTP-analogs

Bovine brain coated vesicles display free calcium-dependent phospholipase C activity. Gpp(NH)p and GTP gamma S inhibited phospholipase C at nanomolar concentrations. Increasing concentrations of Gpp(NH)p and GTP gamma S reversed the inhibitory effects and stimulated phospholipase C activity. Preincubation of coated vesicles with pertussis toxin blocked the poorly-hydrolyzable GTP-analogs' inhibitory effects on phospholipase C. These data indicate that guanine nucleotides exert a dual regulatory control of phospholipase C in coated vesicles and that the inhibitory pathway is mediated by a pertussis toxin-sensitive G-protein.

D-3-deoxy-3-substituted myo-inositol analogues as inhibitors of cell growth

A number of unnatural D-3-deoxy-3-substituted myo-inositols were synthesized and their effects on the growth of wild-type NIH 3T3 cells and oncogene-transformed NIH 3T3 cells were studied. The compounds were found to exhibit a diversity of growth-inhibitory activities and showed selectivity in inhibiting the growth of some transformed cells as compared with wild-type cells. Remarkably, D-3-deoxy-3-azido-myo-inositol exhibited potent growth-inhibitory effects toward v-sis-transformed NIH 3T3 cells but had little effect on the growth of wild-type cells. The growth-inhibitory effects of the myo-inositol analogues were antagonized by myo-inositol. Since [3H]-3-deoxy-3-fluoro-myo-inositol was shown to be taken up by cells and incorporated into cellular phospholipids, we suggest that these unnatural myo-inositol analogues may act as antimetabolites in the phosphatidylinositol intracellular signalling pathways. Because cells transformed by oncogenes often exhibit elevated phosphatidylinositol turnover, the inhibition of signalling pathways that mediate oncogene action could offer new opportunities for controlling the growth of cancer cells.