Activated Fps/Fes tyrosine kinase regulates erythroid differentiation and survival.

OBJECTIVE: A substantial body of evidence implicates the cytoplasmic protein tyrosine kinase Fps/Fes in regulation of myeloid differentiation and survival. In this study we wished to determine if Fps/Fes also plays a role in the regulation of erythropoiesis. METHODS: Mice tissue-specifically expressing a "gain-of-function" mutant fps/fes transgene (fps(MF)) encoding an activated variant of Fps/Fes (MFps), were used to explore the in vivo biological role of Fps/Fes. Erythropoiesis in these mice was assessed by hematological analysis, lineage marker analysis, bone-marrow colony assays, and biochemical approaches. RESULTS: fps(MF) mice displayed reductions in peripheral red cell counts. However, there was an accumulation of immature erythroid precursors, which displayed increased survival. Fps/Fes and the related Fer kinase were both detected in early erythroid progenitors/blasts and in mature red cells. Fps/Fes was also activated in response to erythropoietin (EPO) and stem cell factor (SCF), two critical factors in erythroid development. In addition, increased Stat5A/B activation and reduced Erk1/2 phosphorylation was observed in fps(MF) primary erythroid cells in response to EPO or SCF, respectively. CONCLUSIONS: These data support a role for Fps/Fes in regulating the survival and differentiation of erythroid cells through modulation of Stat5A/B and Erk kinase pathways induced by EPO and SCF. The increased numbers and survival of erythroid progenitors from fps(MF) mice, and their differential responsiveness to SCF and EPO, implicates Fps/Fes in the commitment of multilineage progenitors to the erythroid lineage. The anemic phenotype in fps(MF) mice suggests that downregulation of Fps/Fes activity might be required for terminal erythroid differentiation.

Activated Fps/Fes partially rescues the in vivo developmental potential of Flk1-deficient vascular progenitor cells.

Relatively little is known about the modulators of the vascular endothelial growth factor A (VEGF-A)/Flk1 signaling cascade. To functionally characterize this pathway, VEGF-A stimulation of endothelial cells was performed. VEGF-A-mediated Flk1 activation resulted in increased translocation of the endogenous Fps/Fes cytoplasmic tyrosine kinase to the plasma membrane and increased tyrosine phosphorylation, suggesting a role for Fps/Fes in VEGF-A/Flk1 signaling events. Addition of a myristoylation consensus sequence to Fps/Fes resulted in VEGF-A-independent membrane localization of Fps/Fes in endothelial cells. Expression of the activated Fps/Fes protein in Flk1-deficient embryonic stem (ES) cells rescued their contribution to the developing vascular endothelium in vivo by using ES cell-derived chimeras. Activated Fps/Fes contributed to this rescue event by restoring the migratory potential to Flk1 null progenitors, which is required for movement of hemangioblasts from the primitive streak region into the yolk sac proper. Activated Fps/Fes in the presence of Flk1 increased the number of hemangioblast colonies in vitro and increased the number of mesodermal progenitors in vivo. These results suggest that Fps/Fes may act synergistically with Flk1 to modulate hemangioblast differentiation into the endothelium. We have also demonstrated that activated Fps/Fes causes hemangioma formation in vivo, independently of Flk1, as a result of increasing vascular progenitor density.

Activated Fes protein tyrosine kinase induces terminal macrophage differentiation of myeloid progenitors (U937 cells) and activation of the transcription factor PU.1.

The c-fps/fes proto-oncogene encodes a 92-kDa protein tyrosine kinase that is preferentially expressed in myeloid and endothelial cells. Fes is believed to play a role in vascular development and myelopoiesis and in the inflammatory responses of granulocytes and macrophages. To help define the biological role of this kinase and identify its downstream targets, we have developed a gain-of-function allele of Fes that has potent biological activity in myeloid cell progenitors. Introduction of constitutively active Fes into bipotential U937 cells induced the appearance of fully differentiated macrophages within 6 to 12 days. The Fes-expressing differentiated cells became adherent, had distinctive macrophage morphology, and exhibited increased expression of myelomonocytic differentiation markers, including CD11b, CD11c, CD18, CD14, and the macrophage colony-stimulating factor receptor. These cells acquired phagocytic properties and exhibited NADPH oxidase and nonspecific esterase activities, confirming that they were functionally active macrophages. Concomitantly, there was downregulation of the granulocytic marker granulocyte colony-stimulating factor receptor, indicating that the biological activity of Fes was coordinated in a lineage-specific manner. A constitutively active Src did not induce macrophage morphology or upregulation of myelomonocytic markers in U937 cells, suggesting that the biological activity we observed was not a general consequence of expression of an activated nonreceptor tyrosine kinase. Analysis of possible downstream targets of Fes revealed that this kinase activated the ets family transcription factor PU.1, which is essential for macrophage development. Our results strongly implicate Fes as a key regulator of terminal macrophage differentiation and identify PU.1 as a transcription factor that may mediate some of its biological activities in myeloid cells.

Identification of the human erythropoietin receptor region required for Stat1 and Stat3 activation.

Signal transducers and activators of transcription (Stat) proteins play important roles in the regulation of hematopoiesis as downstream molecules of cytokine signal transduction. It was previously demonstrated that erythropoietin (EPO), a major regulator of erythropoiesis, activates 3 different Stat members, Stat1, Stat3, and Stat5, in a human EPO-dependent cell line, UT-7/EPO. To clarify the mechanism by which EPO activates Stat1 and Stat3 via the EPO receptor (EPOR), a series of chimeric receptors was constructed bearing the extracellular domain of the granulocyte colony-stimulating factor receptor linked to the transmembrane domain of EPOR and the full length or several mutants of the cytoplasmic domain of EPOR, and these chimeric receptor complementary DNAs were introduced into UT-7/EPO cells. Tyr432 on human EPOR was important for activation of Stat1 and Stat3 and c-myc gene induction. In addition, Jak2 and Fes tyrosine kinases were involved in EPO-induced activation of Stat1 and Stat3. These results indicate that Stat1 and Stat3 are activated by EPO via distinct mechanisms from Stat5.

Electrostatic environment surrounding the activation loop phosphotyrosine in the oncoprotein v-Fps.

Autophosphorylation of Tyr-1073 in the activation loop of the oncoprotein v-Fps enhances the phosphoryl transfer reaction without influencing substrate, ATP, or metal ion binding affinities [Saylor, P., et al. (1998) Biochemistry 37, 17875-17881]. A structural model of v-Fps, generated from the insulin receptor, indicates that pTyr-1073 chelates two arginines. Mutation of these residues to alanine (R1042A and R1066A) results in weakly phosphorylated enzymes, indicating that one electropositive center is insufficient for attaining maximum loop phosphorylation and concomitant high catalytic activity. While the turnover rate for R1066A is similar to that for a mutant lacking a phosphorylatable residue in the activation loop, the rate for R1042A is 50-fold slower. While solvent perturbation studies suggest that the former is due to a slow phosphoryl transfer step, the latter effect results from a slow conformational change in the mutant, potentially linked to motions in the catalytic loop. Binding of a stoichiometric quantity of Mg(2+) is essential for ATP binding and catalysis, while binding of an additional Mg(2+) ion activates further the wild-type enzyme. The affinity of the R1066A enzyme for the second Mg(2+) ion is 23-fold higher than that of the phosphorylated or unphosphorylated form of wild-type v-Fps, with substrate binding unaffected. Conversely, the affinity of R1066A for a substrate mimic lacking a phosphorylation site is 12-fold higher than that for the phosphorylated or unphosphorylated form of wild-type v-Fps, with binding of the second Mg(2+) ion unaffected. A comparison of these enzyme-independent parameters indicates that Arg-1042 and Arg-1066 induce strain in the active site in the repressed form of the enzyme. While this strain is not relieved in the phosphorylated form, the improvements in catalysis in activated v-Fps compensate for reduced metal and substrate binding affinities.

Subcellular localization analysis of the closely related Fps/Fes and Fer protein-tyrosine kinases suggests a distinct role for Fps/Fes in vesicular trafficking.

The subcellular localizations of the Fps/Fes and closely related Fer cytoplasmic tyrosine kinases were studied using green fluorescent protein (GFP) fusions and confocal fluorescence microscopy. In contrast to previous reports, neither kinase localized to the nucleus. Fer was diffusely cytoplasmic throughout the cell cycle. Fps/Fes also displayed a diffuse cytoplasmic localization, but in addition it showed distinct accumulations in cytoplasmic vesicles as well as in a perinuclear region consistent with the Golgi. This localization was very similar to that of TGN38, a known marker of the trans Golgi. The localization of Fps/Fes and TGN38 were both perturbed by brefeldin A, a fungal metabolite that disrupts the Golgi apparatus. Fps/Fes was also found to colocalize to various extents with several Rab proteins, which are members of the monomeric G-protein superfamily involved in vesicular transport between specific subcellular compartments. Using Rabs that are involved in endocytosis (Rab5B and Rab7) or exocytosis (Rab1A and Rab3A), we showed that Fps/Fes is localized in both pathways. These results suggest that Fps/Fes may play a general role in the regulation of vesicular trafficking.

Catalytic assessment of the glycine-rich loop of the v-Fps oncoprotein using site-directed mutagenesis.

The three glycine residues in the glycine-rich loop of the oncoprotein, v-Fps, were mutated to determine the function of these highly conserved residues in catalysis. The kinase domains of six mutants (G928A,S, G930A,S, and G933A,S) and the wild-type enzyme were expressed and purified as fusion proteins of glutathione-S-transferase in Escherichia coli, and their catalytic properties were assessed using steady-state kinetic, inhibition, viscosity and autophosphorylation studies. Although both G928A and G930A had no detectable activity toward the substrate peptide (EAEIYEAIE), the other mutants had apparent, but varying activities. G930S lowered the rate of phosphoryl transfer by 130-fold while G928S and G933S had smaller (6-9-fold) reductions in this step. These effects on catalytic function parallel the reductions in turnover and autophosphorylation but, for G933S and G933A, net product release is still rate limiting at saturating substrate and ATP concentrations. On the basis of K(I) measurements, the effects on turnover for these mutants may be due to improved ADP affinity. While ADP affinity is reduced 2- and 3-fold for G928S and G930S, the affinity of this product is increased by 22- and 7-fold for G933S and G933A. In contrast, ATP affinity is enhanced by 5-fold for G928S and G933S and is reduced by less than 2-fold for G930S. These complex, differential effects on nucleotide binding indicate that the glycines influence the relative affinities of ADP and ATP. On the basis of the results of serine replacements, Gly-928 and Gly-930 enhance ADP affinity by 9- and 2-fold compared to ATP affinity whereas Gly-933 diminishes ADP affinity by approximately 4-fold compared to ATP affinity. These findings demonstrate that the functions of the loop lie not only in modulating the rate of the phosphoryl transfer step but also in balancing the relative affinities of ATP and ADP. These effects on nucleotide specificity may be a contributing element for the stabilization of the phosphoryl transition state and may also facilitate quick release of bound products.

Substrate specificity of the oncoprotein v-Fps: site-specific mutagenesis of the putative P+1 pocket.

Based on the X-ray structure of the insulin receptor kinase [Hubbard, S. R. (1997) EMBO J. 16, 5572-5581], Arg-1130 in the oncoprotein v-Fps, a nonreceptor tyrosine protein kinase, is predicted to interact with the P+1 glutamate in substrate peptides. To determine whether this residue is an important recognition element in v-Fps, Arg-1130 was substituted with leucine (R1130L) and glutamic acid (R1130E). The ability of these mutants to phosphorylate the peptide EAEIYXAIE, where X is glutamic acid, alanine, or lysine, was assessed. A comparison of the rates of peptide phosphorylation under limiting substrate concentrations (i.e., k(cat)/K(m) conditions) indicates that substrate specificity is altered by the electrostatic environment of the P+1 pocket. When the pocket displays a positive charge (Arg-1130; wild type), no charge (R1130L), or a negative charge (R1130E), v-Fps prefers to phosphorylate the glutamate peptide over the lysine peptide by a 200:1, 9:1, or 1:1 margin. While k(cat)/K(m) for the glutamate peptide is 50-fold higher for wild type compared to R1130E, k(cat)/K(m) for the lysine peptide is 3-fold higher for R1130E compared to wild type, a 150-fold change in relative substrate specificity. Analysis of the individual steps in the kinetic mechanism using viscosometric techniques indicates that the wild-type enzyme binds the glutamate peptide 3-fold better than the alanine peptide and, at least, 10-fold better than the lysine peptide. For R1130L, this margin range is reduced substantially, and for R1130E, no binding preference is observed. Nonetheless, the lysine peptide binds, at least, 4-fold better to R1130E than to wild type, and the glutamate peptide binds 3-fold poorer to R1130E than to wild type. The mutants lower the phosphoryl transfer rate by 4-30-fold for the three peptides, suggesting that Arg-1130 helps to position the tyrosine for optimum catalysis. The data indicate that a single mutation in v-Fps can alter significantly the relative substrate specificity by about 2 orders of magnitude with, at least, 50% of this effect occurring through relative changes in peptide binding affinity.

The c-Fes family of protein-tyrosine kinases.

The human c-fes protooncogene encodes a protein-tyrosine kinase (c-Fes) distinct from c-Src, c-Abl and other nonreceptor tyrosine kinases. Although originally identified as the cellular homolog of several transforming retroviral oncoproteins, Fes was later found to exhibit strong expression in myeloid hematopoietic cells and to play a direct role in their differentiation. Recent work has shown that Fes exhibits a more widespread expression pattern in both developing and adult tissues, suggesting a general physiological function for this kinase and its closely related homolog, Fer. This review highlights the unique aspects of Fes structure, regulation, and function that set it apart from other tyrosine kinase families.

A second magnesium ion is critical for ATP binding in the kinase domain of the oncoprotein v-Fps.

The activity of the kinase domain of the oncoprotein v-Fps was found to be sensitive to the concentration of magnesium ions. Plots of initial velocity versus free magnesium concentration are hyperbolic and do not extrapolate to the origin at stoichiometric ATP-Mg, indicating that there are two sites for metal chelation on the enzyme and the second is nonessential for catalysis. The second metal is strongly activating and increases the reaction rate constant almost 20-fold from 0.5 to 8.3 s-1 using 0.2 mM ATP-Mg and 1 mM peptide, EAEIYEAIE. This increase in rate is due to a large increase in the apparent affinity of ATP-Mg at high magnesium concentrations. At 0.5 and 10 mM free Mg2+, KATP-Mg is 3.6 and 0.22 mM, respectively. Extrapolation of the observed affinity of ATP-Mg to zero and infinite free metal indicates that KATP-Mg is greater than 8 mM in the absence of the second metal and 0.1 mM in the presence of the second metal, a minimum 80-fold enhancement. By comparison, free levels of the divalent ion do not influence maximum turnover (kcat) and have only a 2-fold effect on the Km for the peptide substrate between 0.5 and 20 mM free Mg2+. Viscosometric studies indicate that free Mg2+ does not influence the rates of phosphoryl transfer or net product release above 0.5 mM but does affect directly the dissociation constant for ATP-Mg. The Kd for ATP-Mg in the absence and presence of the second metal ion is >32 and 0.4 mM, respectively. At high magnesium concentrations, ATP-Mg and the peptide substrate bind independently, while at lower concentrations (0.5 mM), there is significant negative binding synergism suggesting that the second metal may help to reduce charge repulsion between ATP-Mg and the peptide. The data indicate that the first metal is sufficient for phosphoryl transfer. While the second metal could have some influence on phosphoryl transfer or product binding, it is a potent activator that functions minimally by controlling ATP-Mg binding.

v-fps causes transformation by inducing tyrosine phosphorylation and activation of the PDGFbeta receptor.

The v-fps oncogene encodes an activated tyrosine kinase which is capable of transforming fibroblasts. In this report, we provide evidence that within a few minutes of activation of the tyrosine kinase activity of v-Fps, tyrosine phosphorylation of the platelet derived growth factor (PDGF) beta receptor is observed. Further, sustained expression of activated v-Fps results in a down-regulation of the PDGF receptor both at the level of the mRNA (approximately 4-8-fold), but even more markedly at the level of the receptor protein (> 100-fold). The kinase activity of the v-Fps oncoprotein was found to be required for both the induction of PDGF receptor tyrosine phosphorylation and ultimately the reduced receptor protein levels. Tyrosine phosphorylation of a kinase inactive PDGF receptor was also demonstrated in cells which also express v-fps, but this was not sufficient to induce transformation. Only cells expressing both v-fps and a wild type PDGF receptor were able to form colonies in soft agar. These findings suggest that wild type v-fps may use tyrosine phosphorylation of the PDGFbeta receptor to constitutively activate the kinase activity of the receptor, resulting in a sustained proliferative signal and fibroblast transformation.

Mutations in the activation loop tyrosine of the oncoprotein v-Fps.

Mutations were made in the activation loop tyrosine of the kinase domain of the oncoprotein v-Fps to assess the role of autophosphorylation in catalysis. Three mutant proteins, Y1073E, Y1073Q, and Y1073F, were expressed and purified as fusion proteins of glutathione-S-transferase from Escherichia coli and their catalytic properties were evaluated. Y1073E, Y1073Q, and Y1073F have k(cat) values that are reduced by 5-, 35-, and 40-fold relative to the wild-type enzyme, respectively. For all mutant enzymes, the Km values for ATP and a peptide substrate, EAEIYEAIE, are changed by 0.4-2-fold compared to the wild-type enzyme. The slopes for the plots of relative turnover versus solvent viscosity [(k(cat))eta] are 0.71 +/- 0.08, 0.10 +/- 0.06, and approximately 0 for wild type, Y1073Q, and Y1073E, respectively. These results imply that the phosphoryl transfer rate constant is reduced by 19- and 130-fold for Y1073E and Y1073Q compared to the wild-type enzyme. The dissociation constant of the substrate peptide is 1.5-2.5-fold lower for the mutants compared to wild type. The inhibition constant for EAEIFEAIE, a competitive inhibitor, is unaffected for Y1073E and raised 3-fold for Y1073Q compared to wild type. Y1073E and Y1073Q are strongly activated by free magnesium to the same extent and the apparent affinity constant for the metal is similar to that for the wild-type enzyme. The data indicate that the major role of autophosphorylation in the tyrosine kinase domain of v-Fps is to increase the rate of phosphoryl transfer without greatly affecting active-site accessibility or the local environment of the activating metal. Finally, the similar rate enhancements for phosphoryl transfer in v-Fps compared to protein kinase A [Adams et al. (1995) Biochemistry 34, 2447-2454] upon autophosphorylation suggest a conserved mechanism for communication between the activation loop and the catalytic residues of these two enzymes.

Insight into tyrosine phosphorylation in v-Fps using proton inventory techniques.

The phosphoryl group transfer step in the kinase domain of v-Fps, a nonreceptor tyrosine protein kinase, was analyzed using proton inventory and viscosometric techniques. The latter studies show that two nine-residue peptide substrates for v-Fps, peptides I (EAEAYEAIE) and II (EAEIYEAIE), are in rapid equilibrium with the active site and bind with similar affinities (Ks = 2.2 and 1.7 mM for peptides I and II). While phosphoryl group transfer is the rate-limiting step in kcat for peptide I (5 s-1) at neutral pH, peptide II is converted to product by a kinetic mechanism in which phosphoryl group transfer (45 s-1) and product release (20 s-1) partially control this parameter. Significant solvent isotope effects on kcat (k0/kn approximately 1.6) are observed for the phosphorylation of both peptides in 95% D2O. Proton inventories on kcat for peptide I are linear, indicating that the phosphoryl group transfer step is associated with a single proton transfer. Conversely, proton inventories on kcat for peptide II are "bowed" up, consistent with a "virtual" transition state in which phosphoryl group transfer and product release steps partially control this parameter. The lack of solvent isotope effects on kcat/Kpeptide for both peptides can be explained by an equilibrium isotope effect on substrate binding that offsets the kinetic isotope effect for phosphoryl group transfer. In keeping with this proposal, the KI for the inhibitor peptide, EAEIFEAIE, is 11 +/- 0.80 and 6.5 +/- 0.82 mM in 0 and 60% D2O, respectively. Fitting of the proton inventory plots using modified forms of the Gross-Butler equation provide intrinsic isotope effects of 1.7 and 3.6 for peptides I and II. The combined data are consistent with a mechanism involving either an acid-base catalyst or a conformational change preceding the release of products that is accompanied by the disruption of a single hydrogen donor-acceptor pair.

Rate-determining steps for tyrosine phosphorylation by the kinase domain of v-fps.

The rate-determining steps in the phosphorylation of four tyrosine-containing peptides by the kinase domain of the nonreceptor tyrosine protein kinase v-fps were measured using viscosometric methods. The peptides were phosphorylated by a fusion protein of glutathione-S-transferase and the kinase domain of v-fps (GST-kin) and the initial velocities were determined by a coupled enzyme assay. Peptides I (EEEIYEEIE), II (EAEIYEAIE), and III (DADIYDAID) were phosphorylated by GST-kin with similar kinetic constants. The viscosogens, glycerol and sucrose, were found to have intermediate effects on kcat and no effect on kcat/Kpeptide for the phosphorylation of these three peptides. The data are interpreted according to the Stokes-Einstein equation and a simple three-step mechanism involving substrate binding, phosphoryl group transfer, and net product release. Two competitive inhibitors (EAEIFEAIE and DADIFDAID) exhibited K1 values that are 6-10-fold higher than the Kpeptide values for their analogous peptide substrates. The data imply that peptides I-III are in rapid equilibrium with the enzyme and that kcat is partially limited by both phosphoryl group transfer (40-100 s-1) and product release (17-22 s-1). GST-kin phosphorylates peptide IV (R5AENLEYamide) with a low Km (100 microM) and a kcat that is 40-fold lower than that for peptide I. No effect of solvent viscosity was observed for the phosphorylation of this peptide on either kcat or kcat/Kpeptide. This suggests that highly viscous solutions do not perturb structure and that the rate-determining step for this poor substrate is phosphoryl group transfer. The data indicate that the kinase domain of v-fps phosphorylates its best substrate with a chemical rate constant that is at least 5-fold lower than that for the serine-specific cAMP-dependent protein kinase and its best substrate LRRASLG (Adams & Taylor, 1992). Interestingly, both enzymes exhibit a similar affinity for their substrates and both enzymes release their products at a similar rate. This implies that the differences in catalytic efficiency between serine- and tyrosine-specific protein kinases lie exclusively in the rate constants for phosphoryl group transfer and not in substrate absorption or product desorption.

Bacterial expression, purification and preliminary kinetic description of the kinase domain of v-fps.

The gene coding for the tyrosine protein kinase domain of v-fps was subcloned into a plasmid vector expressing glutathione-S-transferase (GST). This new vector expresses a fusion protein in Escherichia coli composed of the kinase domain linked with GST at the N-terminus (GST-kin). A portion of the total expressed protein was soluble upon cell lysis and was purified by affinity chromatography using glutathione cross-linked agarose. GST-kin (M(r) 57,000) is a phosphoprotein as judged by 32P autoradiography, consistent with the known autophosphorylation site within the kinase core [Weinmaster et al. (1984) Cell, 37, 559-568]. Cleavage of the fusion protein with thrombin and purification on phosphocellulose resin yielded the pure kinase domain (M(r) 33,000). The activity of the kinase domain is indistinguishable from that of GST-kin using the peptide substrate EEEIYEEIE, indicating that N-terminal fusion has no effect on the kinase domain. GST-kin phosphorylates a second peptide, EAEIYEAIE, with improved catalytic efficiency. Initial velocity data are consistent with a random bireactant mechanism with no substrate synergism observed in the ternary complex. Steady-state kinetic analyses reveal that this peptide is phosphorylated, with a kcat of 3.6 s-1, a Kpeptide of 500 microM and a KATP of 250 microM. The expression, purification and preliminary kinetic analysis of the kinase domain of v-fps provide the first step in the application of structure-function studies for this oncoprotein.

Leucine zipper-like domain regulates the autophosphorylation and the transforming activity of P130gag-fps.

P130gag-fps, the product of Fujinami sarcoma virus, has a leucine zipper (LZ) motif located in 729-756 amino acid residues. To explore the role of LZ-like domain in the transformation by P130gag-fps, we made a deletion (delta FpsLZ/SH2) and a site-directed substitution mutation (L746P). Deletion mutant did not transform the 3Y1 cells and the resulting protein did not show kinase activity. Substitution of Leu746 with Pro (L746P) reduced the transforming activity by 6-fold. Although the L746P mutant retained intact catalytic activity in vitro, it did not phosphorylate cellular proteins in vivo. We concluded that LZ-like domain might mediate the trans-activation of P130gag-fps tyrosine kinase by autophosphorylation, which is prerequisite for the transforming activity.

Tyrosine phosphorylation of BCR by FPS/FES protein-tyrosine kinases induces association of BCR with GRB-2/SOS.

The human bcr gene encodes a protein with serine/threonine kinase activity, CDC24/dbl homology, a GAP domain, and an SH2-binding region. However, the precise physiological functions of BCR are unknown. Coexpression of BCR with the cytoplasmic protein-tyrosine kinase encoded by the c-fes proto-oncogene in Sf-9 cells resulted in stable BCR-FES protein complex formation and tyrosine phosphorylation of BCR. Association involves the SH2 domain of FES and a novel binding domain localized to the first 347 amino acids of the FES N-terminal region. Deletion of the homologous N-terminal BCR-binding domain from v-fps, a fes-related transforming oncogene, abolished transforming activity and tyrosine phosphorylation of BCR in vivo. Tyrosine phosphorylation of BCR in v-fps-transformed cells induced its association with GRB-2/SOS, the RAS guanine nucleotide exchange factor complex. These data provide evidence that BCR couples the cytoplasmic protein-tyrosine kinase and RAS signaling pathways.

v-Fps-responsiveness in the Egr-1 promoter is mediated by serum response elements.

Egr-1, a mitogen-responsive transcription factor, is rapidly induced by v-Fps in the absence of protein synthesis. Thus, Egr-1 is a primary response to the protein-tyrosine kinase activity of v-Fps. To determine the v-Fps-responsive elements in the Egr-1 promoter, deletion mutants of the Egr-1 promoter were used in transient expression assays. A v-Fps expression vector was contransfected into NIH 3T3 cells with chloramphenicol acetyl transferase (CAT) gene expression vectors under the control of the Egr-1 promoter or the Egr-1 promoter containing various deletions. Responsiveness to v-Fps was restricted to a region that contained repeated CC(A/T)6GG sequences, known as CArG boxes. CArG boxes form the core of serum response element (SREs). v-Fps-induced Egr-1 promoter activation was lost by sequential removal of four tandemly repeated SREs. This region, containing four SREs, was found to be sufficient for maximal Egr-1 induction by v-Fps when placed upstream from a heterologous promoter. Individual SREs from this region were able to respond to v-Fps, however, the activation of the individual SREs was lower than that observed for the clustered SREs. These data suggest that v-Fps-responsiveness in the Egr-1 promoter is mediated by SREs.

Inhibitory effect of myristylation on transrepression by FBR (Gag-Fos) protein.

The myristylated v-fos product, FBR murine sarcoma virus (Gag-Fos) protein, exhibits a lower level of transrepression of the serum response element (SRE) than does c-fos protein (Fos). Mutation of the N-terminal myristylation site in FBR protein restored SRE transrepression. Replacement of N-terminal viral Gag sequences with the Fos N terminus also restored this activity, providing additional evidence that myristylation inhibits transrepression by FBR protein. However, the myristylated Gag domain did not inhibit SRE transrepression when fused to Fos, indicating that myristylation of a fos protein is not by itself sufficient to prevent SRE transrepression and that C-terminal mutation is necessary to inhibit transrepression by N myristylation. Comparison of transfection results with Fos C-terminal deletion mutants and the Fos/FBR chimeric mutant revealed that the FBR C terminus retained the potential for transrepression despite deletion of the normal Fos C terminus, whereas similar Fos deletion mutants did not. These results indicate that both N- and C-terminal mutations are required to inhibit transrepression by FBR protein and that multiple structural mutations accompanied by posttranslational protein modification alter gene regulation by FBR protein.

Phosphorylation of v-mos Ser 47 by the mitotic form of p34cdc2.

P85gag-mos is hyperphosphorylated during mitosis in normal rat kidney (NRK) cells transformed by Moloney murine sarcoma virus ts110. We now report that P85gag-mos is phosphorylated in vitro by the mitotic form of the cdc2 kinase (p34cdc2, known as M-phase kinase) derived from virus-transformed cells. The major site of P85gag-mos phosphorylation by the M-phase kinase in vitro lies within the amino-terminal portion of the viral mos protein sequence spanning residues 45-53, as determined by tryptic peptide mapping. A synthetic peptide corresponding to amino acids 37-55 of v-mos was specifically phosphorylated by the M-phase kinase, whereas v-mos peptides either lacking Ser 47 or substituted with Ala at residue 47 were not phosphorylated. Protein sequencing analyses established that the M-phase kinase specifically phosphorylates Ser 47. Tryptic phosphopeptide mapping of the in vivo-phosphorylated gag-mos protein from mitotic cells indicated that the 45-53 v-mos region was also phosphorylated within mitotic cells. These findings demonstrate that the M-phase kinase phosphorylates the viral mos protein at Ser 47. These results were unexpected in view of earlier reports regarding cdc2 kinase activation/stabilization by the c-mos kinase in maturing oocytes.