Characterization of the major protein-tyrosine-phosphatases of human placenta

Activation of the EGF receptor signaling pathway in human airway epithelial cells exposed to metals

We have previously shown that exposure to combustion-derived metals rapidly (within 20 min) activated mitogen-activated protein kinases (MAPK), including extracellular signal-regulated kinase (ERK), in the human bronchial epithelial cell line BEAS. To study the mechanisms responsible for metal-induced activation of ERK, we examined the effect of noncytotoxic exposures to As, Cu, V, or Zn on the kinases upstream of ERK in the epidermal growth factor (EGF) receptor signaling pathway. Western blotting using phospho-specific ERK1/2 antibody demonstrated the selective MEK1/2 inhibitor PD-98059 blocked metal-induced phosphorylation of ERK1/2. Meanwhile, Western blotting using a phospho-specific MEK1/2 antibody showed that these metals induce a rapid phosphorylation of MEK1/2. Kinase activity assays confirmed the activation of MEK1/2 by metal treatment. Immunoprecipitation studies demonstrated that As, Cu, V, or Zn induces EGF receptor phosphorylation. Furthermore, the EGF receptor-specific tyrosine kinase inhibitor (PD-153035) significantly blocked the phosphorylation of MEK1/2 initiated by metals. Interestingly, we observed low levels of Raf-1 activity that were not increased by metal exposure in these cells through kinase activity assay. Finally, transfection assays showed that MEK1/2 inhibition could inhibit trans-activation of Elk1, a transcription factor in the ERK pathway, in BEAS cells exposed to metals. Together, these data demonstrate that As, Cu, V, and Zn can activate the EGF receptor signaling pathway in BEAS cells and suggest that this mechanism may be involved in pulmonary responses to metal inhalation.

Cell signaling by protein tyrosine phosphorylation

The above data, and others not described herein, indicate the following: First, that phosphatases are not scavenger enzymes, simply there to remove the phosphate groups introduced by the kinases. They cannot be viewed simply as providing an 'off' switch in an 'on/off' kinase/phosphatase system. Kinases and phosphatases do not carry out one-way and opposing reactions. The same enzyme, depending on where it localizes within the cell, or the molecule with which it might interact, can serve either as a positive or negative determinant in defining cell behavior. In many instances, it can act synergistically with the kinases to enhance the phosphorylation reaction. Second, the factors that determine whether a phosphatase would enhance or oppose a kinase reaction would seem to depend less on its state of activity than on its subcellular localization. This would suggest that if one wanted to call upon it to control transformation, one should try to tamper with its localization segments or whatever binding proteins it might be attached to--rather than with its catalytic domains. Displacement of these enzymes from where they are meant to bind would seem a more promising approach than trying to modulate their catalytic activity. Finally, their architectural features are so basically different from those of the kinases, with receptor tyrosine phosphatases displaying all the structural characteristics of cell adhesion molecules, that they must also have a mission of their own in cell development, survival and death, quite apart from that of the kinases.

The next wave: protein tyrosine phosphatases enter T cell antigen receptor signalling

Recent years have seen an exponentially increasing interest in the molecular mechanisms of signal transduction. Much of the focus has been on protein tyrosine kinase-mediated signalling, while the study of protein tyrosine phosphatases has lagged behind. We predict that the phosphatases will become a "hot topic" in the field within the next few years. This review summarizes the current state-of-the-art in our understanding of the structure, regulation and role of protein tyrosine phosphatases in T lymphocyte activation.

A histidine thiol 100 kDa, tetrameric acid phosphatase from lentil, Lens esculenta, seeds with the characteristics of protein tyrosine phosphatases

A non-specific acid phosphatase (APase) hydrolysing L-tyrosine-O-phosphate and 3'-AMP was purified to electrophoretic homogeneity from mature lentil seeds with apparent native molecular mass of 100 kDa and subunit molecular mass of 24 kDa. These activities appear to reside on the same protein which shows a single band in native and SDS-PAGE. The pH optimum is 5.5, while the K(m) (mM) and V(max) (micromoles/min/mg protein) for p-nitrophenyl phosphate (pNPP) are 0.7 and 9.2 and for L-tyrosine-O-phosphate 1.4 and 10.1, respectively, at 30 degrees C and for 3'-AMP, 2 and 4.4 at 37 degrees C. The protein also hydrolyses other phosphomonoesters to a lesser extent. L-Tyrosine-O-phosphate, 3'-AMP and pNPP hydrolysis is potently inhibited by micromolar orthovanadate and also to nearly the same extent by sodium fluoride, potassium tartrate and metal ions. Histidine and cysteine are likely to be involved in the catalysis. Thermal inactivation studies indicate that the active site conformations for pNPP and 3'-AMP hydrolytic activities are different. The enzyme shows the characteristics of the animal protein tyrosine phosphatase.

Reactive Oxygen Intermediates Enhance Fcγ Receptor Signaling and Amplify Phagocytic Capacity

Receptors for the Fc region of IgG (FcγR) mediate internalization of opsonized particles by human neutrophils (PMN) and mononuclear phagocytes. Cross-linking of FcγR leads to activation of protein tyrosine kinases and phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) within FcγR subunits, both obligatory early signals for phagocytosis. Human PMN constitutively express two structurally distinct FcγR, FcγRIIa and FcγRIIIb, and can be induced to express FcγRI by IFN-γ. We have previously shown that stimulation of PMN through FcγRIIIb results in enhanced FcγRIIa-mediated phagocytic activity that is inhibited by catalase. In the present study, we have tested the hypothesis that reactive oxygen intermediates (ROI) have the capacity to regulate FcγR responses and defined a mechanism for this effect. We show that H2O2 augmented phagocytosis mediated by FcγRIIa and FcγRI in PMN and amplified receptor-triggered tyrosine phosphorylation of FcγR-associated ITAMs and signaling elements. Generation of endogenous oxidants in PMN by cross-linking FcγRIIIb similarly enhanced phosphorylation of FcγRIIa and Syk, a tyrosine kinase required for phagocytic function, in a catalase-sensitive manner. Our results provide a mechanism for priming phagocytes for enhanced responses to receptor-driven effects. ROI generated in an inflammatory milieu may stimulate quiescent cells to rapidly increase the magnitude of their effector function. Indeed, human monocytes incubated in the presence of stimulated PMN showed oxidant-induced increases in FcγRIIa-mediated phagocytosis. Definition of the role of oxidants as amplifiers of FcγR signaling identifies a target for therapeutic intervention in immune complex-mediated tissue injury.

Human PIR1 of the protein-tyrosine phosphatase superfamily has RNA 5'-triphosphatase and diphosphatase activities

A human cDNA was isolated encoding a protein with significant sequence similarity (41% identity) to the BVP RNA 5'-phosphatase from the Autographa californica nuclear polyhedrosis virus. This protein is a member of the protein-tyrosine phosphatase (PTP) superfamily and is identical to PIR1, shown by Yuan et al. (Yuan, Y., Da-Ming, L., and Sun, H. (1998) J. Biol. Chem. 272, 20347-20353) to be a nuclear protein that can associate with RNA or ribonucleoprotein complexes. We demonstrate that PIR1 removes two phosphates from the 5'-triphosphate end of RNA, but not from mononucleotide triphosphates. The specific activity of PIR1 with RNA is several orders of magnitude greater than that with the best protein substrates examined, suggesting that RNA is its physiological substrate. A 120-amino acid segment C-terminal to the PTP domain is not required for RNA phosphatase activity. We propose that PIR1 and its closest homologs, which include the metazoan mRNA capping enzymes, constitute a subgroup of the PTP family that use RNA as a substrate.

Lipopolysaccharide-induced priming of the human neutrophil is not associated with a change in phosphotyrosine phosphatase activity

The activation of the neutrophil respiratory burst is a two-step process involving an initial 'priming' phase followed by a 'triggering' event. The biochemical mechanisms which underlie these events are yet to be fully elucidated, but the evidence suggests a crucial role for stimulus-induced tyrosine phosphorylation. The enhanced tyrosine phosphorylation observed upon triggering primed cells may reflect an increase in tyrosine kinase activity or a reduction in the levels of the opposing phosphotyrosine phosphatases (PTPases). We have investigated the latter by examining the possibility that lipopolysaccharide (LPS)-induced priming of the neutrophil respiratory burst involves the suppression of cellular PTPase activity. Purified human neutrophils were incubated for 60 min with and without LPS. Priming of the respiratory burst was confirmed by fMet-Leu-Phe-induced cytochrome c reduction. The level of PTPase activity was assessed by dephosphorylation of [32P]RR-src peptide as substrate. Pretreatment of human neutrophils with 200 ng/ml LPS induced a 2.9 +/- 0.3 (mean +/- SEM, n = 3, P = 0.022) fold increase in the fMet-Leu-Phe-triggered respiratory burst. In the same cells, LPS did not induce a significant change in the total cellular PTPase activity (1.02 +/- 0.02-fold, mean +/- SEM, n = 3, P = 0.63). Similarly, stimulation of neutrophils with fMet-Leu-Phe or phorbol myristate acetate did not significantly affect the cellular PTPase activity (P = 0.94 and 0.68, respectively). Our results suggest that suppression of PTPase activity is not the mechanism underlying the priming and/or triggering of the neutrophil respiratory burst.

Characterization of the interactions between human cdc25C, cdks, cyclins and cdk-cyclin complexes

We have overexpressed and purified human dual-specificity phosphatase cdc25C from a prokaryotic expression system at high levels and in a soluble, active form, and have studied and quantified its potential to interact with cdks, cyclins and preformed cdk-cyclin complexes by fluorescence spectroscopy and size-exclusion chromatography. Our data indicate that human cdc25C forms stable complexes, through hydrophobic contacts, with cdk and cyclin monomers, as well as with preformed cdk-cyclin complexes. In vitro, cdc25C interacts with cyclin monomers with high affinity, with tenfold less affinity with cdks, and with intermediate affinity with cdk-cyclin complexes. Moreover, changes observed in the intrinsic fluorescence of cdks, cyclins and cdk-cyclin complexes upon interaction with cdc25C are indicative of concomitant conformational changes within cdks and cyclins. From our results, we propose that in vitro, in the presence of monomeric cdks and cyclins, cdc25C forms stable ternary complexes, first through a high affinity interaction with a cyclin, which may then help target cdc25C towards a cdk. We discuss the biological relevance of our results and propose that a similar, two-step mechanism of interaction between cdc25C and cdk-cyclin complexes may occur in vivo.

A novel egg-derived tyrosine phosphatase, EDTP, that participates in the embryogenesis of Sarcophaga peregrina (flesh fly)

We have previously reported that cathepsin L mRNA is present in unfertilized eggs of Sarcophaga peregrina (flesh fly) as a maternal mRNA, which suggests that cathepsin L is required for embryogenesis. Now we have identified an egg protein, with a molecular mass of 100 kDa, that is extremely susceptible to cathepsin L digestion and which disappears rapidly as the embryos develop. We purified this protein to homogeneity, cloned its cDNA, and found that it contained a consensus sequence for the active site of tyrosine phosphatase. In fact this protein showed tyrosine phosphatase activity, indicating that it is a novel tyrosine phosphatase. The expression and subsequent disappearance of this protein, which we have named egg-derived tyrosine phosphatase (EDTP), may be indispensable for embryogenesis of Sarcophaga.

Characterization of an exocellular protein phosphatase with dual substrate specificity from the yeast Yarrowia lipolytica

In previous work, the major endocellular protein phosphatase activity has been identified in the secretory yeast Yarrowia lipolytica as a PP2A. The aim of the present work was to seek the presence of one protein phosphatase excreted in the exocellular medium and to study its activity during yeast growth in media supplemented or not supplemented with inorganic phosphate. Protein phosphatase was purified and activity was assayed by following the dephosphorylation of three substrates, [32P]casein, phosphotyrosine and a synthetic tyrosine-phosphorylated peptide. Phosphatase activity recovered in the medium after 25 h culture was greatly enhanced by Pi-deficiency. After several purification steps, the enzyme preparation presents an apparent electrophoretic homogeneity on SDS-PAGE with associated phosphoseryl/threonyl and phosphotyrosyl activities. The kinetic properties exclude contamination by a copurified protein and it is concluded that the two activities are carried by the same single proteic species. It was characterized by gel filtration as a 33 kDa protein with one single subunit demonstrated by SDS-PAGE. An absolute requirement for reducing-agents is observed suggesting that the enzyme contains at least one essential reactive cysteinyl residue. Optimum pH value is 6.1, apparent K(m) for phosphotyrosine was calculated to be 760 microM and Hill coefficient 3.2 indicating a rather high cooperativity. These results showed that the involvement of alkaline and/or acid phosphatase was unlikely. In conclusion, a protein phosphatase distinct from endocellular PP2A is secreted by Yarrowia lipolytica and characterized as a phosphotyrosine protein phosphatase with associated phosphoseryl/threonyl activity.

Role of tyrosine kinase activity of epidermal growth factor receptor in the lysophosphatidic acid-stimulated mitogen-activated protein kinase pathway

Recent evidence indicates that the epidermal growth factor (EGF) receptor mediates a branch of lysophosphatidic acid (LPA)-induced signal transduction pathways that activate mitogen-activated protein (MAP) kinase. However, it is unclear whether the intrinsic tyrosine kinase activity of EGF receptor is involved. We previously showed that reactive oxygen species (ROS) were involved in the LPA-stimulated MAP kinase pathway. Here, we identify tyrosine phosphorylation of EGF receptor as an LPA signaling step that requires ROS. To evaluate the role of the tyrosine kinase activity of EGF receptor in the LPA-stimulated MAP kinase pathway, we examined the effects of an EGF receptor-specific tyrosine kinase inhibitor, PD158780. PD158780 potently inhibited the LPA-stimulated MAP kinase kinase 1/2 (MKK1/2) activation and EGF receptor tyrosine phosphorylation in HeLa cells, while it had no detectable effect on c-Src kinase activity. PD158780 also inhibited LPA-induced MKK1/2 activation and DNA synthesis in NIH 3T3 cells. Furthermore, we compared LPA-stimulated MKK1/2 and MAP kinase activation, transcriptional activity of the c-fos promoter, and DNA synthesis in B82L cells, which lack endogenous EGF receptor, and B82L cells expressing kinase-defective or wild-type human EGF receptor. Results obtained from analysis of these cell lines suggest that the EGF receptor tyrosine kinase contributes to the LPA-stimulated MAP kinase activation, c-fos transcription, and mitogenesis.

Multiple phosphorylation of chicken protein tyrosine phosphatase 1 and human protein tyrosine phosphatase 1B by casein kinase II and p60c-src in vitro

We have cloned a soluble chicken protein tyrosine phosphatase, named CPTP1, from the cDNA library of chicken intestine. The CPTP1 showed 92% sequence identity to the corresponding 321 amino acid residues of human PTP1B (HPTP1B). CPTP1 lacked 13 amino acids of the N-terminal region compared with HPTP1B, while the C-terminal 48 amino acid sequence of this protein was distinct from those of other PTPs. In vitro phosphorylation and phosphoamino acid analysis showed that both CPTP1 and HPTP1B were phosphorylated on serine and threonine residues near their N-terminus by casein kinase II (CKII). Furthermore, phosphorylation of CPTP1 by CKII resulted in an inhibition of its phosphatase activity in vitro. Interestingly, both CPTP1 and HPTP1B were also tyrosine-phosphorylated near their N-terminus by p60c-src. When we examined the vanadate effect, in the absence of vanadate, the tyrosine-phosphorylated CPTP1 by p60c-src was autodephosphorylated by its own phosphatase activity. These results suggest that both CPTP1 and HPTP1B might play an important role in CKII- and p60c-src-induced signal transduction cascades.

Protein tyrosine phosphatase 1B antagonizes signalling by oncoprotein tyrosine kinase p210 bcr-abl in vivo

The p210 bcr-abl protein tyrosine kinase (PTK) appears to be directly responsible for the initial manifestations of chronic myelogenous leukemia (CML). In contrast to the extensive characterization of the PTK and its effects on cell function, relatively little is known about the nature of the protein tyrosine phosphatases (PTPs) that may modulate p210 bcr-abl-induced signalling. In this study, we have demonstrated that expression of PTP1B is enhanced specifically in various cells expressing p210 bcr-abl, including a cell line derived from a patient with CML. This effect on expression of PTP1B required the kinase activity of p210 bcr-abl and occurred rapidly, concomitant with maximal activation of a temperature-sensitive mutant of the PTK. The effect is apparently specific for PTP1B since, among several PTPs tested, we detected no change in the levels of TCPTP, the closest relative of PTP1B. We have developed a strategy for identification of physiological substrates of individual PTPs which utilizes substrate-trapping mutant forms of the enzymes that retain the ability to bind to substrate but fail to catalyze efficient dephosphorylation. We have observed association between a substrate-trapping mutant of PTP1B (PTP1B-D181A) and p210 bcr-abl, but not v-Abl, in a cellular context. Consistent with the trapping data, we observed dephosphorylation of p210 bcr-abl, but not v-Abl, by PTP1B in vivo. We have demonstrated that PTP1B inhibited binding of the adapter protein Grb2 to p210 bcr-abl and suppressed p210 bcr-abl-induced transcriptional activation that is dependent on Ras. These results illustrate selectivity in the effects of PTPs in a cellular context and suggest that PTP1B may function as a specific, negative regulator of p210 bcr-abl signalling in vivo.

Effect of inhibition of tyrosine phosphatases on voltage-operated calcium channel currents in rabbit isolated ear artery cells

1. The effect of increasing cellular tyrosine phosphorylation by inhibiting endogenous tyrosine phosphatases was examined on voltage-operated calcium channel currents in vascular smooth muscle cells. 2. In single ear artery smooth muscle cells of the rabbit, studied by the whole cell voltage clamp technique, intracellular application of the tyrosine phosphatase inhibitors, sodium orthovanadate (100 microM) and peroxyvanadate (100 microM orthovanadate + 1 mM H2O2) increased voltage-operated calcium channel currents by 56% and 83%, respectively. 3. Bath application of two other membrane permeant tyrosine phosphatase inhibitors, phenylarsine oxide (100 microM) and dephostatin (50 microM) also increased voltage-operated calcium channel currents by 48% and 52%, respectively. 4. The selective tyrosine kinase inhibitor, tyrphostin-23 (100 microM) reduced calcium channel currents by 41%. Pre-incubation with tyrphostin-23 abolished the effects of peroxyvanadate, phenylarsine oxide and dephostatin on calcium channels. 5. Western blot analysis of rabbit ear artery cell lysates showed increased tyrosine phosphorylation of several endogenous proteins following treatment with peroxyvanadate. 6. These results indicate that a number of structurally dissimilar inhibitors of tyrosine phosphatases increase voltage-operated calcium channel currents in arterial smooth muscle cells presumably due to increased tyrosine phosphorylation.

Protein-tyrosine phosphatases: biological function, structural characteristics, and mechanism of catalysis

The protein-tyrosine phosphatases (PTPases) superfamily consists of tyrosine-specific phosphatases, dual specificity phosphatases, and the low-molecular-weight phosphatases. They are modulators of signal transduction pathways that regulate numerous cell functions. Malfunction of PTPases have been linked to a number of oncogenic and metabolic disease states, and PTPases are also employed by microbes and viruses for pathogenicity. There is little sequence similarity among the three subfamilies of phosphatases. Yet, three-dimensional structural data show that they share similar conserved structural elements, namely, the phosphate-binding loop encompassing the PTPase signature motif (H/V)C(X)5R(S/T) and an essential general acid/base Asp residue on a surface loop. Biochemical experiments demonstrate that phosphatases in the PTPase superfamily utilize a common mechanism for catalysis going through a covalent thiophosphate intermediate that involves the nucleophilic Cys residue in the PTPase signature motif. The transition states for phosphoenzyme intermediate formation and hydrolysis are dissociative in nature and are similar to those of the solution phosphate monoester reactions. One strategy used by these phosphatases for transition state stabilization is to neutralize the developing negative charge in the leaving group. A conformational change that is restricted to the movement of a flexible loop occurs during the catalytic cycle of the PTPases. However, the relationship between loop dynamics and enzyme catalysis remains to be established. The nature and identity of the rate-limiting step in the PTPase catalyzed reaction requires further investigation and may be dependent on the specific experimental conditions such as temperature, pH, buffer, and substrate used. In-depth kinetic and structural analysis of a representative number of phosphatases from each group of the PTPase superfamily will most likely continue to yield insightful mechanistic information that may be applicable to the rest of the family members.

Properties of pervanadate and permolybdate. Connexin43, phosphatase inhibition, and thiol reactivity as model systems

Pervanadate and permolybdate are irreversible protein-tyrosine phosphatase inhibitors, with IC50 values of 0.3 and 20 microM, respectively, in intact cells. Maximal inhibition was obtained within 1 min at higher concentrations of the compounds. They induced prominent changes in the phosphorylation status of the gap junction protein, connexin43. These effects were utilized as model systems to assess the stability and inactivation of the compounds. Although the concentrated stock solutions were relatively stable, the diluted compounds were unstable. The biological activity had decreased to 20-30% after 6 h of incubation in a phosphate buffer, 1 h in phosphate buffer with 10% fetal calf serum, and 1-3 minutes in culture medium. Thiols reacted rapidly with the compounds and inactivated them (initial reaction rates with cysteine: permolybdate > pervanadate > H2O2). Catalase inactivated the compounds, and permolybdate was the more sensitive. The cells inactivated permolybdate faster than pervanadate. Cellular inactivation of permolybdate, and to a lesser degree pervanadate, appeared to be partly dependent on catalase and thiols. However, a general decrease in cellular thiols was not the mediator of the biological effects of pervanadate or permolybdate. Mathematical modeling of the thiol reactivity suggested that monoperoxovanadate at maximum could possess 20% of the biological activity of diperoxovanadate.

Brazilin inhibits activities of protein kinase C and insulin receptor serine kinase in rat liver

Hypoglycemic action of brazilin was found to be based on the improvement of peripheral glucose utility, and this action might be correlated with the insulin action pathway. In the present study we investigated the effect of brazilin on the insulin receptor autophosphorylation, protein kinase C (PKC), protein phosphatase and insulin receptor serine kinase in order to confirm whether the hypoglycemic mechanism is concerned with insulin action pathway. Brazilin was found to inhibit PKC and insulin receptor serine kinase, which are involved in the regulation of insulin signal pathway. But any significant effect was not shown on insulin receptor tyrosine kinase activity, autophosphorylation and phosphatase activity. These findings suggest that brazilin might enhance insulin receptor function by decreasing serine phosphorylation, which might mediate hypoglycemic effect of brazilin.

Epidermal growth factor receptor and the adaptor protein p52Shc are specific substrates of T-cell protein tyrosine phosphatase

T-cell protein tyrosine phosphatase (TCPTP) exists as two forms generated by alternative splicing: a 48-kDa endoplasmic reticulum (ER)-associated form (TC48) and a 45-kDa nuclear form (TC45). To identify TCPTP substrates, we have generated substrate-trapping mutants, in which the invariant catalytic acid of TCPTP (D182) is mutated to alanine. The TCPTP D182A substrate-trapping mutants were transiently overexpressed in COS cells, and their ability to form complexes with tyrosine-phosphorylated (pTyr) proteins was assessed. No pTyr proteins formed complexes with wild-type TCPTP. In contrast, TC48-D182A formed a complex in the ER with pTyr epidermal growth factor receptor (EGFR). In response to EGF, TC45-D182A exited the nucleus and accumulated in the cytoplasm, where it bound pTyr proteins of approximately 50, 57, 64, and 180 kDa. Complex formation was disrupted by vanadate, highlighting the importance of the PTP active site in the interaction and supporting the characterization of these proteins as substrates. Of these TC45 substrates, the approximately 57- and 180-kDa proteins were identified as p52Shc and EGFR, respectively. We examined the effects of TC45 on EGFR signaling and observed that it did not modulate EGF-induced activation of p42Erk2. However, TC45 inhibited the EGF-induced association of p52Shc with Grb2, which was attributed to the ability of the PTP to recognize specifically p52Shc phosphorylated on Y239. These results indicate that TC45 recognizes not only selected substrates in a cellular context but also specific sites within substrates and thus may regulate discrete signaling events.

Ascaris suum: protein phosphotyrosine phosphatases in oocytes and developing stages

Protein tyrosine phosphatases were analyzed in oocytes of Ascaris suum. Phosphatases dephosphorylating modified acidic lysozyme were present in high-molecular-weight form (M(r) > 600,000) and as a 50- to 55-kDa protein in the soluble fraction. The low-molecular-weight form of the phosphatase cross-reacted with an antiserum raised against human T-cell protein tyrosine phosphatase and was not distinguishable from the 50- to 55-kDa protein tyrosine phosphatase previously described in the muscular layer of the adult worms (B. Schmid et al. 1996, Molecular and Biochemical Parasitology 77, 183-192). The low-molecular-weight form was also present on immunoblots of high-molecular-weight protein tyrosine phosphatase preparations after denaturing electrophoresis. The same or a similar form of the tyrosine phosphatase was also found in detergent extracts from the pelletal fraction. In addition, another tyrosine phosphatase of 180 kDa molecular mass that dephosphorylated myelin basic protein was also found in extracts from the soluble compartment as well as in detergent extracts from the pelletal fraction. It showed no cross-reactivity with antisera raised against soluble mammalian phosphatases and was resistant to inhibition by vanadate. While the activities of the myelin basic protein-dephosphorylating protein phosphatase remained fairly constant during early development of the oocytes, the activity of the enzyme dephosphorylating modified lysozyme in the pelletal fraction decreased to less than 10% of the initial activity between days 3 and 28 of incubation. Immunocytochemical studies of unfertilized and developing Ascaris eggs revealed association of protein tyrosine kinase and protein tyrosine phosphatase with the egg shell, in addition to their presence in the neighborhood of mitochondria. The amount of enzyme changed with the stage of development. In the larval stage (21 days) protein tyrosine kinase had increased in the chitin layer of the shell and in the nuclei while the relative amount of tyrosine phosphatase decreased in accordance with the biochemical data.

Identification of a second aryl phosphate-binding site in protein-tyrosine phosphatase 1B: a paradigm for inhibitor design

The structure of the catalytically inactive mutant (C215S) of the human protein-tyrosine phosphatase 1B (PTP1B) has been solved to high resolution in two complexes. In the first, crystals were grown in the presence of bis-(para-phosphophenyl) methane (BPPM), a synthetic high-affinity low-molecular weight nonpeptidic substrate (Km = 16 microM), and the structure was refined to an R-factor of 18. 2% at 1.9 A resolution. In the second, crystals were grown in a saturating concentration of phosphotyrosine (pTyr), and the structure was refined to an R-factor of 18.1% at 1.85 A. Difference Fourier maps showed that BPPM binds PTP1B in two mutually exclusive modes, one in which it occupies the canonical pTyr-binding site (the active site), and another in which a phosphophenyl moiety interacts with a set of residues not previously observed to bind aryl phosphates. The identification of a second pTyr molecule at the same site in the PTP1B/C215S-pTyr complex confirms that these residues constitute a low-affinity noncatalytic aryl phosphate-binding site. Identification of a second aryl phosphate binding site adjacent to the active site provides a paradigm for the design of tight-binding, highly specific PTP1B inhibitors that can span both the active site and the adjacent noncatalytic site. This design can be achieved by tethering together two small ligands that are individually targeted to the active site and the proximal noncatalytic site.

Characterization of phosphotyrosine phosphatase activity in sheep platelets: amphiphilic and hydrophilic forms

Using O-phosphotyrosine as a substrate, we characterized the phosphotyrosine phosphatase (PTPase; protein-tyrosine-phosphate-phosphohydrolase, EC 3.1.3.48) activity from sheep platelets. PTPase was found to be located in three particulate subcellular fractions and in the cytosol, with K(m) values in the millimolar range. PTPase was strongly inhibited by vanadate, molybdate and HgCl2 and only weakly inhibited by Zn2+. Other divalent cations and NaF had no significant effect on the activity associated with the membrane fraction but were slightly stimulatory as regards cytosolic activity. Heparin inhibited cytosolic activity 2-fold more than membrane-bound activity and dithiothreitol only inhibited cytosolic PTPase. Polycationic compounds were seen to be weak stimulators of all the PTPase activity. Solubilization of the PTPase from membranes always required a detergent. When subjected to Triton X-114 phase partitioning, PTPase was recovered in the detergent-rich (35%) and in the detergent-poor (65%) phases. Sedimentation analysis of the cytosolic PTPase showed a peak of 3.2S that remained unmodified when Triton X-100 or Brij 97 sucrose gradients were used. Sedimentation analysis of the membrane-associated PTPase showed 6S and 3.7S peaks unchanged in Triton X-100 or Brij 97 gradients together with 7.5S and 10.3S shoulders that shifted to smaller sedimentation coefficients in Brij 97 sucrose gradients. These results support the view that sheep platelets contain amphiphilic and hydrophilic forms of PTPase.

Erythrocyte thiol status regulates band 3 phosphotyrosine level via oxidation/reduction of band 3-associated phosphotyrosine phosphatase

Oxidative stress-induced tyrosine phosphorylation has been ascribed to activation of phosphotyrosine kinase or to inhibition of phosphotyrosine phosphatase (PTP). We have previously identified a PTP associated with band 3 in the human erythrocyte membrane, a PTP that is normally highly active and prevents the appearance of band 3 phosphotyrosine. Here we show that treatment of erythrocytes with the thiol-oxidizing agent diamide leads to the formation of PTP disulfides (PTP-band 3 mixed disulfides) and inhibition of dephosphorylation, allowing the accumulation of band 3 phosphotyrosine. Upon reduction of the disulfides, the band 3 phosphotyrosine is dephosphorylated. Erythrocyte thiol alkylation by N-ethylmaleimide results in irreversible PTP inhibition and irreversible phosphorylation. The results are consistent with the notion that alterations in cellular thiol status affect the cell phosphotyrosine status and that oxidative stress-induced tyrosine phosphorylation involves inhibition of PTP.

Protein-tyrosine phosphatases PTP1B and syp are modulators of insulin-stimulated translocation of GLUT4 in transfected rat adipose cells

The protein-tyrosine phosphatases PTP1B and Syp have both been implicated as modulators of the mitogenic actions of insulin. However, the roles of these protein-tyrosine phosphatases in the metabolic actions of insulin are not well characterized. In this study, we directly assessed the ability of PTP1B and Syp to modulate insulin-stimulated translocation of the insulin-responsive glucose transporter GLUT4 in a physiologically relevant insulin target cell. Primary cultures of rat adipose cells were transiently transfected with either wild-type PTP1B (PTP1B-WT), wild-type Syp (Syp-WT), or the catalytically inactive mutants PTP1B-C/S or Syp-C/S. The effects of overexpression of these constructs on insulin-stimulated translocation of a co-transfected epitope-tagged GLUT4 were studied. Cells overexpressing either PTP1B-C/S or Syp-WT had insulin dose-response curves similar to those obtained with control cells expressing only epitope-tagged GLUT4. In contrast, for cells overexpressing PTP1B-WT the level of GLUT4 on the cell surface at each insulin dose (ranging from 0 to 60 nM) was significantly lower than that observed in the control cells. Interestingly, cells overexpressing the dominant inhibitory mutant Syp-C/S also had a small but statistically significant impairment in insulin responsiveness. At a maximally stimulating concentration of insulin (60 nM), cell surface epitope-tagged GLUT4 was approximately 20% less than that of the control cells. It is possible that effects from high level overexpression of Syp and PTP1B constructs may not reflect what occurs under physiological conditions. Nevertheless, our data raise the possibility that PTP1B may be a negative regulator of insulin-stimulated glucose transport, while Syp may have a small role as a positive mediator of the metabolic actions of insulin.

Development of "substrate-trapping" mutants to identify physiological substrates of protein tyrosine phosphatases

The identification of substrates of protein tyrosine phosphatases (PTPs) is an essential step toward a complete understanding of the physiological function of members of this enzyme family. PTPs are defined by a conserved catalytic domain harboring 27 invariant residues. From a mutagenesis study of these invariant residues that was guided by our knowledge of the crystal structure of PTP1B, we have discovered a mutation of the invariant catalytic acid (Asp-181 in PTP1B) that converts an extremely active enzyme into a "substrate trap." Expression of this D181A mutant of PTP1B in COS and 293 cells results in an enzyme that competes with endogenous PTP1B for substrates and promotes the accumulation of phosphotyrosine primarily on the epidermal growth factor (EGF) receptor as well as on proteins of 120, 80, and 70 kDa. The association between the D181A mutant of PTP1B and these substrates was sufficiently stable to allow isolation of the complex by immunoprecipitation. As predicted for an interaction between the substrate-binding site of PTP1B and its substrates, the complex is disrupted by vanadate and, for the EGF receptor, the interaction absolutely requires receptor autophosphorylation. Furthermore, from immunofluorescence studies, the D181A mutant of PTP1B appeared to retain the endogenous EGF receptor in an intracellular complex. These results suggest that the EGF receptor is a bona fide substrate for PTP1B in vivo and that one important function of PTP1B is to prevent the inappropriate, ligand-independent, activation of newly synthesized EGF receptor in the endoplasmic reticulum. This essential catalytic aspartate residue is present in all PTPs and has structurally equivalent counterparts in the dual-specificity phosphatases and the low molecular weight PTPs. Therefore we anticipate that this method may be widely applicable to facilitate the identification of substrates of other members of this enzyme family.

Phosphorylation of protein-tyrosine phosphatase PTP-1B on identical sites suggests activation of a common signaling pathway during mitosis and stress response in mammalian cells

PTP-1B is a widely expressed non-transmembrane tyrosine-specific phosphatase. Previous studies indicated that, at mitosis, PTP-1B undergoes phosphorylation on two sites, 352Ser-Pro-Leu-Asn and 386Ser-Pro-Ala-Lys. Although the Ser-386 site can be phosphorylated by Cyclin B/Cdc2 in vitro, the kinase for the Ser-352 site is unknown. We have found that these phosphorylation events are not unique to normal mitosis. Instead, treatment with many, but not all, stress stimuli, in particular osmotic shock and certain phosphatase and protein synthesis inhibitors, leads to phosphorylation of PTP-1B. Tryptic phosphopeptide and mutant analysis reveals that, as in mitosis, stress-induced PTP-1B phosphorylation involves both Ser-352 and Ser-386. Activation of the proline-directed kinases Erk1/2, JNKs, and p38 was neither necessary nor sufficient for stress-induced PTP-1B phosphorylation. Our data suggest the existence of a novel mitogen-activated protein kinase pathway in mammalian cells, which is activated at mitosis and in response to osmotic shock and other stresses and results in PTP-1B phosphorylation. This pathway may be similar to the recently described Spc1/Sty1 pathway in Schizosaccharomyces pombe.

Protein-tyrosine phosphatase 1B complexes with the insulin receptor in vivo and is tyrosine-phosphorylated in the presence of insulin

In response to insulin, protein-tyrosine phosphatase 1B (PTPase 1B) dephosphorylates 95- and 160-180-kDa tyrosine phosphorylated (PY) proteins (Kenner, K. A., Anyanwu, E., Olefsky, J. M., and Kusari, J. (1996) J. Biol. Chem. 271, 19810-19816). To characterize these proteins, lysates from control and insulin-treated cells expressing catalytically inactive PTPase 1B (CS) were immunoadsorbed and subsequently immunoblotted using various combinations of phosphotyrosine, PTPase 1B, and insulin receptor (IR) antibodies. Anti-PTPase 1B antibodies coprecipitated a 95-kDa PY protein from insulin-stimulated cells, subsequently identified as the IR beta-subunit. Similarly, anti-IR antibodies coprecipitated the 50-kDa PY-PTPase 1B protein from insulin-treated cells. To identify PTPase 1B tyrosine (Tyr) residues that are phosphorylated in response to insulin, three candidate sites (Tyr66, Tyr152, and Tyr153) were replaced with phenylalanine. Replacing Tyr66 or Tyr152 and Tyr153 significantly reduced insulin-stimulated PTPase 1B phosphotyrosine content, as well as its association with the IR. Studies using mutant IRs demonstrated that IR autophosphorylation is necessary for the PTPase 1B-IR interaction. These results suggest that PTPase 1B complexes with the autophosphorylated insulin receptor in intact cells, either directly or within a complex involving additional proteins. The interaction requires multiple tyrosine phosphorylation sites within both the receptor and PTPase 1B.

Mechanism of inhibition of protein-tyrosine phosphatases by vanadate and pervanadate

Vanadate and pervanadate (the complexes of vanadate with hydrogen peroxide) are two commonly used general protein-tyrosine phosphatase (PTP) inhibitors. These compounds also have insulin-mimetic properties, an observation that has generated a great deal of interest and study. Since a careful kinetic study of the two inhibitors has been lacking, we sought to analyze their mechanisms of inhibition. Our results show that vanadate is a competitive inhibitor for the protein-tyrosine phosphatase PTP1B, with a Ki of 0.38+/-0.02 microM. EDTA, which is known to chelate vanadate, causes an immediate and complete reversal of the inhibition due to vanadate when added to an enzyme assay. Pervanadate, by contrast, inhibits by irreversibly oxidizing the catalytic cysteine of PTP1B, as determined by mass spectrometry. Reducing agents such as dithiothreitol that are used in PTP assays to keep the catalytic cysteine reduced and active were found to convert pervanadate rapidly to vanadate. Under certain conditions, slow time-dependent inactivation by vanadate was observed; since catalase blocked this inactivation, it was ascribed to in situ generation of hydrogen peroxide and subsequent formation of pervanadate. Implications for the use of these compounds as inhibitors and rationalization for some of their in vivo effects are considered.

Redox regulation of cellular activation

Growing evidence has indicated that cellular reduction/oxidation (redox) status regulates various aspects of cellular function. Oxidative stress can elicit positive responses such as cellular proliferation or activation, as well as negative responses such as growth inhibition or cell death. Cellular redox status is maintained by intracellular redox-regulating molecules, including thioredoxin (TRX). TRX is a small multifunctional protein that has a redox-active disulfide/dithiol within the conserved active site sequence: Cys-Gly-Pro-Cys. Adult T cell leukemia-derived factor (ADF), which we originally defined as an IL-2 receptor alpha-chain/Tac inducer produced by human T cell lymphotrophic virus-I (HTLV-I)-transformed T cells, has been identified as human TRX. TRX/ADF is a stress-inducible protein secreted from cells. TRX/ADF has both intracellular and extracellular functions as one of the key regulators of signaling in the cellular responses against various stresses. Extracellularly, TRX/ADF shows a cytoprotective activity against oxidative stress-induced apoptosis and a growth-promoting effect as an autocrine growth factor. Intracellularly, TRX/ADF is involved in the regulation of protein-protein or protein-nucleic acid interactions through the reduction/oxidation of protein cysteine residues. For example, TRX/ADF translocates from the cytosol into the nucleus by a variety of cellular stresses, to regulate the expression of various genes through the redox factor-1 (Ref-1)/APEX. Further studies to clarify the regulatory roles of TRX/ADF and its target molecules may elucidate the intracellular signaling pathways in the responses against various stresses. The concept of "redox regulation" is emerging as an understanding of the novel mechanisms in the pathogenesis of several disorders, including viral infections, immunodeficiency, malignant transformation, and degenerative disease.

Direct binding of the proline-rich region of protein tyrosine phosphatase 1B to the Src homology 3 domain of p130(Cas)

Protein tyrosine phosphatase 1B (PTP1B) is an abundant intracellular enzyme that is thought to act as a negative regulator of certain signaling pathways. The C terminus of PTP1B contains two proline-rich regions which conform to the canonical class II Src homology 3 domain binding motif, Pro-X-X-Pro-X-Arg. In this study, we establish that PTP1B interacts with Crk, Grb2, and p130(Cas) in vitro and with at least one of these, p130(Cas), in intact cells. The interaction of PTP1B and p130(Cas) is independent of tyrosine phosphorylation but can be disrupted by replacing two critical proline residues in the proline-rich domain of PTP1B between amino acids 301 and 315. When wild-type PTP1B is expressed in 3Y1-v-crk cells, p130(Cas) shows substantial dephosphorylation, whereas the PTP1B proline mutant does not have this effect. In 3Y1 and 3Y1 v-crk-transformed fibroblasts, almost all of the total PTP1B and about 40% of total p130(Cas) co-sediment with membranes composed primarily of endoplasmic reticulum. These results suggest that the proline-rich domain between amino acids 301 and 315 in PTP1B binds Src homology 3-containing proteins and that p130(Cas) may be a physiological target of this phosphatase in cells.

Site-specific tyrosine phosphorylation of IkappaBalpha negatively regulates its inducible phosphorylation and degradation

The transcription factor NF-kappaB is retained in the cytoplasm by its interaction with the inhibitory subunit known as IkappaB. Signal-induced serine phosphorylation and subsequent ubiquitination of IkappaBalpha target it for degradation by the 26 S proteasome. Recently, pervanadate, a protein-tyrosine phosphatase inhibitor, was shown to block the degradation of IkappaBalpha, thus inhibiting NF-kappaB activation. We investigated the mechanism by which pervanadate inhibits the degradation of IkappaBalpha. Western blot analysis of IkappaBalpha from tumor necrosis factor-treated cells revealed a slower migrating IkappaBalpha species that was subsequently degraded. However, pervanadate-treated cells also revealed a slower migrating species of IkappaBalpha that appeared in a time- and dose-dependent manner and was not degraded by tumor necrosis factor. The slower migrating species of IkappaBalpha from pervanadate-treated cells was tyrosine-phosphorylated as revealed by cross-reactivity with anti-phosphotyrosine antibodies, by the ability of the specific tyrosine phosphatase PTP1B to dephosphorylate it, and by phosphoamino acid analysis of IkappaBalpha immunoprecipitated from 32P-labeled cells. By site-specific mutagenesis and deletion analysis, we identified Tyr-42 on IkappaBalpha as the phosphoacceptor site. Furthermore, in an in vitro reconstitution system, tyrosine-phosphorylated IkappaBalpha was protected from degradation. Our results demonstrate that inducible phosphorylation and degradation of IkappaBalpha are negatively regulated by phosphorylation at Tyr-42, thus preventing NF-kappaB activation.

Association of human protein-tyrosine phosphatase kappa with members of the armadillo family

We have identified a human receptor-like protein-tyrosine phosphatase (PTP) in the mammary carcinoma cell line SK-BR-3, which represents the human homolog of murine PTPkappa (Jiang, Y.-P., Wang, H., D'Eustachio, P., Musacchio, J. M., Schlessinger, J., and Sap, J. (1993) Mol. Cell. Biol. 13, 2942-2951) and was therefore termed hPTPkappa. We show here that hPTPkappa expression is dependent on cell density and find it colocalized with two members of the arm family of proteins, beta-catenin and gamma-catenin/plakoglobin, at adherens junctions. Using both in vitro and in vivo binding assays, we demonstrate specific complex formation between endogenous hPTPkappa and beta- and gamma-catenin/plakoglobin. In addition, we present evidence that suggests that beta-catenin may represent a substrate for the catalytic activity of hPTPkappa. The identification of specific binding partners for this receptor-like PTP provides insight into the mechanisms of its biological action and suggests a role for hPTPkappa in the regulation of processes involving cell contact and adhesion such as growth control, tumor invasion, and metastasis.

Protein phosphotyrosine phosphatases in Ascaris suum muscle

Two forms of protein tyrosine phosphatases were partially purified from the musculo-cutaneous layer of Ascaris suum. A 50-55-kDa soluble form of the phosphatase cross-reacted with antisera raised against human PTP-1B and TC-PTP. Like the enzyme of human origin the phosphatase from Ascaris exhibited a preference for anionic substrates (tyrosine-phosphorylated carboxymethylated and maleylated lysozyme) and was inhibited by micromolar concentrations of vanadate, molybdate, Zn2+, heparin, and poly(Glu4Tyr). As revealed by immuno-cytochemistry, the phosphatase was mainly localized and appeared equally distributed in the cytoplasm, apart from the myofibrils, possibly in loose association with cytoskeletal elements. A second tyrosine phosphatase of 180 kDa molecular mass was mainly found in detergent extracts from a microsomal fraction. It showed no cross-reactivity with antisera raised against soluble mammalian phosphatases and dephosphorylated a basic substrate (Tyr-phosphorylated myelin basic protein). It was resistant to common inhibitors of mammalian tyrosine phosphatases except Zn2+ and thiol reagents.

Thermosensitive mutants of the MPTP and hPTP1B protein tyrosine phosphatases: isolation and structural analysis

A PCR-based random mutagenesis procedure was employed to identify several thermosensitive mutants of the MPTP enzyme, the murine homologue of the human T-cell PTPase and rat PTP-S enzymes. Four mutants with varying degrees of thermosensitivity were characterized for their thermostability and refolding properties following incubation at the nonpermissive temperature. Structure analysis of these mutations based on the hPTP1B co-ordinate structure demonstrates a clear relationship between the position of each mutated amino acid relative to the catalytic cysteine residue and their thermostability. Introduction of two of these mutations in the related enzyme hPTP1B suggests that the structural defects and the resulting thermosensitivity of these mutations may represent an intrinsic property of all PTPase catalytic domains.

Surgical denervation increases protein tyrosine phosphatase activity in skeletal muscle

Protein tyrosine phosphorylation, which plays an important role in synapse formation at the neuromuscular junction, appears to be regulated by presynaptic neurons. Innervation increases whereas denervation decreases the phosphotyrosine content at the neuromuscular junction. The innervation-dependent tyrosine phosphorylation may result from elevated activity of protein tyrosine kinases; alternatively innervation may down-regulate the protein tyrosine phosphatase activity in the skeletal muscle. To investigate the possible neuronal control of protein tyrosine phosphatase activity at the neuromuscular junction, we have characterized protein tyrosine phosphatase activity in rat skeletal muscle and studied the effects of surgical denervation on the phosphatase activity. Protein tyrosine phosphatase activity in the skeletal muscle, assayed using src [32P]-phosphorylated myelin basic protein as a substrate, was both time- and protein concentration-dependent and was inhibited by micromolar concentrations of vanadate and zinc ion, both of which are known to inhibit tyrosine phosphatases specifically. It was not affected, however, by chemicals known to inhibit acid and alkaline phosphatases or serine/threonine phosphatases. Surgical denervation caused an increase in protein tyrosine phosphatase activity in rat hindlimb muscles. The increase in phosphatase activity reached a maximum (2-fold above the normal) 4 days post-denervation and maintained a plateau for up to 24 days. The biochemical properties of the phosphatase activity in denervated muscle were similar to those of the phosphatase activity in the innervated muscles. These results demonstrate that protein tyrosine phosphatase activity in skeletal muscle is regulated by motoneurons.

The active site specificity of the Yersinia protein-tyrosine phosphatase

Yersinia protein-tyrosine phosphatase substrates have been synthesized employing an expedient methodology that incorporates phosphorylated non-amino acid residues into an active site-directed peptide. While the peptidic portion of these compounds serves an enzyme targeting role, the nonpeptidic component provides a critical assessment of the range of functionality that can be accommodated within the active site region. We have found that the Yersinia phosphatase hydrolyzes both L- and D-stereoisomers of phosphotyrosine in active site-directed peptides, with the former serving as a 10-fold more efficient substrate than the latter. In addition, this enzyme catalyzes the hydrolysis of a variety of aromatic and aliphatic phosphates. Indeed, a peptide bearing the achiral phosphotyrosine analog, phosphotyramine, is not only the most efficient substrate described in this study, it is also one of the most efficient substrates ever reported for the Yersinia phosphatase. Straight chain peptide-bound aliphatic phosphates of the general structure, (Glu)4-NH-(CH2)n-OPO3(2-) (n = 2-8), are also hydrolyzed, where the most efficient substrate contains seven methylene groups. Finally, a comparison of the substrate efficacy of the peptide-bound species with that of the corresponding non-peptidic analogs, reveals that the peptide component enhances kcat/Km by up to nearly 3 orders of magnitude.

Identification of an essential acidic residue in Cdc25 protein phosphatase and a general three-dimensional model for a core region in protein phosphatases

The reaction mechanism of protein tyrosine phosphatases (PTPases) and dual-specificity protein phosphatases is thought to involve a catalytic aspartic acid residue. This residue was recently identified by site-directed mutagenesis in Yersinia PTPase, VHR protein phosphatase, and bovine low molecular weight protein phosphatase. Herein we identify aspartic acid 383 as a potential candidate for the catalytic acid in human Cdc25A protein phosphatase, using sequence alignment, structural information, and site-directed mutagenesis. The D383N mutant enzyme exhibits a 150-fold reduction in kcat, with Kw only slightly changed. Analysis of sequence homologies between several members of the Cdc25 family and deletion mutagenesis substantiate the concept of a two-domain structure for Cdc25, with a regulatory N-terminal and a catalytic C-terminal domain. Based on the alignment of catalytic residues and secondary structure elements, we present a three-dimensional model for the core region of Cdc25. By comparing this three-dimensional model to the crystal structures of PTP1b, Yersinia PTPase, and bovine low molecular weight PTPase, which share only very limited amino acid sequence similarities, we identify a general architecture of the protein phosphatase core region, encompassing the active site loop motif HCXXXXXR and the catalytic aspartic acid residue.

Characterization of a life-cycle-stage-regulated membrane protein tyrosine phosphatase in Trypanosoma brucei

We report the first characterization of plasma-membrane-bound tyrosine phosphatase activity in the haemoprotozoan. Trypanosoma brucei. Several enzymic properties of the membrane fraction were identical to other protein tyrosine phosphatases (PTPases), such as (a) insensitivity to inhibitors of other protein phosphatases, including tetramisole, sodium tartrate and okadaic acid, (b) inhibition by sodium vanadate, and (c) activation by spermidine. Additionally, T. brucei PTPase activity presented two novel features, an acidic pH optimum at pH 4.0-5.0 and a very low Km value (2.5 nM) for the specific synthetic substrate, Tyr(P)Raytide. Higher Km values of 170 nM for Tyr(P)-RCML (RCML, reduced, carboxamidomethylated and maleylated lysozyme) and of 3 mM for the non-specific inorganic substrate p-nitrophenyl phosphate, suggested that the PTPase activity of T. brucei was substrate specific. Reconstitution experiments on bloodstream-stage membrane proteins revealed that three polypeptides of 148, 115 and 72 kDa contained vanadate-inhibitable PTPase activity. Modulator assays revealed that the 72-kDa protein was responsible for the observed spermidine stimulation, but indicated that the modulator profile of the 148-kDa protein was most similar to the whole membrane fraction. Furthermore, the PTPase activity of T. brucei was life-cycle-stage regulated. Neither the whole membrane fraction nor the reconstituted proteins of the procyclic insect stage dephosphorylated tyrosine residues.

Protein tyrosine phosphatases take off

Protein tyrosine phosphatases (PTPs) are a family of signal transduction enzymes that dephosphorylate phosphotyrosine containing proteins. Structural and kinetic studies provide a molecular understanding of how these enzymes regulate a wide range of intracellular processes.

A heteromorphic protein-tyrosine phosphatase, PTP phi, is regulated by CSF-1 in macrophages

A novel protein-tyrosine phosphatase, PTP phi, was cloned from a murine macrophage cDNA library. As a result of alternative splicing, macrophage PTP phi mRNAs are predicted to encode two membrane-spanning molecules and a cytosolic enzyme with identical catalytic domains. The membrane-spanning forms differ in the juxtamembrane region, while a start codon downstream of this region is utilized in the translation of the putative cytosolic form. Expression of PTP phi mRNA is low and restricted to macrophage cell lines, macrophage-rich tissues, and brain, kidney, and heart. The mRNA in macrophages and heart is approximately 2.8 kilobases (kb). However, a approximately 5.5-kb transcript in brain and kidney indicates a fourth isoform encoding a large extracellular domain. The approximately 5.5-kb PTP phi brain mRNA encodes the mouse homolog of GLEPP1, a recently reported glomerular epithelial protein. The level of expression of the mRNA encoding the cytosolic form was very low, and only the membrane-spanning proteins (43 and 47 kDa) could be detected in macrophages. Following addition of colony stimulating factor-1 to quiescent BAC1.2F5 macrophages, PTP phi mRNA and protein were down-regulated. The restricted expression of the shorter isoforms of PTP phi and their regulation by colony stimulating factor-1 in macrophages suggest that PTP phi may play a role in mononuclear phagocyte survival, proliferation, and/or differentiation.

Arrest at the G2/M transition of the cell cycle by protein-tyrosine phosphatase inhibition: studies on a neuronal and a glial cell line

The addition of the peroxovanadium (pV) derivatives potassium bisperoxo(1,10-phenanthroline)oxovanadate(v) (bpV[phen]) or potassium bisperoxo(pyridine-2-carboxylato) oxovanadate(v) (bpV[pic]), both of which are potent inhibitors of protein tyrosine phosphatases (PTPs) [Posner et al. (1994): J Biol Chem 269:4596-4604], to the culture medium of neuroblastoma NB 41 and glioma C6 cells resulted in a marked decrease in their proliferation rates and a progressive accumulation at the G2/M transition of the cell cycle. The effect was dependent on dose, cell type, and a pV compound employed. Mean values of the RNA-to-DNA and RNA-to-protein ratios in NB cells treated for 48 h with increased doses of bpV[phen] showed that general synthetic functions were not altered, nor did we observe oxidative damage to DNA using a sensitive DNA-nick detection assay. No changes in the expression and localization of vimentin, a component of the intermediate filament cytoskeleton, were observed by indirect immunofluorescence, showing that treatment did not disturb the cytoskeleton network. Measurements of BrdU incorporation into newly synthesized DNA showed that cells treated were not totally arrested. Furthermore, cells arrested G2/M were able to reenter the cycle rapidly after the release of inhibition. This progressive accumulation of G2/M coincided with the detection of tyrosine-phosphorylated p34cdc2 and a dramatic reduction in its kinase activity toward histone H1 by 48 h of culture. Both compounds were equally potent in inhibiting the catalytic activity of a yeast and the structurally distant mouse cdc25B in vitro, suggesting that augmented tyrosine phosphorylation of p34cdc2 derived from the in vivo inhibition of cdc25. Their equal in vitro potency contrasted with the considerably greater potency of bpV[phen] in vivo, in vivo suggesting that factors regulating the intracellular access of these compounds to cdc25 might be critical in determining in vivo specificity. In conclusion the final consequence of long-term exposure to potent and structurally defined PTP inhibitors on two highly proliferative nerve cell lines is to restrict cell growth. The corresponding hyperphosphorylation and reduced activity of p34cdc2 likely reflects the unusual sensitivity of cdc25 as an in vivo target for peroxovanadium compounds.

The yeast immunophilin Fpr3 is a physiological substrate of the tyrosine-specific phosphoprotein phosphatase Ptp1

The tyrosine-specific phosphoprotein phosphatase encoded by the Saccharomyces cerevisiae PTP1 gene dephosphorylates artificial substrates in vitro, but little is known about its functions and substrates in vivo. The presence of Ptp1 resulted in dephosphorylation of multiple tyrosine-phosphorylated proteins in yeast expressing a heterologous tyrosine-specific protein kinase, indicating that Ptp1 can dephosphorylate a broad range of substrates in vivo. Correspondingly, several proteins phosphorylated at tyrosine by endogenous protein kinases exhibited a marked increase in tyrosine phosphorylation in ptp1 mutant cells. One of these phosphotyrosyl proteins (p70) was also dephosphorylated in vitro when incubated with recombinant Ptp1. p70 was purified to homogeneity; analysis of four tryptic peptides revealed that p70 is identical to the recently described FPR3 gene product, a nucleolarly localized proline rotamase of the FK506- and rapamycin-binding family. The identity of p70 with Fpr3 was confirmed in the demonstration that the abundance of tyrosine-phosphorylated p70 in ptp1 mutants was strictly correlated with the level of FPR3 expression; immobilized phosphotyrosyl Fpr3 was directly dephosphorylated by recombinant Ptp1. Site-directed mutagenesis demonstrated that the site of tyrosine phosphorylation is Tyr-184, which resides within the nucleolin-like amino-terminal domain of Fpr3. Protein kinase activities from yeast cell extracts can bind to and phosphorylate the immobilized amino-terminal domain of Fpr3 on serine, threonine, and tyrosine. Fpr3 represents the first phosphotyrosyl protein identified in S. cerevisiae that is not itself a protein kinase and is as yet the only known physiological substrate of Ptp1.

Identification of a putative Syp substrate, the PDGF beta receptor

Because the protein-tyrosine phosphatase (PTP) Syp associates with the tyrosine-phosphorylated platelet-derived growth factor beta receptor (beta PDGFR), the beta PDGFR is a likely Syp substrate. We tested this hypothesis by determining whether recombinant Syp (rSyp) and a control PTP, recombinant PTP1B (rPTP1B), were able to dephosphorylate the beta PDGFR. The beta PDGFR was phosphorylated at multiple tyrosine residues in an in vitro kinase assay and then incubated with increasing concentrations of rSyp or rPTP1B. While the receptor was nearly completely dephosphorylated by high concentrations of rPTP1B, receptor dephosphorylation by rSyp plateaued at approximately 50%. Two-dimensional phosphopeptide maps of the beta PDGFR demonstrated that rSyp displayed a clear preference for certain receptor phosphorylation sites; the most efficiently dephosphorylated sites were phosphotyrosines (Tyr(P)-771 and -751, followed by Tyr(P)740, while Tyr(P)-1021 and Tyr(P)-1009 were very poor substrates. In contrast, rPTP1B displayed no selectivity for the various rPTP1B displayed no selectivity for the various beta PDGFR tyrosine phosphorylation sites and dephosphorylated all of them with comparable efficiency. A Syp construct that lacked the SH2 domains was still able to discriminate between the various receptor phosphorylation sites, although less effectively than full-length Syp. These in vitro studies predicted that Syp can dephosphorylate the receptor in vivo. Indeed, we found that a beta PDGFR mutant (F1009) that associates poorly with Syp, had a much slower in vivo rate of receptor dephosphorylation than the wild type receptor. In addition, the GTPase-activating protein of Ras (GAP) and phosphatidylinositol 3-kinase were less stably associated with the wild type beta PDGFR than with the F1009 receptor. These findings are consistent with the in vitro experiments showign that Syp prefers to dephosphorylate sites on the beta PDGFR, that are important for binding phosphatidylinositol 3-kinase (Tyr(P)-740 and Tyr(P)-751) and GAP (Tyr(P)-771). These studies reveal that Syp is a substrate-selective PTP and that both the catalytic domain and the SH2 domains contribute to Syp's ability to choose substrates. Furthermore, it appears that Syp plays a role in PDGF-dependent intracellular signal relay by selectively dephosphorylating the beta PDGFR and thereby regulating the binding of a distinct group of receptor-associated signal relay enzymes.

RK-682, a potent inhibitor of tyrosine phosphatase, arrested the mammalian cell cycle progression at G1phase

A specific inhibitor of protein tyrosine phosphatase (PTPase), RK-682 (3-hexadecanoyl-5-hydroxymethyl-tetronic acid) was isolated from microbial metabolites. In vitro, RK-682 inhibited dephosphorylation activity of CD45 and VHR with IC50 54 and 2.0 microM, respectively. In situ, sodium orthovanadate and RK-682 enhanced the phosphotyrosine level of Ball-1 cells, a human B cell leukemia, but not the phosphoserine/threonine level. The PTPase inhibitors, however, had the different arrest point on the cell cycle progression. Sodium orthovanadate inhibited the cell cycle progression at G2/M boundary phase, on the other hand, RK-682 inhibited the G1/S transition.

Association of SH2 domain protein tyrosine phosphatases with the epidermal growth factor receptor in human tumor cells. Phosphatidic acid activates receptor dephosphorylation by PTP1C

The SH2 domain protein tyrosine phosphatases (PTPases) PTP1C and PTP1D were found associated with epidermal growth factor (EGF) receptor which was purified from A431 cell membranes by several steps of chromatography. Both PTPases also associated with the EGF receptor upon exposure of immunoprecipitated receptor to lysates of MCF7 mammary carcinoma cells. The associated PTPases had little activity toward the bound receptor when it was autophosphorylated in vitro. Receptor dephosphorylation could, however, be initiated by treatment of the receptor-PTPase complex with phosphatidic acid (PA). When autophosphorylated EGF receptor was exposed to lysates of PTP1C or PTP1D overexpressing 293 cells, the association of PTP1C but not of PTP1D was enhanced in the presence of PA. In intact A431 cells, an association of PTP1C and PTP1D with the EGF receptor was detectable by coimmunoprecipitation experiments. PA treatment reduced the phosphorylation state of ligand activated EGF receptors in A431 cells and in 293 cells overexpressing EGF receptors together with PTP1C but not in 293 cells overexpressing EGF receptors alone or together with PTP1D. We conclude that PTP1C but not PTP1D participates in dephosphorylation of activated EGF receptors. A possible role of PA for physiological modulation of EGF receptor signaling is discussed.