Kinetic analysis of two closely related receptor-like protein-tyrosine-phosphatases, PTP alpha and PTP epsilon

Clustering of phosphatase RPTPα promotes Src signaling and the arthritogenic action of synovial fibroblasts

Receptor-type protein phosphatase α (RPTPα) promotes fibroblast-dependent arthritis and fibrosis, in part, by enhancing the activation of the kinase SRC. Synovial fibroblasts lining joint tissue mediate inflammation and tissue damage, and their infiltration into adjacent tissues promotes disease progression. RPTPα includes an ectodomain and two intracellular catalytic domains (D1 and D2) and, in cancer cells, undergoes inhibitory homodimerization, which is dependent on a D1 wedge motif. Through single-molecule localization and labeled molecule interaction microscopy of migrating synovial fibroblasts, we investigated the role of RPTPα dimerization in the activation of SRC, the migration of synovial fibroblasts, and joint damage in a mouse model of arthritis. RPTPα clustered with other RPTPα and with SRC molecules in the context of actin-rich structures. A known dimerization-impairing mutation in the wedge motif (P210L/P211L) and the deletion of the D2 domain reduced RPTPα-RPTPα clustering; however, it also unexpectedly reduced RPTPα-SRC association. The same mutations also reduced recruitment of RPTPα to actin-rich structures and inhibited SRC activation and cellular migration. An antibody against the RPTPα ectodomain that prevented the clustering of RPTPα also inhibited RPTPα-SRC association and SRC activation and attenuated fibroblast migration and joint damage in arthritic mice. A catalytically inactivating RPTPα-C469S mutation protected mice from arthritis and reduced SRC activation in synovial fibroblasts. We conclude that RPTPα clustering retains it to actin-rich structures to promote SRC-mediated fibroblast migration and can be modulated through the extracellular domain.

Macromolecular crowding amplifies allosteric regulation of T-cell protein tyrosine phosphatase

T-cell protein tyrosine phosphatase (TC-PTP) is a negative regulator of T-cell receptor and oncogenic receptor tyrosine kinase signaling and implicated in cancer and autoimmune disease. TC-PTP activity is modulated by an intrinsically disordered C-terminal region (IDR) and suppressed in cells under basal conditions. In vitro structural studies have shown that the dynamic reorganization of IDR around the catalytic domain, driven by electrostatic interactions, can lead to TC-PTP activity inhibition; however, the process has not been studied in cells. Here, by assessing a mutant (³⁷⁸KRKRPR³⁸³ mutated into ³⁷⁸EAAAPE³⁸³, called TC45^E/A) with impaired tail-PTP domain interaction, we obtained evidence that the downmodulation of TC-PTP enzymatic activity by the IDR occurs in cells. However, we found that the regulation of TC-PTP by the IDR is only recapitulated in vitro when crowding polymers that mimic the intracellular environment are present in kinetic assays using a physiological phosphopeptide. Our FRET-based assays in vitro and in cells confirmed that the effect of the mutant correlates with an impairment of the intramolecular inhibitory remodeling of TC-PTP by the IDR. This work presents an early example of the allosteric regulation of a protein tyrosine phosphatase being controlled by the cellular environment and provides a framework for future studies and targeting of TC-PTP function.

RPTPα phosphatase activity is allosterically regulated by the membrane-distal catalytic domain

Receptor-type protein tyrosine phosphatase α (RPTPα) is an important positive regulator of SRC kinase activation and a known promoter of cancer growth, fibrosis, and arthritis. The domain structure of RPTPs comprises an extracellular region, a transmembrane helix, and two tandem intracellular catalytic domains referred to as D1 and D2. The D2 domain of RPTPs is believed to mostly play a regulatory function; however, no regulatory model has been established for RPTPα-D2 or other RPTP-D2 domains. Here, we solved the 1.8 Å resolution crystal structure of the cytoplasmic region of RPTPα, encompassing D1 and D2, trapped in a conformation that revealed a possible mechanism through which D2 can allosterically inhibit D1 activity. Using a D2-truncation RPTPα variant and mutational analysis of the D1/D2 interfaces, we show that D2 inhibits RPTPα phosphatase activity and identified a ⁴⁰⁵PFTP⁴⁰⁸ motif in D1 that mediates the inhibitory effect of D2. Expression of the gain-of-function F406A/T407A RPTPα variant in HEK293T cells enhanced SRC activation, supporting the relevance of our proposed D2-mediated regulation mechanism in cell signaling. There is emerging interest in the development of allosteric inhibitors of RPTPs but a scarcity of validated allosteric sites for RPTPs. The results of our study not only shed light on the regulatory role of RPTP-D2 domains, but also provide a potentially useful tool for the discovery of chemical probes targeting RPTPα and other RPTPs.

Tuning the Protein Phosphorylation by Receptor Type Protein Tyrosine Phosphatase Epsilon (PTPRE) in Normal and Cancer Cells

Tyrosine phosphorylation is an important post-translation modification of proteins that is controlled by tyrosine kinases and phosphatases. Disruption of the balance between the activity of tyrosine kinases and phosphatases may result in diseases. Receptor type protein tyrosine phosphatase epsilon (PTPRE) is closely related with receptor type protein tyrosine phosphatase alpha (PTPRA). PTPRE has been studied in osteoclast cells, nerve cells, hematopoietic cells, cancer cells and others, and it has different functions among various tissues. In this review, we summarized the current knowledge about the regulation of PTPRE on cellular signal transduction and its function under normal and pathological conditions.

High-resolution crystal structures of the D1 and D2 domains of protein tyrosine phosphatase epsilon for structure-based drug design

Here, new crystal structures are presented of the isolated membrane-proximal D1 and distal D2 domains of protein tyrosine phosphatase epsilon (PTPℇ), a protein tyrosine phosphatase that has been shown to play a positive role in the survival of human breast cancer cells. A triple mutant of the PTPℇ D2 domain (A455N/V457Y/E597D) was also constructed to reconstitute the residues of the PTPℇ D1 catalytic domain that are important for phosphatase activity, resulting in only a slight increase in the phosphatase activity compared with the native D2 protein. The structures reported here are of sufficient resolution for structure-based drug design, and a microarray-based assay for high-throughput screening to identify small-molecule inhibitors of the PTPℇ D1 domain is also described.

A 1-phytase type III effector interferes with plant hormone signaling

Most Gram-negative phytopathogenic bacteria inject type III effector (T3E) proteins into plant cells to manipulate signaling pathways to the pathogen's benefit. In resistant plants, specialized immune receptors recognize single T3Es or their biochemical activities, thus halting pathogen ingress. However, molecular function and mode of recognition for most T3Es remains elusive. Here, we show that the Xanthomonas T3E XopH possesses phytase activity, i.e., dephosphorylates phytate (myo-inositol-hexakisphosphate, InsP6), the major phosphate storage compound in plants, which is also involved in pathogen defense. A combination of biochemical approaches, including a new NMR-based method to discriminate inositol polyphosphate enantiomers, identifies XopH as a naturally occurring 1-phytase that dephosphorylates InsP6 at C1. Infection of Nicotiana benthamiana and pepper by Xanthomonas results in a XopH-dependent conversion of InsP6 to InsP5. 1-phytase activity is required for XopH-mediated immunity of plants carrying the Bs7 resistance gene, and for induction of jasmonate- and ethylene-responsive genes in N. benthamiana.

Role of PTPα in the destruction of periodontal connective tissues

IL-1β contributes to connective tissue destruction in part by up-regulating stromelysin-1 (MMP-3), which in fibroblasts is a focal adhesion-dependent process. Protein tyrosine phosphatase-α (PTPα) is enriched in and regulates the formation of focal adhesions, but the role of PTPα in connective tissue destruction is not defined. We first examined destruction of periodontal connective tissues in adult PTPα^+/+ and PTPα^−/− mice subjected to ligature-induced periodontitis, which increases the levels of multiple cytokines, including IL-1β. Three weeks after ligation, maxillae were processed for morphometry, micro-computed tomography and histomorphometry. Compared with unligated controls, there was ∼1.5–3 times greater bone loss as well as 3-fold reduction of the thickness of the gingival lamina propria and 20-fold reduction of the amount of collagen fibers in WT than PTPα^−/− mice. Immunohistochemical staining of periodontal tissue showed elevated expression of MMP-3 at ligated sites. Second, to examine mechanisms by which PTPα may regulate matrix degradation, human MMP arrays were used to screen conditioned media from human gingival fibroblasts treated with vehicle, IL-1β or TNFα. Although MMP-3 was upregulated by both cytokines, only IL-1β stimulated ERK activation in human gingival fibroblasts plated on fibronectin. TIRF microscopy and immunoblotting analyses of cells depleted of PTPα activity with the use of various mutated constructs or with siRNA or PTPα^KO and matched wild type fibroblasts were plated on fibronectin to enable focal adhesion formation and stimulated with IL-1β. These data showed that the catalytic and adaptor functions of PTPα were required for IL-1β-induced focal adhesion formation, ERK activation and MMP-3 release. We conclude that inflammation-induced connective tissue degradation involving fibroblasts requires functionally active PTPα and in part is mediated by IL-1β signaling through focal adhesions.

Receptor-like protein-tyrosine phosphatase α enhances cell surface expression of neural adhesion molecule NB-3

Neural adhesion molecule NB-3 plays an important role in the apical dendrite development of layer V pyramidal neurons in the visual cortex, and receptor-like protein-tyrosine phosphatase α (PTPα) mediates NB-3 signaling in this process. Here we investigated the role of PTPα in regulating cell surface expression of NB-3. We found that cortical neurons from PTPα knock-out mice exhibited a lower level of NB-3 at the cell surface. When expressed in COS1 cells, NB-3 was enriched in the Golgi apparatus with a low level of cell surface expression. However, co-expression of PTPα increased the cell surface distribution of NB-3. Further analysis showed that PTPα facilitated Golgi exit of NB-3 and stabilized NB-3 protein at the cell surface by preventing its release from the plasma membrane. The extracellular region of PTPα but not its catalytic activity is necessary for its effect on NB-3 expression. Thus, the PTPα-mediated increase of NB-3 level at the cell surface represents a novel function of PTPα in NB-3 signaling in neural development.

Serine dephosphorylation of receptor protein tyrosine phosphatase alpha in mitosis induces Src binding and activation

Receptor protein tyrosine phosphatase alpha (RPTPalpha) is the mitotic activator of the protein tyrosine kinase Src. RPTPalpha serine hyperphosphorylation was proposed to mediate mitotic activation of Src. We raised phosphospecific antibodies to the two main serine phosphorylation sites, and we discovered that RPTPalpha Ser204 was almost completely dephosphorylated in mitotic NIH 3T3 and HeLa cells, whereas Ser180 and Tyr789 phosphorylation were only marginally reduced in mitosis. Concomitantly, Src pTyr527 and pTyr416 were dephosphorylated, resulting in 2.3-fold activation of Src in mitosis. Using inhibitors and knockdown experiments, we demonstrated that dephosphorylation of RPTPalpha pSer204 in mitosis was mediated by PP2A. Mutation of Ser204 to Ala did not activate RPTPalpha, and intrinsic catalytic activity of RPTPalpha was not affected in mitosis. Interestingly, binding of endogenous Src to RPTPalpha was induced in mitosis. GRB2 binding to RPTPalpha, which was proposed to compete with Src binding to RPTPalpha, was only modestly reduced in mitosis, which could not account for enhanced Src binding. Moreover, we demonstrate that Src bound to mutant RPTPalpha-Y789F, lacking the GRB2 binding site, and mutant Src with an impaired Src homology 2 (SH2) domain bound to RPTPalpha, illustrating that Src binding to RPTPalpha is not mediated by a pTyr-SH2 interaction. Mutation of RPTPalpha Ser204 to Asp, mimicking phosphorylation, reduced coimmunoprecipitation with Src, suggesting that phosphorylation of Ser204 prohibits binding to Src. Based on our results, we propose a new model for mitotic activation of Src in which PP2A-mediated dephosphorylation of RPTPalpha pSer204 facilitates Src binding, leading to RPTPalpha-mediated dephosphorylation of Src pTyr527 and pTyr416 and hence modest activation of Src.

Catalytically active membrane-distal phosphatase domain of receptor protein-tyrosine phosphatase alpha is required for Src activation

Receptor protein-tyrosine phosphatase alpha (RPTPalpha) is a transmembrane protein with tandem cytoplasmic phosphatase domains. Most of the catalytic activity is contained by the membrane-proximal catalytic domain (D1). We found a spontaneous Arg554 to His mutation in the pTyr recognition loop of the membrane-distal phosphatase domain (D2) of a human patient. This mutation was not linked to the disease. Here, we report that the R554H mutation abolished RPTPalpha-D2 catalytic activity. The R554H mutation impaired Src binding to RPTPalpha. RPTPalpha, with a catalytic site cysteine to serine mutation in D2, also displayed diminished binding to Src. Concomitant with decreased Src binding of the R554H and C723S mutants compared with wild-type RPTPalpha, enhanced phosphorylation of the inhibitory Src Tyr527 site was observed, as well as reduced Src activation. To confirm that catalytic activity of RPTPalpha-D2 was required for these effects, we analyzed a third mutant, RPTPalpha-R729K, which had an inactive D2. Again, Src binding was reduced and Tyr527 phosphorylation was enhanced. Our results suggest that a catalytically active D2 is required for RPTPalpha to bind and dephosphorylate its well-characterized substrate, Src.

Protein-tyrosine phosphatase-alpha and Src functionally link focal adhesions to the endoplasmic reticulum to mediate interleukin-1-induced Ca2+ signaling

Calcium (Ca2+) signaling by the pro-inflammatory cytokine interleukin-1 (IL-1) is dependent on focal adhesions, which contain diverse structural and signaling proteins including protein phosphatases. We examined here the role of protein-tyrosine phosphatase (PTP) alpha in regulating IL-1-induced Ca2+ signaling in fibroblasts. IL-1 promoted recruitment of PTPalpha to focal adhesions and endoplasmic reticulum (ER) fractions, as well as tyrosine phosphorylation of the ER Ca2+ release channel IP3R. In response to IL-1, catalytically active PTPalpha was required for Ca2+ release from the ER, Src-dependent phosphorylation of IP3R1 and accumulation of IP3R1 in focal adhesions. In pulldown assays and immunoprecipitations PTPalpha was required for the association of PTPalpha with IP3R1 and c-Src, and this association was increased by IL-1. Collectively, these data indicate that PTPalpha acts as an adaptor to mediate functional links between focal adhesions and the ER that enable IL-1-induced Ca2+ signaling.

Functions and mechanisms of redox regulation of cysteine-based phosphatases

Reactive oxygen species (ROS) have been implicated as mediators of cell-signaling responses, particularly in pathways involving protein tyrosine phosphorylation. One mechanism by which ROS are thought to exert their effects is through the reversible regulation of cysteine-based phosphatases (CBPs). The CBPs, which include protein tyrosine phosphatases (PTPs), dual-specificity phosphatases, low-molecular-weight PTPs, and the lipid phosphatase PTEN, all contain a nucleophilic catalytic cysteine within a conserved motif that enables these enzymes to dephosphorylate phosphoproteins or phospholipids. In addition to enabling phosphatase activity, the nucleophilic catalytic cysteines of CBPs are also highly susceptible to oxidation, a property that permits redox regulation of this enzyme family. In this review, we discuss the evidence implicating ROS as mediators of CBP activity within signaling pathways and discuss how specificity of ROS-dependent signaling involving CBPs may be achieved. We also discuss the molecular mechanisms that facilitate the stabilization of a reversibly oxidized form of the catalytic cysteine. These mechanisms include the formation of disulfide bonds or the formation of a sulfenamide bond, a novel mechanism that was identified for PTP1B. Formation of either type of covalent bond may be accompanied by dramatic structural rearrangements that can affect downstream signaling events and allow for multitiered enzyme regulation.

Expression, purification, and characterization of rat protein tyrosine phosphatase η catalytic domain

Receptor-like protein tyrosine phosphatases generally contain one or two conserved intracellular catalytic domains with a conserved sequence motif ([I/V]HCXAGXXR[S/T]G), a single transmembrane domain, and an external highly variable part. Here, we describe cloning of the intracellular catalytic domain of the rat protein tyrosine phosphatase eta (rPTPetaCD) into pET28a(+) vector, its expression in Escherichia coli, purification and initial characterization. The purification of His6-tagged rPTPetaCD to near homogeneity was achieved by a combination of affinity and size exclusion chromatography. The His-tag was subsequently removed by thrombin digestion. PhastGel IEF electrophoresis demonstrated that the isoelectric point of this 41 kDa His6-tag free recombinant protein was 7.3, which is just slightly higher than the theoretically predicted value of 7.2. To assess the functionality of the rPTPetaCD we used the pNPP hydrolysis assay and observed that the enzyme has a specific activity of 9 nmol/min/mug. The secondary structure and stability of the recombinant protein was also analyzed by circular dichroism and fluorescence spectroscopy. In summary, the rPTPetaCD is stable at 18 degrees C, properly folded, and fully active, which makes it a suitable candidate for structural and functional studies.

Dimerization in vivo and inhibition of the nonreceptor form of protein tyrosine phosphatase epsilon

cyt-PTP epsilon is a naturally occurring nonreceptor form of the receptor-type protein tyrosine phosphatase (PTP) epsilon. As such, cyt-PTP epsilon enables analysis of phosphatase regulation in the absence of extracellular domains, which participate in dimerization and inactivation of the receptor-type phosphatases receptor-type protein tyrosine phosphatase alpha (RPTPalpha) and CD45. Using immunoprecipitation and gel filtration, we show that cyt-PTP epsilon forms dimers and higher-order associations in vivo, the first such demonstration among nonreceptor phosphatases. Although cyt-PTP epsilon readily dimerizes in the absence of exogenous stabilization, dimerization is increased by oxidative stress. Epidermal growth factor receptor stimulation can affect cyt-PTP epsilon dimerization and tyrosine phosphorylation in either direction, suggesting that cell surface receptors can relay extracellular signals to cyt-PTP epsilon, which lacks extracellular domains of its own. The inactive, membrane-distal (D2) phosphatase domain of cyt-PTP epsilon is a major contributor to intermolecular binding and strongly interacts in a homotypic manner; the presence of D2 and the interactions that it mediates inhibit cyt-PTP epsilon activity. Intermolecular binding is inhibited by the extreme C and N termini of D2. cyt-PTP epsilon lacking these regions constitutively dimerizes, and its activities in vitro towards para-nitrophenylphosphate and in vivo towards the Kv2.1 potassium channel are markedly reduced. We conclude that physiological signals can regulate dimerization and phosphorylation of cyt-PTP epsilon in the absence of direct interaction between the PTP and extracellular molecules. Furthermore, dimerization can be mediated by the D2 domain and does not strictly require the presence of PTP extracellular domains.

Enzyme kinetic characterization of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) play a central role in cellular signaling processes, resulting in an increased interest in modulating the activities of PTPs. We therefore decided to undertake a detailed enzyme kinetic evaluation of various transmembrane and cytosolic PTPs (PTPalpha, PTPbeta, PTPepsilon, CD45, LAR, PTP1B and SHP-1), using pNPP as substrate. Most noticeable is the increase in the turnover number for PTPbeta with increasing pH and the weak pH-dependence of the turnover number of CD45. The kinetic data for PTPalpha-D1 and PTPalpha-D1D2 suggest that D2 affects the catalysis of pNPP. PTPepsilon and the closely homologous PTPalpha behave differently. The K(m) data were lower for PTPepsilon than those for PTPalpha, while the inverse was observed for the catalytic efficiencies.

PTP alpha regulates integrin-stimulated FAK autophosphorylation and cytoskeletal rearrangement in cell spreading and migration

We investigated the molecular and cellular actions of receptor protein tyrosine phosphatase (PTP) alpha in integrin signaling using immortalized fibroblasts derived from wild-type and PTP alpha-deficient mouse embryos. Defects in PTP alpha-/- migration in a wound healing assay were associated with altered cell shape and focal adhesion kinase (FAK) phosphorylation. The reduced haptotaxis to fibronectin (FN) of PTP alpha-/- cells was increased by expression of active (but not inactive) PTP alpha. Integrin-mediated formation of src-FAK and fyn-FAK complexes was reduced or abolished in PTP alpha-/- cells on FN, concomitant with markedly reduced phosphorylation of FAK at Tyr397. Reintroduction of active (but not inactive) PTP alpha restored FAK Tyr-397 phosphorylation. FN-induced cytoskeletal rearrangement was retarded in PTP alpha-/- cells, with delayed filamentous actin stress fiber assembly and focal adhesion formation. This mimicked the effects of treating wild-type fibroblasts with the src family protein tyrosine kinase (Src-PTK) inhibitor PP2. These results, together with the reduced src/fyn tyrosine kinase activity in PTP alpha-/- fibroblasts (Ponniah et al., 1999; Su et al., 1999), suggest that PTP alpha functions in integrin signaling and cell migration as an Src-PTK activator. Our paper establishes that PTP alpha is required for early integrin-proximal events, acting upstream of FAK to affect the timely and efficient phosphorylation of FAK Tyr-397.

Regulation of protein-tyrosine phosphatases alpha and epsilon by calpain-mediated proteolytic cleavage

The precise subcellular localization of non-receptor tyrosine phosphatases is a major factor in regulating their physiological functions. We have previously shown that cellular processing of protein-tyrosine phosphatase epsilon (PTP epsilon) generates a physiologically distinct, cytoplasmic form of this protein, p65 PTP epsilon. Here we describe a novel protein form of the related receptor-type tyrosine phosphatase alpha (RPTP alpha), p66 PTP alpha, which is detected in nearly all cell types where RPTP alpha is expressed. Both p66 PTP alpha and p65 PTP epsilon are produced by calpain-mediated proteolytic cleavage in vivo. Cleavage is inhibited in living cells by a variety of calpain inhibitors, can be induced in primary cortical neurons treated with calcium chloride, and is observed in lysates of brain or of cultured cells following addition of purified calpain. Cleavage occurs within the intracellular juxtamembrane domain of RPTP alpha, releasing the phosphatase catalytic domains from their membranal anchors and translocating them to the cytoplasm. Translocation reduces the ability of PTPalpha to act on membrane-associated substrates, as it loses its ability to dephosphorylate Src at its C-terminal regulatory site, and its ability to dephosphorylate the Kv2.1 voltage-gated potassium channel is severely impaired. In all, the data indicate that control of phosphatase function via post-translational processing occurs also among receptor-type phosphatases, and demonstrate the molecular complexity of regulating these parameters within the PTP alpha/PTP epsilon phosphatase subfamily.

PTP-epsilon, a tyrosine phosphatase expressed in endothelium, negatively regulates endothelial cell proliferation

The vascular endothelium is a dynamic interface between the blood vessel and circulating factors and, as such, plays a critical role in vascular events like inflammation, angiogenesis, and hemostasis. Whereas specific protein tyrosine kinases have been identified in these processes, less is known about their protein tyrosine phosphatase (PTP) counterparts. We utilized a RT-PCR/differential hybridization assay to identify PTP-epsilon as a highly abundant endothelial cell PTP. PTP-epsilon mRNA expression is growth factor responsive, suggesting a role for this enzyme in endothelial cell proliferation. Overexpression of PTP-epsilon decreases proliferation by 60% in human umbilical vein endothelial cells (HUVEC) but not in smooth muscle cells or fibroblasts. In contrast, overexpression of PTP-epsilon (D284A), a catalytically inactive mutant, has no significant effect on HUVEC proliferation. These data provide the first functional characterization of PTP-epsilon in endothelial cells and identify a novel pathway that negatively regulates endothelial cell growth. Such a pathway may have important implications in vascular development and angiogenesis.

Calmodulin binds to and inhibits the activity of the membrane distal catalytic domain of receptor protein-tyrosine phosphatase alpha

cDNA expression library screening revealed binding between the membrane distal catalytic domain (D2) of protein-tyrosine phosphatase alpha (PTPalpha) and calmodulin. Characterization using surface plasmon resonance showed that calmodulin bound to PTPalpha-D2 in a Ca(2+)-dependent manner but did not bind to the membrane proximal catalytic domain (D1) of PTPalpha, to the two tandem catalytic domains (D1D2) of PTPalpha, nor to the closely related D2 domain of PTPepsilon. Calmodulin bound to PTPalpha-D2 with high affinity, exhibiting a K(D) approximately 3 nm. The calmodulin-binding site was localized to amino acids 520-538 in the N-terminal region of D2. Site-directed mutagenesis showed that Lys-521 and Asn-534 were required for optimum calmodulin binding and that restoration of these amino acids to the counterpart PTPepsilon sequence could confer calmodulin binding. The overlap of the binding site with the predicted lip of the catalytic cleft of PTPalpha-D2, in conjunction with the observation that calmodulin acts as a competitive inhibitor of D2-catalyzed dephosphorylation (K(i) approximately 340 nm), suggests that binding of calmodulin physically blocks or distorts the catalytic cleft of PTPalpha-D2 to prevent interaction with substrate. When expressed in cells, full-length PTPalpha and PTPalpha lacking only D1, but not full-length PTPepsilon, bound to calmodulin beads in the presence of Ca(2+). Also, PTPalpha was found in association with calmodulin immunoprecipitated from cell lysates. Thus calmodulin does associate with PTPalpha in vivo but not with PTPalpha-D1D2 in vitro, highlighting a potential conformational difference between these forms of the tandem catalytic domains. The above findings suggest that calmodulin is a possible specific modulator of PTPalpha-D2 and, via D2, of PTPalpha.

Receptor-like protein tyrosine phosphatase alpha homodimerizes on the cell surface

We reported previously that the N-terminal D1 catalytic domain of receptor protein-tyrosine phosphatase alpha (RPTPalpha) forms a symmetrical, inhibited dimer in a crystal structure, in which a helix-turn-helix wedge element from one monomer is inserted into the catalytic cleft of the other monomer. Previous functional studies also suggested that dimerization inhibits the biological activity of a CD45 chimeric RPTP and the catalytic activity of an isolated RPTPsigma D1 catalytic domain. Most recently, we have also shown that enforced dimerization inhibits the biological activity of full-length RPTPalpha in a wedge-dependent manner. The physiological significance of such inhibition is unknown, due to a lack of understanding of how RPTPalpha dimerization is regulated in vivo. In this study, we show that transiently expressed cell surface RPTPalpha exists predominantly as homodimers, suggesting that dimerization-mediated inhibition of RPTPalpha biological activity is likely to be physiologically relevant. Consistent with our published and unpublished crystallographic data, we show that mutations in the wedge region of D1 catalytic domain and deletion of the entire D2 catalytic domain independently reduced but did not abolish RPTPalpha homodimerization, suggesting that both domains are critically involved but that neither is essential for homodimerization. Finally, we also provide evidence that both the RPTPalpha extracellular domain and the transmembrane domain were independently able to homodimerize. These results lead us to propose a zipper model in which inactive RPTPalpha dimers are stabilized by multiple, relatively weak dimerization interfaces. Dimerization in this manner would provide a potential mechanism for negative regulation of RPTPalpha. Such RPTPalpha dimers could be activated by extracellular ligands or intracellular binding proteins that induce monomerization or by intracellular signaling events that induce an open conformation of the dimer.

Protein-tyrosine phosphatases: structure, mechanism, and inhibitor discovery

Protein-tyrosine kinases (PTKs) and their associated signaling pathways are crucial for the regulation of numerous cell functions including growth, mitogenesis, motility, cell-cell interactions, metabolism, gene transcription, and the immune response. Since tyrosine phosphorylation is reversible and dynamic in vivo, the phosphorylation states of proteins are governed by the opposing actions of PTKs and protein-tyrosine phosphatases (PTPs). In this light, both PTKs and PTPs play equally important roles in signal transduction in eukaryotic cells, and comprehension of mechanisms behind the reversible pTyr-dependent modulation of protein function and cell physiology must necessarily encompass the characterization of PTPs as well as PTKs. In spite of the large number of PTPs identified to date and the emerging role played by PTPs in disease, a detailed understanding of the role played by PTPs in signaling pathways has been hampered by the absence of PTP-specific agents. Such PTP-specific inhibitors could potentially serve as useful tools in determining the physiological significance of protein tyrosine phosphorylation in complex cellular signal transduction pathways and may constitute valuable therapeutics in the treatment of several human diseases. The goal of this review is therefore to summarize current understandings of PTP structure and mechanism of catalysis and the relationship of these to PTP inhibitor development. The review is organized such that enzyme structure is covered first, followed by mechanisms of catalysis then PTP inhibitor development. In discussing PTP inhibitor development, nonspecific inhibitors and those obtained by screening methods are initially presented with the focus then shifting to inhibitors that utilize a more structure-based rationale.

A phosphotyrosine displacement mechanism for activation of Src by PTPalpha

Protein tyrosine phosphatase alpha (PTPalpha) is believed to dephosphorylate physiologically the Src proto-oncogene at phosphotyrosine (pTyr)527, a critical negative-regulatory residue. It thereby activates Src, and PTPalpha overexpression neoplastically transforms NIH 3T3 cells. pTyr789 in PTPalpha is constitutively phosphorylated and binds Grb2, an interaction that may inhibit PTPalpha activity. We show here that this phosphorylation also specifically enables PTPalpha to dephosphorylate pTyr527. Tyr789-->Phe mutation abrogates PTPalpha-Src binding, dephosphorylation of pTyr527 (although not of other substrates), and neoplastic transformation by overexpressed PTPalpha in vivo. We suggest that pTyr789 enables pTyr527 dephosphorylation by a pilot binding with the Src SH2 domain that displaces the intramolecular pTyr527-SH2 binding. Consistent with model predictions, we find that excess SH2 domains can disrupt PTPalpha-Src binding and can block PTPalpha-mediated dephosphorylation and activation in proportion to their affinity for pTyr789. Moreover, we show that, as predicted by the model, catalytically defective PTPalpha has reduced Src binding in vivo. The displacement mechanism provides another potential control point for physiological regulation of Src-family signal transduction pathways.

The carboxyl-terminal tyrosine residue of protein-tyrosine phosphatase alpha mediates association with focal adhesion plaques

The receptor protein-tyrosine phosphatase alpha (PTPalpha) is involved in the activation of c-Src kinase as well as in down-regulation of the insulin signal. To investigate the role of PTPalpha in activation of the Src kinase in more detail we tried to overexpress this phosphatase in NIH3T3 fibroblasts. Although PTPalpha has been overexpressed in rat embryonic fibroblasts and in embryonic carcinoma cells and should increase mitogenic responses we were not able to achieve a detectable overexpression. In contrast, expression of partially (C442S) or completely inactive (C442S,C732S) PTPalpha or of phosphatase active PTPalpha containing mutation Y781F or Y798F was possible. The level of expression, however, was reduced to background after several passages of lines expressing PTPalphaC442S,C732S and PTPalphaY781F. When employed in a focus formation assay, only infection with virus encoding PTPalphaY798F induced Src-dependent formation of foci. In immunofluorescence studies, PTPalphaC442S and PTPalphaY781F but not PTPalphaY798F colocalized with proteins found in focal adhesion plaques. Treatment of PTPalphaC442S-overexpressing cells with vanadate abolished this colocalization and led to proteolytic processing of the phosphatase. We conclude that tyrosine 798 in PTPalpha is important for localization at focal adhesion plaques. Inhibition of phosphatases by vanadate treatment releases PTPalpha from focal adhesions.

Protein tyrosine phosphatase alpha (PTPalpha) and contactin form a novel neuronal receptor complex linked to the intracellular tyrosine kinase fyn

Glycosyl phosphatidylinositol (GPI)-linked receptors and receptor protein tyrosine phosphatases (RPTPs), both play key roles in nervous system development, although the molecular mechanisms are largely unknown. Despite lacking a transmembrane domain, GPI receptors can recruit intracellular src family tyrosine kinases to receptor complexes. Few ligands for the extracellular regions of RPTPs are known, relegating most to the status of orphan receptors. We demonstrate that PTPalpha, an RPTP that dephosphorylates and activates src family kinases, forms a novel membrane-spanning complex with the neuronal GPI-anchored receptor contactin. PTPalpha and contactin associate in a lateral (cis) complex mediated through the extracellular region of PTPalpha. This complex is stable to isolation from brain lysates or transfected cells through immunoprecipitation and to antibody-induced coclustering of PTPalpha and contactin within cells. This is the first demonstration of a receptor PTP in a cis configuration with another cell surface receptor, suggesting an additional mode for regulation of a PTP. The transmembrane and catalytic nature of PTPalpha indicate that it likely forms the transducing element of the complex, and we postulate that the role of contactin is to assemble a phosphorylation-competent system at the cell surface, conferring a dynamic signal transduction capability to the recognition element.

Catalytic activation of the membrane distal domain of protein tyrosine phosphatase epsilon, but not CD45, by two point mutations

Most, if not all, of the catalytic activity of the tandem catalytic domain-containing receptor-like protein tyrosine phosphatases (PTPs) resides in the membrane proximal domains (D1), with little to no activity associated with the membrane distal domains (D2). Two point mutations in the D2 domain of PTPalpha, which restore invariant amino acids found in the KNRY motif and WPD loop of all active D1 domains, synergistically confer D1-equivalent kinetic properties towards the phosphotyrosine analogue pNPP, and activate PTPalpha-D2 catalysed phosphopeptide hydrolysis (Lim et al., J. Biol. Chem. 273 (1998) 28986-28993; Buist et al., Biochemistry 38 (1999) 914-922). As all D2 domains lack these two D1-invariant amino acids, we have investigated whether other D2 domains are activated by such point mutations. Mutant PTPepsilon-D2, closely related to PTPalpha-D2 and belonging to a subgroup of D2 domains with minimal and conservative substitutions of D1-invariant amino acids, exhibits synergistic activation towards pNPP but not towards a phosphopeptide substrate. CD45-D2, belonging to another subgroup of D2 domains with considerable substitutions in D1-invariant amino acids, is not activated by these mutations, even in the context of a third mutation which restores the minimal essential active site sequence C(X(5))R, indicating that additional defects are sufficient to preclude catalysis. The ability of the KNRY and WPD replacements to activate PTPepsilon-D2 and PTPalpha-D2, but not CD45-D2, in conjunction with the extent and nature of their wild-type amino acid substitutions, suggests that these D2 domains are representative of two functionally distinct groups of D2 domain.

The transmembranal and cytoplasmic forms of protein tyrosine phosphatase epsilon physically associate with the adaptor molecule Grb2

The protein tyrosine phosphatase Epsilon (PTPepsilon) gene gives rise to two physiologically-distinct protein products - a transmembranal, receptor-like form and a cytoplasmic, non-receptor form. Previous studies have suggested a link between expression of transmembranal PTPepsilon and transformation of mouse mammary epithelium specifically by ras or neu, although little is known about the underlying molecular mechanisms; cytoplasmic PTPepsilon is believed to function mainly in hematopoietic tissues. As part of our efforts to understand PTPepsilon function at the molecular level, we demonstrate here that both forms of PTPepsilon associate with the adaptor molecule Grb2 in vivo. Binding is mediated by the SH2 domain of Grb2; this domain binds exclusively to the carboxy-terminal phosphotyrosine of cytoplasmic PTPepsilon(Y638), and probably to additional phosphotyrosine residues in transmembranal PTPepsilon. Through its SH2 domain, Grb2 can constitutively associate with transmembranal PTPepsilon in mammary tumors initiated by ras or neu, and can be induced to associate with cytoplasmic PTPepsilon in Jurkat T-cells following stimulation of T-cell receptor signaling by pervanadate. These findings indicate that tyrosine phosphorylation of PTPepsilon and subsequent binding to Grb may link this phosphatase to downstream events which transduce signals from the cell membrane to its interior.

Overexpression of protein tyrosine phosphatase-alpha (PTP-alpha) but not PTP-kappa inhibits translocation of GLUT4 in rat adipose cells

Protein tyrosine phosphatases (PTPases) are likely to play important roles in insulin action. We recently demonstrated that the nontransmembrane PTPase PTP1B can act as a negative modulator of insulin-stimulated translocation of GLUT4. We now examine the role of PTP-alpha and PTP-kappa (two transmembrane PTPases) in this metabolic action of insulin. Rat adipose cells were transfected with either PTP-alpha or PTP-kappa and effects of these PTPases on the translocation of a cotransfected epitope-tagged GLUT4 were studied. Cells overexpressing wild-type PTP-alpha had significantly lower levels of cell surface GLUT4 in response to insulin and a threefold decrease in insulin sensitivity when compared with control cells expressing only tagged GLUT4. Co-overexpression of PTP-alpha and PTP1B did not have additive effects, suggesting that these PTPases share common substrates. Cells overexpressing either wild-type PTP-kappa or catalytically inactive mutants of PTP-alpha had dose-response curves similar to those of control cells. Since overexpression of PTP-alpha, but not PTP-kappa, had effects on translocation of GLUT4, our data suggest that PTPalpha may be a specific negative modulator of insulin-stimulated glucose transport.

Insulin secretagogues activate the secretory granule receptor-like protein-tyrosine phosphatase IAR

To investigate the potential role of protein-tyrosine phosphatases (PTPs) in regulated secretion, cellular PTP activity was measured in pancreatic beta cell lines after exposure to insulin secretagogues. A peak of elevated PTP activity was detected in whole cell lysates after 15-20 min of treatment of the cells with high KCl, glucose, or TPA, which did not appear upon treatment with control compounds. Neither was it detected in cells that do not undergo regulated secretion. The PTP activation was transient, SDS-resistant, and localized to the cytoskeleton fraction of cells. The cytoskeletal localization of IAR, a receptor-like PTP associated with secretory granules of neuroendocrine cells, suggested the possibility that IAR is the secretagogue-activated PTP. The transient expression of human IAR in betaTC3 and HIT-T15 beta cells, followed by treatment with secretagogues or control compounds and immunoprecipitation of human IAR, showed that immunoprecipitates from the secretagogue-treated cells contained an elevated PTP activity. The secretagogue-induced activation of IAR had identical kinetics to that of the endogenous PTP. Although ectopic IAR was present in membrane and cytoskeletal fractions from the cells, only the cytoskeleton-associated IAR could be activated. Thus IAR represents the endogenous secretagogue-responsive PTP, or at least a component of it, and is one of the few receptor-like PTPs for which enzymatic activation has been demonstrated. Insulin secretion is detected prior to IAR activation, suggesting that IAR is not required for immediate secretion but likely plays a role in events downstream of insulin secretion or in another pathway related to the specialized function of secretory cells.

Interconversion of the kinetic identities of the tandem catalytic domains of receptor-like protein-tyrosine phosphatase PTPalpha by two point mutations is synergistic and substrate-dependent

The two tandem homologous catalytic domains of PTPalpha possess different kinetic properties, with the membrane proximal domain (D1) exhibiting much higher activity than the membrane distal (D2) domain. Sequence alignment of PTPalpha-D1 and -D2 with the D1 domains of other receptor-like PTPs, and modeling of the PTPalpha-D1 and -D2 structures, identified two non-conserved amino acids in PTPalpha-D2 that may account for its low activity. Mutation of each residue (Val-536 or Glu-671) to conform to its invariant counterpart in PTPalpha-D1 positively affected the catalytic efficiency of PTPalpha-D2 toward the in vitro substrates para-nitrophenylphosphate and the phosphotyrosyl-peptide RR-src. Together, they synergistically transformed PTPalpha-D2 into a phosphatase with catalytic efficiency for para-nitrophenylphosphate equal to PTPalpha-D1 but not approaching that of PTPalpha-D1 for the more complex substrate RR-src. In vivo, no gain in D2 activity toward p59(fyn) was effected by the double mutation. Alteration of the two corresponding invariant residues in PTPalpha-D1 to those in D2 conferred D2-like kinetics toward all substrates. Thus, these two amino acids are critical for interaction with phosphotyrosine but not sufficient to supply PTPalpha-D2 with a D1-like substrate specificity for elements of the phosphotyrosine microenvironment present in RR-src and p59(fyn). Whether the structural features of D2 can uniquely accommodate a specific phosphoprotein substrate or whether D2 has an alternate function in PTPalpha remains an open question.

Gathering STYX: phosphatase-like form predicts functions for unique protein-interaction domains

The effects of tyrosine phosphorylation are manifested and regulated through protein domains that bind to specific phosphotyrosine motifs. STYX is a unique modular domain found within proteins implicated in mediating the effects of tyrosine phosphorylation in vivo. Individual STYX domains are not catalytically active; however, they resemble protein tyrosine phosphatase (PTP) domains and, like PTPs, contain core sequences that recognize phosphorylated substrates. Thus, the STYX domain adds to the repertoire of modular domains that can mediate intracellular signaling in response to protein 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.

Physical and functional interactions between receptor-like protein-tyrosine phosphatase alpha and p59fyn

We have examined the in vivo activity of receptor-like protein-tyrosine phosphatase alpha (PTPalpha) toward p59(fyn), a widely expressed Src family kinase. In a coexpression system, PTPalpha effected a dose-dependent tyrosine dephosphorylation and activation of p59(fyn), where maximal dephosphorylation correlated with a 5-fold increase in kinase activity. PTPalpha expression resulted in increased accessibility of the p59(fyn) SH2 domain, consistent with a PTPalpha-mediated dephosphorylation of the regulatory C-terminal tyrosine residue of p59(fyn). No p59(fyn) dephosphorylation was observed with an enzymatically inactive mutant form of PTPalpha or with another receptor-like PTP, CD45. Many enzyme-linked receptors are complexed with their substrates, and we examined whether PTPalpha and p59(fyn) underwent association. Reciprocal immunoprecipitations and assays detected p59(fyn) and an appropriate kinase activity in PTPalpha immunoprecipitates and PTPalpha and PTP activity in p59(fyn) immunoprecipitates. No association between CD45 and p59(fyn) was detected in similar experiments. The PTPalpha-mediated activation of p59(fyn) is not prerequisite for association since wild-type and inactive mutant PTPalpha bound equally well to p59(fyn). Endogenous PTPalpha and p59(fyn) were also found in association in mouse brain. Together, these results demonstrate a physical and functional interaction of PTPalpha and p59(fyn) that may be of importance in PTPalpha-initiated signaling events.

Mutant forms of the protein tyrosine phosphatase alpha show differential activities towards intracellular substrates

BHK cells overexpressing five million IR (BHK-IR) respond to insulin with reduced growth and detachment from the dish surface. We have recently identified protein tyrosine phosphatase (PTP) alpha as a negative regulator of the insulin receptor (IR) tyrosine kinase that is able to rescue BHK-IR cells from the insulin effect. In this report we describe the effect of several point mutations in PTP alpha on the phosphatase activity and regulation of insulin signaling in BHK-IR cells. Analysis of total cellular phosphotyrosine protein revealed several molecules that were dephosphorylated when PTP alpha or a phosphatase active mutant was overexpressed. By contrast, some proteins were tyrosine phosphorylated as strong or to an even higher extent as in the parental line when PTP alpha Y798F was present. We conclude that mutation of the carboxyterminal tyrosine in PTP alpha uncovers a dual function of this phosphatase in BHK cells: reduction of the IR signal and activation of an endogenous kinase.