Structural basis for inhibition of receptor protein-tyrosine phosphatase-alpha by dimerization

A monoclonal antibody against CD148, a receptor-like tyrosine phosphatase, inhibits endothelial-cell growth and angiogenesis

Angiogenesis contributes to a wide range of neoplastic, ischemic, and inflammatory disorders. Definition of the intrinsic molecular controls in angiogenic vessel growth promises novel therapeutic approaches for angiogenesis-related diseases. CD148 (also named DEP-1/PTP eta) is a receptor-like protein tyrosine phosphatase that is abundantly expressed in vascular endothelial cells. To explore a role of CD148 in endothelial vessel formation, we generated a monoclonal antibody, Ab1, against the ectodomain sequence of CD148 and examined its effects on endothelial-cell growth and vessel formation. Here we report that a bivalent, but not a monovalent, form of the Ab1 antibody inhibits endothelial-cell growth and blocks angiogenesis in mouse cornea in vivo. We further demonstrate that (1) bivalent Ab1 arrests cell-cycle progression of CD148-transfected CHO cells at G(0)/G(1) phase, (2) coexpression of catalytically inactive CD148 mutants attenuates the Ab1-cell growth inhibition, and (3) bivalent Ab1 suppresses phosphorylation of ERK1/2 kinases and Met tyrosine kinase as activated CD148 does, with an increase in CD148-associated tyrosine phosphatase activity. Taken together, these findings demonstrate that Ab1-induced ectodomain oligomerization arrests endothelial-cell growth through catalytic activity of the CD148 cytoplasmic domain. The present study defines CD148 as a valuable molecular target for antiangiogenesis therapy.

Protein-tyrosine phosphatases and cancer

Tyrosine phosphorylation is an important signalling mechanism in eukaryotic cells. In cancer, oncogenic activation of tyrosine kinases is a common feature, and novel anticancer drugs have been introduced that target these enzymes. Tyrosine phosphorylation is also controlled by protein-tyrosine phosphatases (PTPs). Recent evidence has shown that PTPs can function as tumour suppressors. In addition, some PTPs, including SHP2, positively regulate the signalling of growth-factor receptors, and can be oncogenic. An improved understanding of how these enzymes function and how they are regulated might aid the development of new anticancer agents.

Molecular analysis of receptor protein tyrosine phosphatase mu-mediated cell adhesion

Type IIB receptor protein tyrosine phosphatases (RPTPs) are bi-functional cell surface molecules. Their ectodomains mediate stable, homophilic, cell-adhesive interactions, whereas the intracellular catalytic regions can modulate the phosphorylation state of cadherin/catenin complexes. We describe a systematic investigation of the cell-adhesive properties of the extracellular region of RPTPmu, a prototypical type IIB RPTP. The crystal structure of a construct comprising its N-terminal MAM (meprin/A5/mu) and Ig domains was determined at 2.7 A resolution; this assigns the MAM fold to the jelly-roll family and reveals extensive interactions between the two domains, which form a rigid structural unit. Structure-based site-directed mutagenesis, serial domain deletions and cell-adhesion assays allowed us to identify the four N-terminal domains (MAM, Ig, fibronectin type III (FNIII)-1 and FNIII-2) as a minimal functional unit. Biophysical characterization revealed at least two independent types of homophilic interaction which, taken together, suggest that there is the potential for formation of a complex and possibly ordered array of receptor molecules at cell contact sites.

Mutation of Thr466 in SHP2 abolishes its phosphatase activity, but provides a new substrate-trapping mutant

Most classical phosphotyrosyl phosphatases (PTPs), including the Src homology phosphotyrosyl phosphatase 2 (SHP2) possess a Thr or a Ser residue immediately C-terminal to the invariant Arg in the active site consensus motif (H/V-C-X5-R-S/T), also known as the "signature motif". SHP2 has a Thr (Thr466) at this position, but its importance in catalysis has not been investigated. By employing site-directed mutagenesis, phosphatase assays and substrate-trapping studies, we demonstrate that Thr466 is critical for the catalytic activity of SHP2. Its mutation to Ala abolishes phosphatase activity, but provides a new substrate-trapping mutant. We further show that the nucleophilic Cys459 is not involved in substrate trapping by Thr466Ala-SHP2 (T/A-SHP2). Mutation of Thr466 does not cause significant structural changes in the active site as revealed by the trapping of the epidermal growth factor receptor (EGFR), the physiological substrate of SHP2, and by orthovanadate competition experiments. Based on these results and previous other works, we propose that the role of Thr466 in the catalytic process of SHP2 could be stabilizing the sulfhydryl group of Cys459 in its reduced state, a state that enables nucleophilic attack on the phosphate moiety of the substrate. The T/A-SHP2 harbors a single mutation and specifically interacts with the EGFR. Since the nucleophilic Cys459 and the proton donor Asp425 are intact in the T/A-SAHP2, it offers an excellent starting material for solving the structure of SHP2 in complex with its physiological substrate.

Pleiotrophin stimulates tyrosine phosphorylation of β-adducin through inactivation of the transmembrane receptor protein tyrosine phosphatase β/ζ

Pleiotrophin (PTN the protein, Ptn the gene) signals through a unique mechanism; it inactivates the tyrosine phosphatase activity of its receptor, the transmembrane receptor protein tyrosine phosphatase (RPTP)beta/zeta, and increases tyrosine phosphorylation of the substrates of RPTPbeta/zeta through the continued activity of a yet to be described protein tyrosine kinase(s) in PTN-stimulated cells. We have now found that the cytoskeletal protein beta-adducin interacts with the intracellular domain of RPTPbeta/zeta in a yeast two-hybrid system, that beta-adducin is a substrate of RPTPbeta/zeta, that beta-adducin is phosphorylated in tyrosine in cells not stimulated by PTN, and that tyrosine phosphorylation of beta-adducin is sharply increased in PTN-stimulated cells, suggesting that beta-adducin is a downstream target of and regulated by the PTN/RPTPbeta/zeta signaling pathway. beta-Catenin was the first downstream target of the PTN/RPTPbeta/zeta signaling pathway to be identified; these data thus also suggest that PTN coordinately regulates steady state levels of tyrosine phosphorylation of the important cytoskeletal proteins beta-adducin and beta-catenin and, through PTN-stimulated tyrosine phosphorylation, beta-adducin may contribute to the disruption of cytoskeletal structure, increased plasticity, and loss of homophilic cell-cell adhesion that are the consequences of PTN stimulation of cells and a characteristic feature of different malignant cells with mutations that activate constitutive expression of the endogenous Ptn gene.

Transmembrane homodimerization of receptor-like protein tyrosine phosphatases

Receptor-like protein tyrosine phosphatases (RPTPs) are type I integral membrane proteins. Together with protein tyrosine kinases, RPTPs regulate the phosphotyrosine levels in the cell. Studies of two RPTPs, CD45 and PTPalpha, have provided strong evidence that dimerization leads to inactivation of the receptors, and that the dimerization of PTPalpha involves interactions in the transmembrane domain (TMD). Using the TOXCAT assay, a genetic approach for analyzing TM interactions in Escherichia coli membranes, we show that the TMD of RPTPs interact in the membrane, albeit to different extents. Using fusion proteins of TMDs, we also observe an equilibrium between monomer and dimer in sodium dodecyl sulfate (SDS) micelles. Through a mutational study of the DEP1 TMD, we demonstrate that these interactions are specific. Taken together, our results define a subset of the RPTP family in which TM homodimerization may act as a mediator of protein function.

6,8-Difluoro-4-methylumbiliferyl phosphate: a fluorogenic substrate for protein tyrosine phosphatases

The fluorogenic substrate 6,8-difluoro-4-methylumbiliferyl phosphate (DIFMUP) has been widely used for the detection of serine and threonine phosphatase activities. Here we describe the use of this substrate for the characterization of protein tyrosine phosphatases (PTPs) and for the screening for PTP inhibitors. The measured kinetic and inhibitor constants for DIFMUP cleavage were comparable with those of the widely used but less discriminative and practicable substrates, para-nitrophenylphosphate and phosphotyrosine-containing peptides, respectively. Furthermore, the continuous and highly sensitive assay allows fast and accurate investigations of the type, kinetic behavior, and binding mode of small-molecule inhibitors. We discuss the validation of this assay system for various PTPs and its use in inhibitor screening for PTP1B.

Integrin CD11a cytoplasmic tail interacts with the CD45 membrane-proximal protein tyrosine phosphatase domain 1

Leucocyte adhesion receptor integrin CD11aCD18 and the transmembrane receptor-like protein tyrosine phosphatase (RPTP) CD45 mediate immune synapse formation and signalling during antigen presentation. Previous cocapping studies on human naïve T cells demonstrate an interaction between CD11aCD18 and CD45. CD45 cross-linking also has an effect on the ligand-binding activity of CD11aCD18. However, the mode of interaction between CD11aCD18 and CD45 remains unclear. Herein, yeast two-hybrid analysis identified a partial CD45 cytoplasmic tail interacting with that of CD11a. The CD45 cytoplasmic tail comprises a membrane proximal (Mp) region, protein tyrosine phosphatase domain 1 (D1), spacer, D2, and carboxyl terminus. CD45 Mp-D1 was found to be the main interacting region for the CD11a cytoplasmic tail. In contrast, the full-length CD45 cytoplasmic tail interacted weakly with that of CD11a. It has been reported that CD45 Mp-D1 but not the full-length cytoplasmic tail forms a homodimer whose enzymatic activity is inhibited. Our in vitro binding and enzymatic assays showed that the homodimeric CD45 cytoplasmic tail interacts with that of CD11a. The biological function of CD45 dimerization and its association with CD11a remains to be investigated.

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.

Crystal structure of low-molecular-weight protein tyrosine phosphatase from Mycobacterium tuberculosis at 1.9-A resolution

The low-molecular-weight protein tyrosine phosphatase (LMWPTPase) belongs to a distinctive class of phosphotyrosine phosphatases widely distributed among prokaryotes and eukaryotes. We report here the crystal structure of LMWPTPase of microbial origin, the first of its kind from Mycobacterium tuberculosis. The structure was determined to be two crystal forms at 1.9- and 2.5-A resolutions. These structural forms are compared with those of the LMWPTPases of eukaryotes. Though the overall structure resembles that of the eukaryotic LMWPTPases, there are significant changes around the active site and the protein tyrosine phosphatase (PTP) loop. The variable loop forming the wall of the crevice leading to the active site is conformationally unchanged from that of mammalian LMWPTPase; however, differences are observed in the residues involved, suggesting that they have a role in influencing different substrate specificities. The single amino acid substitution (Leu12Thr [underlined below]) in the consensus sequence of the PTP loop, CTGNICRS, has a major role in the stabilization of the PTP loop, unlike what occurs in mammalian LMWPTPases. A chloride ion and a glycerol molecule were modeled in the active site where the chloride ion interacts in a manner similar to that of phosphate with the main chain nitrogens of the PTP loop. This structural study, in addition to identifying specific mycobacterial features, may also form the basis for exploring the mechanism of the substrate specificities of bacterial LMWPTPases.

Differential Oxidation of Protein-tyrosine Phosphatases

Oxidation is emerging as an important regulatory mechanism of protein-tyrosine phosphatases (PTPs). Here we report that PTPs are differentially oxidized, and we provide evidence for the underlying mechanism. The membrane-proximal RPTPalpha-D1 was catalytically active but not readily oxidized as assessed by immunoprobing with an antibody that recognized oxidized catalytic site cysteines in PTPs (oxPTPs). In contrast, the membrane-distal RPTPalpha-D2, a poor PTP, was readily oxidized. Oxidized catalytic site cysteines in PTP immunoprobing and mass spectrometry demonstrated that mutation of two residues in the Tyr(P) loop and the WPD loop that reverse catalytic activity of RPTPalpha-D1 and RPTPalpha-D2 also reversed oxidizability, suggesting that oxidizability and catalytic activity are coupled. However, catalytically active PTP1B and LAR-D1 were readily oxidized. Oxidizability was strongly dependent on pH, indicating that the microenvironment of the catalytic cysteine has an important role. Crystal structures of PTP domains demonstrated that the orientation of the absolutely conserved PTP loop arginine correlates with oxidizability of PTPs, and consistently, RPTPmu-D1, with a similar conformation as RPTPalpha-D1, was not readily oxidized. In conclusion, PTPs are differentially oxidized at physiological pH and H(2)O(2) concentrations, and the PTP loop arginine is an important determinant for susceptibility to oxidation.

Receptor protein tyrosine phosphatase ζ as a therapeutic target for glioblastoma therapy

Astrocytomas are the most frequent brain tumour type in adults. The most common astrocytoma is the glioblastoma (GBM), which is also the most malignant and refractory to treatment--ultimately leading to the patient's death within a year of diagnosis. Neither the classical nor more experimental therapeutic approaches have significantly improved the clinical outcome of this disease. Expression profile analysis of primary tumours has provided recent insight into the identification of new GBM therapeutic targets. These proteins serve as excellent candidates to either inhibit the target molecule's functions (e.g., angiogenesis, migration or proliferation) or, coupled with a toxin or radionucleotide, to bind and exterminate the tumour cells. The receptor protein tyrosine phosphatase zeta (RPTPzeta) and one of its main ligands, pleiotropin (Ptn), are overexpressed in GBMs, thus making them potentially very good targets for the development of new immunotherapeutics. This review will summarise recent advances in GBM therapies focusing on RPTPzeta as a target for immunotherapeutics.

Protein tyrosine phosphatases: their role in insulin action and potential as drug targets

Protein tyrosine phosphatases (PTPases) are the enzymes responsible for the selective dephosphorylation of tyrosine residues. PTPases function to regulate a wide array of biological responses mediated by growth factors and other stimuli by balancing the cellular level of phosphotyrosine in concert with their counterparts, protein tyrosine kinases. The important roles which PTPases play in regulating intracellular signalling and, ultimately, biological function along with the recent availability of information regarding their structural features has highlighted them as potential targets for pharmacological modulation. This is demonstrated by the increased level of activity directed towards the identification of novel small-molecule PTPase inhibitors. The rationale and potential utility of this drug discovery approach is discussed here, with particular emphasis on its application for the treatment of insulin resistance and Type 2 diabetes.

Redox regulation of protein-tyrosine phosphatases

The protein-tyrosine phosphatases (PTPs) form a large family of signaling proteins with essential functions in embryonic development and adult physiology. The PTPs are characterized by an absolutely conserved catalytic site cysteine with a low pK(a) due to its microenvironment, making it vulnerable to oxidation. PTPs are differentially oxidized and inactivated in vitro and in living cells. Many cellular stimuli induce a shift in the cellular redox state towards oxidation and evidence is accumulating that at least part of the cellular responses to these stimuli are due to specific, transient inactivation of PTPs, indicating that PTPs are important sensors of the cellular redox state.

Structural basis for the function and regulation of the receptor protein tyrosine phosphatase CD45

CD45 is the prototypic member of transmembrane receptor-like protein tyrosine phosphatases (RPTPs) and has essential roles in immune functions. The cytoplasmic region of CD45, like many other RPTPs, contains two homologous protein tyrosine phosphatase domains, active domain 1 (D1) and catalytically impaired domain 2 (D2). Here, we report crystal structure of the cytoplasmic D1D2 segment of human CD45 in native and phosphotyrosyl peptide-bound forms. The tertiary structures of D1 and D2 are very similar, but doubly phosphorylated CD3ζ immunoreceptor tyrosine-based activation motif peptide binds only the D1 active site. The D2 “active site” deviates from the other active sites significantly to the extent that excludes any possibility of catalytic activity. The relative orientation of D1 and D2 is very similar to that observed in leukocyte common antigen–related protein with both active sites in an open conformation and is restrained through an extensive network of hydrophobic interactions, hydrogen bonds, and salt bridges. This crystal structure is incompatible with the wedge model previously suggested for CD45 regulation.

Expression of a catalytically inactive transmembrane protein tyrosine phosphatase epsilon (tm-PTP epsilon) delays optic nerve myelination

Reversible tyrosine phosphorylation is integral to the process of oligodendrocyte differentiation. To interfere with the subset of the phosphorylation cycle overseen by protein tyrosine phosphatase epsilon (PTP epsilon) in oligodendrocytes, we applied a substrate-trapping approach in the development of transgenic mice overexpressing a catalytically inactive, transmembrane PTP epsilon-hemaglutinin (tm-PTP epsilon-HA) from the dual promoter element of the gene encoding the myelin protein 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP). Transgene expression peaked during the active myelinating period, at 2-3 weeks postnatal. Two tyrosine phosphoproteins, alpha-enolase and beta-actin, were phosphorylated to a greater degree in transgenic mice. Despite a high degree of tm-PTP epsilon-HA expression, myelin was grossly normal in nearly all axonal tracts. Phenotypic abnormalities were limited to optic nerve, where a decrease in the degree of myelination was reflected by reduced levels of myelin proteins on postnatal day 21 (PND21), as well as a decrease in the density of differentiated oligodendrocytes. The optic chiasm was reduced in thickness in transgenic mice; optic nerves similarly exhibited a reduction in transverse width. Further analyses of the optic pathway demonstrated that transgenic protein was unexpectedly present in retinal ganglion cells, whose axons are the targets of myelination by optic nerve oligodendrocytes. On PND28, transgenic protein declined dramatically in both oligodendrocytes and retinal ganglion cells contributing to the recovery of optic nerve myelination. Thus, delayed myelination arises only when tm-PTP epsilon-HA is simultaneously expressed in myelin-forming glia and their neuronal targets. While tm-PTP epsilon related signaling pathways may figure in axon-glial interactions, interfering with tm-PTP epsilon activity does not perceptibly affect the development or myelinating capacity of most oligodendrocytes.

Trimeric Structure of PRL-1 Phosphatase Reveals an Active Enzyme Conformation and Regulation Mechanisms

The PRL phosphatases, which constitute a subfamily of the protein tyrosine phosphatases (PTPs), are implicated in oncogenic and metastatic processes. Here, we report the crystal structure of human PRL-1 determined at 2.7A resolution. The crystal structure reveals the shallow active-site pocket with highly hydrophobic character. A structural comparison with the previously determined NMR structure of PRL-3 exhibits significant differences in the active-site region. In the PRL-1 structure, a sulfate ion is bound to the active-site, providing stabilizing interactions to maintain the canonically found active conformation of PTPs, whereas the NMR structure exhibits an open conformation of the active-site. We also found that PRL-1 forms a trimer in the crystal and the trimer exists in the membrane fraction of cells, suggesting the possible biological regulation of PRL-1 activity by oligomerization. The detailed structural information on the active enzyme conformation and regulation of PRL-1 provides the structural basis for the development of potential inhibitors of PRL enzymes.

The Conserved Immunoglobulin Domain Controls the Subcellular Localization of the Homophilic Adhesion Receptor Protein-tyrosine Phosphatase μ

The receptor protein-tyrosine phosphatase mu (PTPmu) is a homophilic adhesion protein thought to regulate cell-cell adhesion in the vascular endothelium through dephosphorylation of cell junction proteins. In subconfluent cell cultures, PTPmu resides in an intracellular membrane pool; however, as culture density increases and cell contacts form, the phosphatase localizes to sites of cell-cell contact, and its expression level increases. These characteristics of PTPmu, which are consistent with a role in cell-cell adhesion, suggest that control of subcellular localization is an important mechanism to regulate the function of this phosphatase. To gain a better understanding of how PTPmu is regulated, we examined the importance of the conserved immunoglobulin domain, containing the homophilic binding site, in control of the localization of the enzyme. Deletion of the immunoglobulin domain impaired localization of PTPmu to the cell-cell contacts in endothelial and epithelial cells. In addition, deletion of the immunoglobulin domain affected the distribution of PTPmu in subconfluent endothelial cells when homophilic binding to another PTPmu molecule on an apposing cell was not possible, resulting in an accumulation of the mutant phosphatase at the cell surface with a concentration at the cell periphery in the region occupied by focal adhesions. This aberrant localization correlated with reduced survival and alterations in normal focal adhesion and cytoskeleton morphology. This study therefore illustrates the critical role of the immunoglobulin domain in regulation of the localization of PTPmu and the importance of such control for the maintenance of normal cell physiology.

Receptor protein tyrosine phosphatases regulate neural development and axon guidance

The regulation of tyrosine phosphorylation is recognized as an important developmental mechanism. Both addition and removal of phosphate moieties on tyrosine residues are tightly regulated during development. Originally, most attention focused on the role of tyrosine kinases during development, but more recently, the developmental importance of tyrosine phosphatases has been gaining interest. Receptor protein tyrosine phosphatases (RPTPs) are of particular interest to developmental biologists because the extracellular domains of RPTPs are similar to those of cell adhesion molecules (CAMs). This suggests that RPTPs may have functions in development similar to CAMs. This review focuses on the role of RPTPs in development of the nervous system in processes such as axon guidance, synapse formation, and neural tissue morphogenesis.

Crystal structure of the PTPL1/FAP-1 human tyrosine phosphatase mutated in colorectal cancer: evidence for a second phosphotyrosine substrate recognition pocket

Protein-tyrosine phosphatase-L1 (PTPL1, also known as FAP-1, PTP1E, PTP-BAS, and PTPN13) is mutated in a significant number of colorectal tumors and may play a role in down-regulating signaling responses mediated by phosphatidylinositol 3-kinase, although the precise substrates are as yet unknown. In this study, we describe a 1.8 A resolution crystal structure of a fully active fragment of PTPL1 encompassing the catalytic domain. PTPL1 adopts the standard PTP fold, albeit with an unusually positioned additional N-terminal helix, and shows an ordered phosphate in the active site. Interestingly, a positively charged pocket is located near the PTPL1 catalytic site, reminiscent of the second phosphotyrosine binding site in PTP1B, which is required to dephosphorylate peptides containing two adjacent phosphotyrosine residues (as occurs for example in the activated insulin receptor). We demonstrate that PTPL1, like PTP1B, interacts with and dephosphorylates a bis-phosphorylated insulin receptor peptide more efficiently than monophosphorylated peptides, indicating that PTPL1 may down-regulate the phosphatidylinositol 3-kinase pathway, by dephosphorylating insulin or growth factor receptors that contain tandem phosphotyrosines. The structure also reveals that four out of five PTPL1 mutations found in colorectal cancers are located on solvent-exposed regions remote from the active site, consistent with these mutants being normally active. In contrast, the fifth mutation, which changes Met-2307 to Thr, is close to the active site cysteine and decreases activity significantly. Our studies provide the first molecular description of the PTPL1 catalytic domain and give new insight into the function of PTPL1.

The tyrosine phosphatase PTPRJ/DEP-1 genotype affects thyroid carcinogenesis

We recently isolated the r-PTPeta gene, which encodes a receptor-type tyrosine phosphatase protein that suppresses the neoplastic phenotype of retrovirally transformed rat thyroid cells. The human homologue gene PTPRJ/DEP-1 is deleted in various tumors. Moreover, the Gln276Pro polymorphism, located in the extracellular region of the gene, seems to play a critical role in susceptibility to some human neoplasias. Here we report the loss of heterozygosity (LOH) of PTPRJ in 11/76 (14.5%) informative thyroid tumors (including adenomas and carcinomas). We also looked for the Gln276Pro, Arg326Gln and Asp872Glu polymorphisms in exons 5, 6 and 13 of PTPRJ in 88 patients with thyroid tumors and in 54 healthy individuals. We found that the PTPRJ genotypes homozygous for the Gln276Pro and Arg326Gln polymorphisms, and the Asp872 allele were more frequent in thyroid carcinoma patients than in healthy individuals (P=0.032). In addition, PTPRJ LOH was more frequent in thyroid carcinomas of heterozygotes for Gln276Pro and Arg326Gln compared with homozygotes (P=0.006). This suggests that the presence of hemizygosity for these polymorphisms in the tumor facilitates tumor progression. These results indicate that the genotypic profile of PTPRJ affects susceptibility to thyroid carcinomas, and that allelic loss of this gene is involved in thyroid carcinogenesis.

Functional significance of the LAR receptor protein tyrosine phosphatase family in development and diseases

The protein tyrosine phosphatases (PTPs) have emerged as critical players in diverse cellular functions. The focus of this review is the leukocyte common antigen-related (LAR) subfamily of receptor PTPs (RPTPs). This subfamily is composed of three vertebrate homologs, LAR, RPTP-sigma, and RPTP-delta, as well as few invertebrates orthologs such as Dlar. LAR-RPTPs have a predominant function in nervous system development that is conserved throughout evolution. Proteolytic cleavage of LAR-RPTP proproteins results in the noncovalent association of an extracellular domain resembling cell adhesion molecules and intracellular tandem PTPs domains, which is likely regulated via dimerization. Their receptor-like structures allow them to sense the extracellular environment and transduce signals intracellularly via their cytosolic PTP domains. Although many interacting partners of the LAR-RPTPs have been identified and suggest a role for the LAR-RPTPs in actin remodeling, very little is known about the mechanisms of action of RPTPs. LAR-RPTPs recently raised a lot of interest when they were shown to regulate neurite growth and nerve regeneration in transgenic animal models. In addition, LAR-RPTPs have also been implicated in metabolic regulation and cancer. This RPTP subfamily is likely to become important as drug targets in these various human pathologies, but further understanding of their complex signal transduction cascades will be required.Key words: protein tyrosine phosphatase, LAR, signal transduction, nervous system development.

Noonan syndrome type I with PTPN11 3 bp deletion: Structure-function implications

Noonan syndrome was recently reported to be caused by mutations in the PTPN11 gene in 40% of the cases. This gene encodes the nonreceptor-type protein tyrosine phosphatase SHP-2 and has been shown to be self down-regulated with the concurrency of two SH2 domains. Insertion of a specific loop (D'EF) from N-terminal SH2 domain into the SHP-2 active-site is responsible for the reversible inhibition of the phosphatase activity. Here we report the first in frame trinucleotide deletion resulting in the removal of Aspartate 61 (D61del), a key residue of the N-terminal SH2 D'EF loop. Energetic-based structural analysis and electrostatic calculations carried out on the wild-type and mutant proteins predict lower stability of the D'EF loop for the D61del variant as compared to the wild type indicating better access to the active site and most likely an enzyme activated for longer extent. Similar computations were performed on the previously functionally characterized gain-of-function D61Y mutant and similar behaviors were observed. The simulation data for the D61del and D61Y mutants suggest that both variants could yield more catalytic cycles than the wild-type molecule in the same timespan because of the opening of the active site. It also supports the notion that D61 plays a major role for proper down-regulation of the protein tyrosine phosphatase activity of SHP-2.

H2O2-induced Intermolecular Disulfide Bond Formation between Receptor Protein-tyrosine Phosphatases

Receptor protein-tyrosine phosphatase alpha (RPTPalpha) belongs to the subfamily of receptor-like protein-tyrosine phosphatases that are characterized by two catalytic domains of which only the membrane-proximal one (D1) exhibits appreciable catalytic activity. The C-terminal catalytic domain (D2) regulates RPTPalpha catalytic activity by controlling rotational coupling within RPTPalpha dimers. RPTPalpha-D2 changes conformation and thereby rotational coupling within RPTPalpha dimers in response to changes in the cellular redox state. Here we report a decrease in motility of RPTPalpha from cells treated with H2O2 on non-reducing SDS-polyacrylamide gels to a position that corresponds to RPTPalpha dimers, indicating intermolecular disulfide bond formation. Using mutants of all individual cysteines in RPTPalpha and constructs encoding the individual protein-tyrosine phosphatase domains, we located the intermolecular disulfide bond to the catalytic Cys-723 in D2. Disulfide bond formation and dimer stabilization showed similar levels of concentration and time dependence. However, treatment of lysates with dithiothreitol abolished intermolecular disulfide bonds but not stable dimer formation. Intermolecular disulfide bond formation and rotational coupling were also found using a chimera of the extracellular domain of RPTPalpha fused to the transmembrane and intracellular domain of the leukocyte common antigen-related protein (LAR). These results suggest that H2O2 treatment leads to oxidation of the catalytic Cys in D2, which then rapidly forms a disulfide bond with the D2 catalytic Cys of the dyad-related monomer, rendering an inactive RPTP dimer. Recovery from oxidative stress first leads to the reduction of the disulfide bond followed by a slower refolding of the protein to the active conformation.

The oxidative mechanism of action of ortho-quinone inhibitors of protein-tyrosine phosphatase α is mediated by hydrogen peroxide

Here, we report the identification and characterization of five ortho-quinone inhibitors of PTPalpha. We observed that the potency of these compounds in biochemical assays was markedly enhanced by the presence of DTT. A kinetic analysis suggested that they were functioning as irreversible inhibitors and that the inhibition was targeted to the catalytic site of PTPalpha. The inhibition observed by these compounds was sensitive to superoxide dismutase and catalase, suggesting that reactive oxygen species may be mediators of their inhibition. We observed that in the presence of DTT, these compounds would produce up to 2.5mM hydrogen peroxide (H(2)O(2)). The levels of H(2)O(2) produced were sufficient to completely inactivate PTPalpha. In contrast, without a reducing agent the compounds did not generate H(2)O(2) and showed little activity towards PTPalpha. In addition, these compounds inhibited PTPalpha-dependent cell spreading in NIH 3T3 cells at concentrations that were similar to their activity in biochemical assays. The biological implications of these results are discussed as they support growing evidence that H(2)O(2) is a key regulator of PTPs.

Organization and expression of thirteen alternatively spliced exons in catfish CD45 homologs

CD45, also known as LCA, is a transmembrane protein tyrosine phosphatase encoded by the PTPRC gene. In mammals, it plays an important role in T and B cell receptor and cytokine signaling by maintaining receptor associated kinases in an active state. A prominent CD45 feature is alternative splicing of exons encoding the N-terminus, resulting in the generation of several isoforms. The expression of isoforms is tightly regulated and dependent on the developmental/activation state of the lymphocyte. Nevertheless, the significance of these multiple isoforms in mammals is poorly understood. In this study, the channel catfish CD45 homolog was sequenced and found to be similar to CD45 of other species. However, unlike mammalian CD45, it appears that up to 13 exons are used in producing multiple alternatively spliced CD45 variants in catfish cells. These 13 alternatively spliced exons variably encode for O-linked glycosylation sites. Several of the exons are identical or very similar, suggesting gene duplication of a block of four exons. As demonstrated by RT-PCR, many of the alternatively spliced forms of catfish CD45 are differentially expressed in lymphoid cell lines with B cells expressing larger isoforms than do T cells. Furthermore, immunoprecipitation experiments utilizing anti-catfish CD45 mAbs substantiated that different size CD45 isoforms are expressed at the protein level on catfish T and B cells.

The minimal essential core of a cysteine-based protein-tyrosine phosphatase revealed by a novel 16-kDa VH1-like phosphatase, VHZ

The smallest active protein-tyrosine phosphatase yet (only 16 kDa) is described here and given the name VHZ for VH1-like member Z because it belongs to the group of small Vaccinia virus VH1-related dual specific phosphatases exemplified by VHR, VHX, and VHY. Human VHZ is remarkably well conserved through evolution as it has species orthologs in frogs, fish, fly, and Archaea. The gene for VHZ, which we designate as DUSP25, is located on human chromosome 1q23.1 and consists of only two coding exons. VHZ is broadly expressed in tissues and cells, including resting blood lymphocytes, Jurkat T cells, HL-60, and RAMOS. In transfected cells, VHZ was located in the cytosol and in other cells also in the nucleoli. Endogenous VHZ showed a similar but more granular distribution. We show that VHZ is an active phosphatase and analyze its structure by computer modeling, which shows that in comparison with the 185-amino acid residue VHR, the 150-residue VHZ is a shortened version of VHR and contains the minimal set of secondary structure elements conserved in all known phosphatases from this class. The surface charge distribution of VHZ differs from that of VHR and is therefore unlikely to dephosphorylate mitogen-activated protein kinases. The remarkably high degree of conservation of VHZ through evolution may indicate a role in some ancient and fundamental physiological process.

CD45-associated protein inhibits CD45 dimerization and up-regulates its protein tyrosine phosphatase activity

CD45, a receptor-like protein tyrosine phosphatase (PTP), plays an essential role in lymphocyte development and immune responses. Recent evidence suggests that dimerization of CD45 down-regulates its function. However, the mechanisms by which CD45 dimerization is regulated remain unclear, and there is no direct evidence that the PTP activity of CD45 dimers is less than that of monomers. CD45 in lymphocytes associates with CD45-AP (CD45-associated protein). Here we show that T cells from CD45-AP-null mice have a much higher level of CD45 dimers than those of wild-type mice, suggesting that CD45-AP inhibits CD45 dimer formation. This was confirmed with the use of a novel CD45-AP-null T-cell line, ALST-1, that we established from a spontaneous thymic tumor found in a CD45-AP-null mouse. Transfected CD45-AP inhibited CD45 dimer formation in ALST-1 cells in proportion to the amount of CD45-AP expressed. Finally, with the use of microsomal fractions from both mouse thymocytes and ALST-1 transfectants, the PTP activity of CD45 was found to be significantly lower in CD45-AP-negative cells than in CD45-AP-positive cells. Therefore, our results support a model in which binding of CD45-AP to inactive CD45 dimers converts them to active monomers. (Blood. 2004;103:3440-3447)

The MAM (meprin/A5-protein/PTPmu) domain is a homophilic binding site promoting the lateral dimerization of receptor-like protein-tyrosine phosphatase mu

The MAM (meprin/A5-protein/PTPmu) domain is present in numerous proteins with diverse functions. PTPmu belongs to the MAM-containing subclass of protein-tyrosine phosphatases (PTP) able to promote cell-to-cell adhesion. Here we provide experimental evidence that the MAM domain is a homophilic binding site of PTPmu. We demonstrate that the MAM domain forms oligomers in solution and binds to the PTPmu ectodomain at the cell surface. The presence of two disulfide bridges in the MAM molecule was evidenced and their integrity was found to be essential for MAM homophilic interaction. Our data also indicate that PTPmu ectodomain forms oligomers and mediates the cellular adhesion, even in the absence of MAM domain homophilic binding. Reciprocally, MAM is able to interact homophilically in the absence of ectodomain trans binding. The MAM domain therefore contains independent cis and trans interaction sites and we predict that its main role is to promote lateral dimerization of PTPmu at the cell surface. This finding contributes to the understanding of the signal transduction mechanism in MAM-containing PTPs.

Suramin Derivatives as Inhibitors and Activators of Protein-tyrosine Phosphatases

Protein-tyrosine phosphatases (PTPs) are important signaling enzymes that have emerged within the last decade as a new class of drug targets. It has previously been shown that suramin is a potent, reversible, and competitive inhibitor of PTP1B and Yersinia PTP (YopH). We therefore screened 45 suramin analogs against a panel of seven PTPs, including PTP1B, YopH, CD45, Cdc25A, VHR, PTPalpha, and LAR, to identify compounds with improved potency and specificity. Of the 45 compounds, we found 11 to have inhibitory potency comparable or significantly improved relative to suramin. We also found suramin to be a potent inhibitor (IC(50) = 1.5 microm) of Cdc25A, a phosphatase that mediates cell cycle progression and a potential target for cancer therapy. In addition we also found three other compounds, NF201, NF336, and NF339, to be potent (IC(50) < 5 microm) and specific (at least 20-30-fold specificity with respect to the other human PTPs tested) inhibitors of Cdc25A. Significantly, we found two potent and specific inhibitors, NF250 and NF290, for YopH, the phosphatase that is an essential virulence factor for bubonic plague. Two of the compounds tested, NF504 and NF506, had significantly improved potency as PTP inhibitors for all phosphatases tested except for LAR and PTPalpha. Surprisingly, we found that a significant number of these compounds activated the receptor-like phosphatases, PTPalpha and LAR. In further characterizing this activation phenomenon, we reveal a novel role for the membrane-distal cytoplasmic PTP domain (D2) of PTPalpha: the direct intramolecular regulation of the activity of the membrane-proximal cytoplasmic PTP domain (D1). Binding of certain of these compounds to PTPalpha disrupts D1-D2 basal state contacts and allows new contacts to occur between D1 and D2, which activates D1 by as much as 12-14-fold when these contacts are optimized.

Dynamic regulation of Src-family kinases by CD45 in B cells

CD45 is a key protein tyrosine phosphatase regulating Src-family protein tyrosine kinases (Src-PTKs) in lymphocytes; precisely how it exerts its effect remains controversial, however. We previously demonstrated that CD45 negatively regulates Lyn in the WEHI-231 B-cell line. Here we show that negative regulation by CD45 is physiologically significant in B cells and that some CD45 is constitutively associated with glycolipid-enriched microdomains (GEMs), where it inhibits Src-PTKs by dephosphorylating both the negative and the positive regulatory sites. Upon B-cell receptor (BCR) ligation, however, CD45 dissociates from GEMs within 30 seconds, inducing phosphorylation of 2 regulatory sites and activation of Src-PTKs, but subsequently reassociates with the GEMs within 15 minutes. Disruption of GEMs with methyl-β-cyclodextrin results in abrogation of BCR-induced apoptosis in WEHI-231 cells, suggesting GEMs are critical to signals leading to the fate determination. We propose that the primary function of CD45 is inhibition of Src-PTKs and that the level of Src-PTK activation and the B-cell fate are determined in part by dynamic behavior of CD45 with respect to GEMs.

Preferential oxidation of the second phosphatase domain of receptor-like PTP-alpha revealed by an antibody against oxidized protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) constitute a large enzyme family with important biological functions. Inhibition of PTP activity through reversible oxidation of the active-site cysteine residue is emerging as a general, yet poorly characterized, regulatory mechanism. In this study, we describe a generic antibody-based method for detection of oxidation-inactivated PTPs. Previous observations of oxidation of receptor-like PTP (RPTP) alpha after treatment of cells with H(2)O(2) were confirmed. Platelet-derived growth factor (PDGF)-induced oxidation of endogenous SHP-2, sensitive to treatment with the phosphatidylinositol 3-kinase inhibitor LY294002, was demonstrated. Furthermore, oxidation of RPTPalpha was shown after UV-irradiation. Interestingly, the catalytically inactive second PTP domain of RPTPalpha demonstrated higher susceptibility to oxidation. The experiments thus demonstrate previously unrecognized intrinsic differences between PTP domains to susceptibility to oxidation and suggest mechanisms for regulation of RPTPs with tandem PTP domains. The antibody strategy for detection of reversible oxidation is likely to facilitate further studies on regulation of PTPs and might be applicable to analysis of redox regulation of other enzyme families with active-site cysteine residues.

Evolution of the multifunctional protein tyrosine phosphatase family

The protein tyrosine phosphatase (PTP) family plays a central role in signal transduction pathways by controlling the phosphorylation state of serine, threonine, and tyrosine residues. PTPs can be divided into dual specificity phosphatases and the classical PTPs, which can comprise of one or two phosphatase domains. We studied amino acid substitutions at functional sites in the phosphatase domain and identified putative noncatalytic phosphatase domains in all subclasses of the PTP family. The presence of inactive phosphatase domains in all subclasses indicates that they were invented multiple times in evolution. Depending on the domain composition, loss of catalytic activity can result in different consequences for the function of the protein. Inactive single-domain phosphatases can still specifically bind substrate and protect it from dephosphorylation by other phosphatases. The inactive domains of tandem phosphatases can be further subdivided. The first class is more conserved, still able to bind phosphorylated tyrosine residues and might recruit multiphosphorylated substrates for the adjacent active domain. The second has accumulated several variable amino acid substitutions in the catalytic center, indicating a complete loss of tyrosine-binding capabilities. To study the impact of substitutions in the catalytic center to the evolution of the whole domain, we examined the evolutionary rates for each individual site and compared them between the classes. This analysis revealed a release of evolutionary constraint for multiple sites surrounding the catalytic center only in the second class, emphasizing its difference in function compared with the first class. Furthermore, we found a region of higher conservation common to both domain classes, suggesting a new regulatory center. We discuss the influence of evolutionary forces on the development of the phosphatase domain, which has led to additional functions, such as the specific protection of phosphorylated tyrosine residues, substrate recruitment, and regulation of the catalytic activity of adjacent domains.

Protein tyrosyl phosphatases in T cell activation: implication for human immunodeficiency virus transcriptional activity

The protein tyrosine phosphatases (PTPs) superfamily is a large group of enzymes showing a wide diversity of structure and biological functions. Their implication in the regulation of signal transduction processes is critical for homeostasis and efficient cellular activation. Disturbance of the delicate balance between protein tyrosine kinase and protein tyrosine phosphatase activities is at the heart of a large number of diseases. Control of cellular activation is especially important for human immunodeficiency virus type 1 (HIV-1) since this retrovirus requires activated T cells in order to replicate efficiently. Identification of PTPs implicated in signaling pathways leading to upregulation of HIV-1 gene transcription therefore contributes to the general understanding of cellular factors needed for strong HIV-1 replication and progression to AIDS. The use of bisperoxovanadium compounds as potent, specific, and highly purified PTP inhibitors releases HIV-1 from PTP control and strongly increases HIV-1 gene expression. These inhibitors can thus be used to study signal transduction mechanisms regulated by PTP activity that are important for HIV-1 replication and provide new and interesting therapeutic avenues for the efficient control of this debilitating retroviral infection.

Structure-based active site profiles for genome analysis and functional family subclassification

In previous work, structure-based functional site descriptors, fuzzy functional forms (FFFs), were developed to recognize structurally conserved active sites in proteins. These descriptors identify members of protein families according to active-site structural similarity, rather than overall sequence or structure similarity. FFFs are defined by a minimal number of highly conserved residues and their three-dimensional arrangement. This approach is advantageous for function assignment across broad families, but is limited when applied to detailed subclassification within these families. In the work described here, we developed a method of three-dimensional, or structure-based, active-site profiling that utilizes FFFs to identify residues located in the spatial environment around the active site. Three-dimensional active-site profiling reveals similarities and differences among active sites across protein families. Using this approach, active-site profiles were constructed from known structures for 193 functional families, and these profiles were verified as distinct and characteristic. To achieve this result, a scoring function was developed that discriminates between true functional sites and those that are geometrically most similar, but do not perform the same function. In a large-scale retrospective analysis of human genome sequences, this profile score was shown to identify specific functional families correctly. The method is effective at recognizing the likely subtype of structurally uncharacterized members of the diverse family of protein kinases, categorizing sequences correctly that were misclassified by global sequence alignment methods. Subfamily information provided by this three-dimensional active-site profiling method yields key information for specific and selective inhibitor design for use in the pharmaceutical industry.

Signaling at the growth cone: ligand-receptor complexes and the control of axon growth and guidance

The guidance of axons during the establishment of the nervous system is mediated by a variety of extracellular cues that govern cytoskeletal dynamics in axonal growth cones. A large number of these guidance cues and their cell-surface receptors have now been identified, and the intracellular signaling pathways by which these cues induce cytoskeletal rearrangements are becoming defined. This review summarizes our current understanding of the major families of axon guidance cues and their receptors, with a particular emphasis on receptor signaling mechanisms. We also discuss recent advances in understanding receptor cross talk and how the activities of guidance cues and their receptors are modulated during neural development.

Functions of the ectodomain and cytoplasmic tyrosine phosphatase domains of receptor protein tyrosine phosphatase Dlar in vivo

The receptor protein tyrosine phosphatase (PTPase) Dlar has an ectodomain consisting of three immunoglobulin (Ig)-like domains and nine fibronectin type III (FnIII) repeats and a cytoplasmic domain consisting of two PTPase domains, membrane-proximal PTP-D1 and C-terminal PTP-D2. A series of mutant Dlar transgenes were introduced into the Drosophila genome via P-element transformation and were then assayed for their capacity to rescue phenotypes caused by homozygous loss-of-function genotypes. The Ig-like domains, but not the FnIII domains, are essential for survival. Conversely, the FnIII domains, but not the Ig-like domains, are required during oogenesis, suggesting that different domains of the Dlar ectodomain are involved in distinct functions during Drosophila development. All detectable PTPase activity maps to PTP-D1 in vitro. The catalytically inactive mutants of Dlar were able to rescue Dlar(-/-) lethality nearly as efficiently as wild-type Dlar transgenes, while this ability was impaired in the PTP-D2 deletion mutants DlarDeltaPTP-D2 and Dlar(bypass). Dlar-C1929S, in which PTP-D2 has been inactivated, increases the frequency of bypass phenotype observed in Dlar(-/-) genotypes, but only if PTP-D1 is catalytically active in the transgene. These results indicate multiple roles for PTP-D2, perhaps by acting as a docking domain for downstream elements and as a regulator of PTP-D1.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

Either of the CD45RB and CD45RO isoforms are effective in restoring T cell, but not B cell, development and function in CD45-null mice

The protein tyrosine phosphatase CD45 is expressed as a series of isoforms whose tissue and differentiation stage specificity is broadly conserved in evolution. CD45 has been shown to be an important regulator of a variety of functions in many different hemopoietic lineages. We have chosen an in vivo genetic complementation strategy to investigate the differential functions between isoforms. In this study, we report the characterization of transgenic mice which express the isoforms CD45RO or CD45RB as their only CD45 molecules, at a variety of expression levels and in the majority of hemopoietic lineages. Both CD45RO and CD45RB isoforms reconstitute thymocyte development in a CD45-null mouse background when expressed above a threshold level. The resulting mature T cells populate the peripheral lymphoid organs where they are found at normal frequency. Both CD45RO and CD45RB isoforms also permit T cell function in the periphery, although the threshold for normal function here appears to be set higher than in the thymus. In contrast, neither isoform is capable of fully restoring peripheral B cell maturation, even at levels approaching those in heterozygous CD45(+/-) mice in which maturation is normal. In vitro activation of B cells by Ag-receptor stimulation is only minimally complemented by these CD45RO and CD45RB transgenes. Our results suggest that CD45 isoforms play unique roles which differ between the T and B lineages.

A novel mechanism for Wnt activation of canonical signaling through the LRP6 receptor

LDL receptor-related protein 6 (LRP6) is a Wnt coreceptor in the canonical signaling pathway, which plays essential roles in embryonic development. We demonstrate here that wild-type LRP6 forms an inactive dimer through interactions mediated by epidermal growth factor repeat regions within the extracellular domain. A truncated LRP6 comprising its transmembrane and cytoplasmic domains is expressed as a constitutively active monomer whose signaling ability is inhibited by forced dimerization. Conversely, Wnts are shown to activate canonical signaling through LRP6 by inducing an intracellular conformational switch which relieves allosteric inhibition imposed on the intracellular domains. Thus, Wnt canonical signaling through LRP6 establishes a novel mechanism for receptor activation which is opposite to the general paradigm of ligand-induced receptor oligomerization.

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.

Protein tyrosine phosphatase RQ is a phosphatidylinositol phosphatase that can regulate cell survival and proliferation

Protein tyrosine phosphatase RQ (PTPRQ) was initially identified as a protein tyrosine phosphatase (PTPase)-like protein that is upregulated in a model of renal injury. Here we present evidence that, like PTEN, the biologically important enzymatic activity of PTPRQ is as a phosphatidylinositol phosphatase (PIPase). The PIPase specificity of PTPRQ is broader than that of PTEN and depends on different amino acid residues in the catalytic domain. In vitro, the recombinant catalytic domain of PTPRQ has low PTPase activity against tyrosine-phosphorylated peptide and protein substrates but can dephosphorylate a broad range of phosphatidylinositol phosphates, including phosphatidylinositol 3,4,5-trisphosphate and most phosphatidylinositol monophosphates and diphosphates. Phosphate can be hydrolyzed from the D3 and D5 positions in the inositol ring. PTPRQ does not have either of the basic amino acids in the catalytic domain that are important for the PIPase activity of PTEN or the sequence motifs that are characteristic of type II phosphatidylinositol 5-phosphatases. Instead, the PIPase activity depends on the WPE sequence present in the catalytic cleft of PTPRQ, and in the "inactive" D2 domains of many dual-domain PTPases, in place of the WPD motif present in standard active PTPases. Overexpression of PTPRQ in cultured cells inhibits proliferation and induces apoptosis. An E2171D mutation that retains or increases PTPase activity but eliminates PIPase activity, eliminates the inhibitory effects on proliferation and apoptosis. These results indicate that PTPRQ represents a subtype of the PTPases whose biological activities result from its PIPase activity rather than its PTPase activity.

Redox-regulated rotational coupling of receptor protein-tyrosine phosphatase alpha dimers

Receptor protein-tyrosine phosphatase alpha (RPTP alpha) constitutively forms dimers in the membrane, and activity studies with forced dimer mutants of RPTP alpha revealed that rotational coupling of the dimer defines its activity. The hemagglutinin (HA) tag of wild type RPTP alpha and of constitutively dimeric, active RPTP alpha-F135C with a disulfide bond in the extracellular domain was not accessible for antibody, whereas the HA tag of constitutively dimeric, inactive RPTP alpha-P137C was. All three proteins were expressed on the plasma membrane to a similar extent, and the accessibility of their extracellular domains did not differ as determined by biotinylation studies. Dimerization was required for masking the HA tag, and we identified a region in the N terminus of RPTP alpha that was essential for the effect. Oxidative stress has been shown to induce a conformational change of the membrane distal PTP domain (RPTP alpha-D2). Here we report that H(2)O(2) treatment of cells induced a change in rotational coupling in RPTP alpha dimers as detected using accessibility of an HA tag in the extracellular domain as a read-out. The catalytic site Cys(723) in RPTP alpha-D2, which was required for the conformational change of RPTP alpha-D2 upon H(2)O(2) treatment, was essential for the H(2)O(2)-induced increase in accessibility. These results show for the first time that a conformational change in the intracellular domain of RPTP alpha led to a change in conformation of the extracellular domains, indicating that RPTPs have the capacity for inside-out signaling.

Structure-based design and discovery of novel inhibitors of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are important in the regulation of signal transduction processes. Certain enzymes of this class are considered as potential therapeutic targets in the treatment of a variety of diseases such as diabetes, inflammation, and cancer. However, many PTP inhibitors identified to date are peptide-based and contain a highly charged phosphate-mimicking component. These compounds usually lack membrane permeability and this limits their utility in the inhibition of intracellular phosphatases. In the present study, we have used structure-based design and modeling techniques to explore catalytic-site directed, reversible inhibitors of PTPs. Employing a non-charged phosphate mimic and non-peptidyl structural components, we have successfully designed and synthesized a novel series of trifluoromethyl sulfonyl and trifluoromethyl sulfonamido compounds as PTP inhibitors. This is the first time that an uncharged phosphate mimic is reported in the literature for general, reversible, and substrate-competitive inhibition of PTPs. It is an important discovery because the finding may provide a paradigm for the development of phosphatase inhibitors that enter cells and modify signal transduction.

Receptor protein tyrosine phosphatases in nervous system development

Receptor protein tyrosine phosphatases (RPTPs) are key regulators of neuronal morphogenesis in a variety of different vertebrate and invertebrate systems, yet the mechanisms by which these proteins regulate central nervous system development are poorly understood. In the past few years, studies have begun to outline possible models for RPTP function by demonstrating in vivo roles for RPTPs in axon outgrowth, guidance, and synaptogenesis. In addition, the crystal structures of several RPTPs have been solved, numerous downstream effectors of RPTP signaling have been identified, and a small number of RPTP ligands have been described. In this review, we focus on how RPTPs transduce signals from the extracellular environment to the cytoplasm, using a detailed comparative analysis of the different RPTP subfamilies. Focusing on the roles RPTPs play in the development of the central nervous system, we discuss how the elucidation of RPTP crystal structures, the biochemical analysis of phosphatase enzyme catalysis, and the characterization of complex signal transduction cascades downstream of RPTPs have generated testable models of RPTP structure and function.

Multimerization of the protein-tyrosine phosphatase (PTP)-like insulin-dependent diabetes mellitus autoantigens IA-2 and IA-2beta with receptor PTPs (RPTPs). Inhibition of RPTPalpha enzymatic activity

Most receptor-type protein-tyrosine phosphatases (RPTPs) contain two tandem PTP domains. For some RPTPs the enzymatically inactive membrane-distal phosphatase domains (D2) were found to bind enzymatically active membrane proximal PTP (D1) domains, and oligomerization has been proposed as a general regulatory mechanism. The RPTP-like proteins IA-2 and IA-2beta, major autoantigens in insulin-dependent diabetes mellitus, contain just a single enzymatically inactive PTP-like domain. Their physiological role is as yet enigmatic. To investigate whether the catalytically inactive cytoplasmic domains of IA-2 and IA-2beta are involved in oligomerization, we exploited interaction trap assay in yeast and glutathione S-transferase pull-down and co-immunoprecipitation strategies on lysates of transfected COS-1 cells. The results show that IA-2 and IA-2beta are capable of homo- and heterodimerization to which both the juxtamembrane region and the phosphatase-like segment can contribute. Furthermore, they can form heterodimers with some other RPTP members, most notably RPTPalpha and RPTPepsilon, and down-regulate RPTPalpha enzymatic activity. Thus, in addition to homo-dimerization, the enzymatic activity of receptor-type PTPs can be regulated through heterodimerization with other RPTPs, including the catalytically inactive IA-2 and IA-2beta.

Intra- and intermolecular interactions between intracellular domains of receptor protein-tyrosine phosphatases

The presence of two protein-tyrosine phosphatase (PTP) domains is a striking feature in most transmembrane receptor PTPs (RPTPs). The generally inactive membrane-distal PTP domains (RPTP-D2s) bind and are proposed to regulate the membrane-proximal PTP domains (RPTP-D1s). We set out to characterize the interactions between RPTP-D1s and RPTP-D2s in vivo by co-immunoprecipitation of hemagglutinin-tagged fusion proteins encoding the transmembrane domain and RPTP-D1 and myc-tagged RPTP-D2. Seven RPTPs from four different subfamilies were used: RPTPalpha, RPTPepsilon, LAR, RPTPvarsigma, RPTPdelta, CD45, and RPTP(mu). We found that RPTP-D2s bound to RPTPs with different affinities. The presence of intrinsic RPTP-D2 altered the binding specificity toward other RPTP-D2s positively or negatively, depending on the identity of the RPTPs. Furthermore, the C terminus of RPTP-D2s and the "wedge" in RPTP-D1s played a central role in binding specificity. Finally, full-length RPTPalpha and LAR heterodimerized in an oxidative stress-dependent manner. Like RPTPalpha-D2, the LAR-D2 conformation was affected by oxidative stress, suggesting a common regulatory mechanism for RPTP complex formation. Taken together, interactions between RPTP-D1s and RPTP-D2s are a common but specific mechanism that is likely to be regulated. The RPTP-D2s and the wedge structures are crucial determinants of binding specificity, thus regulating cross-talk between RPTPs.

Negative regulation of CD45 by differential homodimerization of the alternatively spliced isoforms

The regulation of receptor-like protein tyrosine phosphatases (RPTPs) is not well understood. Although CD45 can be negatively regulated by dimerization, how dimerization is modulated is unclear. Here we show that various isoforms of CD45 differentially homodimerize in T cells. The dimerization is modulated by the sialylation and O-glycosylation of alternatively spliced CD45 exons in the extracellular domain. Thus, the smallest isoform, CD45RO--which undergoes the least extracellular sialylation and O-glycosylation--homodimerizes with the highest efficiency, resulting in decreased signaling via the T cell receptor. Because CD45 is required for T cell activation, our findings may reveal a mechanism that contributes to the termination of the primary T cell response. Our results not only demonstrate the biological significance of alternative splicing in the immune system, but also suggest a model for regulating RPTP dimerization and function.