Protein tyrosine phosphatases take off

LAR tyrosine phosphatase receptor: proximal membrane alternative splicing is coordinated with regional expression and intraneuronal localization

Examination of null-mutant Drosophila and Leukocyte Common Antigen-Related (LAR)-deficient transgenic mice has demonstrated that the LAR protein tyrosine phosphatase (PTP) receptor promotes neurite outgrowth. In the absence of known ligands, the mechanisms by which LAR-type PTP receptors are regulated are unknown. We hypothesized that an alternatively spliced eleven amino acid proximal membrane segment of LAR (LAR alternatively spliced element-a; LASE-a) contributes to regulation of LAR function. Human, rat and mouse LAR cDNA sequences demonstrated that the predicted eleven amino acid inserts in rat and mouse are identical and share nine of eleven residues with the human insert. LASE-a splicing led to the introduction of a Ser residue into LAR at a position analogous to Ser residues undergoing regulated phosphorylation in other PTPs. In-situ studies revealed increasingly region-specific expression of LASE-a containing LAR transcripts during postnatal development. RT-PCR analysis of cortical and hippocampal tissue confirmed that the proportion of LAR transcripts containing LASE-a decreases during development. Immunostaining of cultured PC12 cells, cerebellar granule neurons, dorsal root ganglia and sciatic nerve sections with antibody directed against the LASE-a insert demonstrated signal in cell bodies but little if any along neurites. In contrast, staining with antibody directed to a separate domain of LAR showed accumulation of LAR along neurites. The findings that LASE-a splicing is conserved across human, rat and mouse, that the LASE-a insert introduces a Ser at a site likely to be targeted for regulated phosphorylation and that developmentally regulated splicing is coordinated with specific regional and intraneuronal localization point to important novel potential mechanisms regulating LAR-type tyrosine phosphatase receptor function in the nervous system.

A novel phosphatase regulating neurite extension on CNS inhibitors

The inability of injured axons to regenerate in the adult mammalian central nervous system is thought to be in part due to inhibitory molecules synthesized by oligodendrocytes and present in myelin. We describe the cloning of a cDNA encoding a novel neuronal protein, named NERPP-2C, which is distantly related to protein phosphatase 2C and plays a role in the inhibitory response pathway to myelin inhibitors. NERPP-2C is expressed in neuronal cell lines and in rat brain. Expression in rat is detectable at E15, increases with age, and is highest in adulthood. Exposure of NG108-15 cells to antisense oligonucleotides reduces NERPP-2C expression and overcomes the inhibition of neurite extension on CNS myelin substrates in vitro. Antibodies to NERPP-2C detect two proteins, approximately 55 and 80 kDa in size, the smaller of which is found in the cytoplasm, and the larger is associated with the membrane fraction. The antibodies specifically immunoprecipitate a protein which exhibits serine/threonine and tyrosine phosphatase activity. NERPP-2C is localized in neurites and in growth cones, as well as in the cell nucleus. We hypothesize that NERPP-2C is a component in the signal transduction pathway for neuronal growth inhibitory factors in CNS myelin.

Pten is essential for embryonic development and tumour suppression

The PTEN gene encodes a dual-specificity phosphatase mutated in a variety of human cancers. PTEN germline mutations are found in three related human autosomal dominant disorders, Cowden disease (CD), Lhermitte-Duclos disease (LDD) and Bannayan-Zonana syndrome (BZS), characterized by tumour susceptibility and developmental defects. To examine the role of PTEN in ontogenesis and tumour suppression, we disrupted mouse Pten by homologous recombination. Pten inactivation resulted in early embryonic lethality. Pten-/- ES cells formed aberrant embryoid bodies and displayed an altered ability to differentiate into endodermal, ectodermal and mesodermal derivatives. Pten+/- mice and chimaeric mice derived from Pten+/- ES cells showed hyperplastic-dysplastic changes in the prostate, skin and colon, which are characteristic of CD, LDD and BZS. They also spontaneously developed germ cell, gonadostromal, thyroid and colon tumours. In addition, Pten inactivation enhanced the ability of ES cells to generate tumours in nude and syngeneic mice, due to increased anchorage-independent growth and aberrant differentiation. These results support the notion that PTEN haploinsufficiency plays a causal role in CD, LDD and BZS pathogenesis, and demonstrate that Pten is a tumour suppressor essential for embryonic development.

Q: a molecular dynamics program for free energy calculations and empirical valence bond simulations in biomolecular systems

A new molecular dynamics program for free energy calculations in biomolecular systems is presented. It is principally designed for free energy perturbation simulations, empirical valence bond calculations, and binding affinity estimation by linear interaction energy methods. Evaluation of ligand-binding selectivity and free energy profiles for nucleophile activation in two protein tyrosine phosphatases as well as absolute binding affinity estimation for a lysine-binding protein are given as examples.

Characterization of recombinant CD45 cytoplasmic domain proteins. Evidence for intramolecular and intermolecular interactions

CD45 is a transmembrane two-domain tyrosine phosphatase required for efficient signal transduction initiated by lymphocyte antigen receptors. As with most transmembrane two-domain phosphatases, the role of the second phosphatase domain is unclear. In this study, recombinant CD45 cytoplasmic domain proteins purified from bacteria were used to evaluate the function of the individual phosphatase domains. A recombinant protein expressing the membrane-proximal region, first phosphatase domain, and spacer region of CD45 (rD1) was catalytically active and found to exist primarily as a dimer. In contrast to this, a recombinant protein expressing the spacer region, the second phosphatase domain and the carboxy tail of CD45 (rD2) existed as a monomer and had no catalytic activity against any of the substrates tested. Comparison of rD1 with the recombinant protein expressing the entire cytoplasmic domain of CD45 (rD1/D2) indicated that rD1/D2 was 2-3-fold more catalytically active, was more thermostable, and existed primarily as a monomer. Limited trypsin digestion of rD1/D2 provided evidence for a noncovalent association between an N-terminal 27-kDa fragment and a C-terminal 53-kDa fragment, suggesting an intramolecular interaction. Furthermore, rD1 was found to specifically associate with rD2 in an in vitro binding assay. Taken together, these data provide evidence for an intramolecular interaction occurring in the cytoplasmic domain of CD45. In the absence of the C-terminal region containing the second phosphatase domain, intermolecular interactions occur, resulting in dimer formation.

The structure and mechanism of protein phosphatases: insights into catalysis and regulation

Eukaryotic protein phosphatases are structurally and functionally diverse enzymes that are represented by three distinct gene families. Two of these, the PPP and PPM families, dephosphorylate phosphoserine and phosphothreonine residues, whereas the protein tyrosine phosphatases (PTPs) dephosphorylate phosphotyrosine amino acids. A subfamily of the PTPs, the dual-specificity phosphatases, dephosphorylate all three phosphoamino acids. Within each family, the catalytic domains are highly conserved, with functional diversity endowed by regulatory domains and subunits. The protein Ser/Thr phosphatases are metalloenzymes and dephosphorylate their substrates in a single reaction step using a metal-activated nucleophilic water molecule. In contrast, the PTPs catalyze dephosphorylation by use of a cysteinyl-phosphate enzyme intermediate. The crystal structures of a number of protein phosphatases have been determined, enabling us to understand their catalytic mechanisms and the basis for substrate recognition and to begin to provide insights into molecular mechanisms of protein phosphatase regulation.

Inhibition of protein tyrosine phosphatases PTP1B and CD45 by sulfotyrosyl peptides

Sulfotyrosyl peptides corresponding to the known high-affinity substrate phosphotyrosyl peptide sequences in casein and the autophosphorylation sites of insulin receptor and EGF receptor were investigated as inhibitors of protein tyrosine phosphatases PTP1B and CD45. These peptides inhibit both PTP1B and CD45 in the micromolar range competitively and reversibly. The elements required for inhibition were investigated by truncation and substitution of these peptides. Acidic residues N-terminal to the sulfotyrosyl residues are essential for high-affinity binding to PTP1B. The recognition elements required for inhibition of PTP1B and CD45 are different and this suggests the possibility of identifying selective active-site-directed inhibitors for these enzymes.

Visualization of the cysteinyl-phosphate intermediate of a protein-tyrosine phosphatase by x-ray crystallography

Protein-tyrosine phosphatases (PTPs) are signal transduction enzymes that catalyze the dephosphorylation of phosphotyrosine residues via the formation of a transient cysteinyl-phosphate intermediate. The mechanism of hydrolysis of this intermediate has been examined by generating a Gln-262 --> Ala mutant of PTP1B, which allows the accumulation and trapping of the intermediate within a PTP1B crystal. The structure of the intermediate at 2.5-A resolution reveals that a conformationally flexible loop (the WPD loop) is closed over the entrance to the catalytic site, sequestering the phosphocysteine intermediate and catalytic site water molecules and preventing nonspecific phosphoryltransfer reactions to extraneous phosphoryl acceptors. One of the catalytic site water molecules, the likely nucleophile, forms a hydrogen bond to the putative catalytic base, Asp-181. In the wild-type enzyme, the nucleophilic water molecule would be coordinated by the side chain of Gln-262. In combination with our previous structural data, we can now visualize each of the reaction steps of the PTP catalytic pathway. The hydrolysis of the cysteinyl-phosphate intermediate of PTPs is reminiscent of GTP hydrolysis by the GTPases, in that both families of enzymes utilize an invariant Gln residue to coordinate the attacking nucleophilic water molecule.

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.

Altering the nucleophile specificity of a protein-tyrosine phosphatase-catalyzed reaction. Probing the function of the invariant glutamine residues

Protein-tyrosine phosphatases (PTPases) catalysis involves a cysteinyl phosphate intermediate, in which the phosphoryl group cannot be transferred to nucleophiles other than water. The dual specificity phosphatases and the low molecular weight phosphatases utilize the same chemical mechanism for catalysis and contain the same (H/V)C(X)5R(S/T) signature motif present in PTPases. Interestingly, the latter two groups of phosphatases do catalyze phosphoryl transfers to alcohols in addition to water. Unique to the PTPase family are two invariant Gln residues which are located at the active site. Mutations at Gln-446 (and to a much smaller extent Gln-450) to Ala, Asn, or Met (but not Glu) residues disrupt a bifurcated hydrogen bond between the side chain of Gln-446 and the nucleophilic water and confer phosphotransferase activity to the Yersinia PTPase. Thus, the conserved Gln-446 residue is responsible for maintaining PTPases' strict hydrolytic activity and for preventing the PTPases from acting as kinases to phosphorylate undesirable substrates. This explains why phosphoryl transfer from the phosphoenzyme intermediate in PTPases can only occur to water and not to other nucleophilic acceptors. Detailed kinetic analyses also suggest roles for Gln-446 and Gln-450 in PTPase catalysis. Although Gln-446 is not essential for the phosphoenzyme formation step, it plays an important role during the hydrolysis of the intermediate by sequestering and positioning the nucleophilic water in the active site for an in-line attack on the phosphorus atom of the cysteinyl phosphate intermediate. Gln-450 interacts through a bound water molecule with the phosphoryl moiety and may play a role for the precise alignment of active site residues, which are important for substrate binding and transition state stabilization for both of the chemical steps.

Purification and crystallization of the CDK-associated protein phosphatase KAP expressed in Escherichia coli

The kinase associated phosphatase (KAP) is a human dual specificity protein phosphatase that dephosphorylates the cell cycle control protein, cyclin dependent kinase-2 on Thr 160 in a cyclin dependent manner (Poon & Hunter, 1995). We report here the over-expression of KAP in Escherichia coli as an N-terminal His-tagged protein using a modified pET-28a T7-expression vector. The recombinant protein was purified to homogeneity and crystallized. The crystals diffract to 2.3 A resolution when exposed to synchrotron radiation and belong to space group P6(1)22, or its enantiomorph P6(5)22, with unit cell dimensions a = b = 74.5 A, c = 139.5 A.

The catalytic subunits of Ser/Thr protein phosphatases from Caenorhabditis elegans

The catalytic activities of protein phosphatase 1, 2A, 2B, and 2C were detected in crude extracts of Caenorhabditis elegans with different phosphoprotein substrates and specific inhibitors or activators. The enzymological properties of protein phosphatase 2B as well as those of the catalytic subunits of protein phosphatase 1 and protein phosphatase 2A were determined after partial purification. Gene fragments encoding the catalytic subunits of the protein phosphatase 1-2A-2B superfamily were amplified by polymerase chain reaction and were identified by DNA sequencing. Besides the homologs of protein phosphatase 1, 2B, and X, five protein phosphatase 1-type sequences and four novel protein phosphatase sequences were found. Our data, together with the results of the C. elegans genome project, suggest that this nematode contains an extensive family of Ser/Thr specific protein phosphatases including several up to now biochemically uncharacterized members.

CD45 and RPTPalpha display different protein tyrosine phosphatase activities in T lymphocytes

To examine the substrate specificity and function of two receptor protein tyrosine phosphatases, CD45 and RPTPalpha, RPTPalpha was expressed in a CD45(-), T-cell receptor (TCR)+, BW5147 T-lymphoma cell. High levels of expression of RPTPalpha did not fully restore either proximal or distal TCR-mediated signalling events. RPTPalpha was unable to reconstitute the phosphorylation of CD3zeta and did not increase the expression of the activation marker, CD69, on stimulation with TCR/CD3. RPTPalpha did not significantly alter the phosphorylation state or kinase activity of two CD45 substrates, p56(lck) or p59(fyn), suggesting that RPTPalpha does not have the same specificity or function as CD45 in T-cells. Further comparison of the two phosphatases indicated that immunoprecipitated RPTPalpha was approx. one-seventh to one-tenth as active as CD45 when tested against artificial substrates. This difference in activity was also observed in vitro with purified recombinant enzymes at physiological pH. Additional analysis with Src family phosphopeptides and recombinant p56(lck) as substrates indicated that CD45 was consistently more active than RPTPalpha, having both higher Vmax and lower Km values. Thus CD45 is intrinsically a much more active phosphatase than RPTPalpha, which provides one reason why RPTPalpha cannot effectively dephosphorylate p56(lck) and substitute for CD45 in T-cells. This work establishes that these two related protein tyrosine phosphatases are not interchangeable in T-cells and that this is due, at least in part, to quantitative differences in phosphatase activity.

Comparison between the loss of platelet membrane asymmetry, microvesiculation and the tyrosine phosphorylation of proteins

Platelet activation by agents such as the Ca2+-ionophore A23187 or Ca2+-ATPase inhibitors leads to the generation of a procoagulant surface and the formation of microparticles. These responses are late events of platelet activation and readily detected by flow cytometry using annexin V-FITC as an aminophospholipid probe. One Ca2+-ATPase inhibitor, 2,5-di-(tertbutyl)-1,4-benzohydroquinone induced aminophospholipid exposure without microparticle formation. Previous work has shown that microparticle formation is strictly linked to the activation of calpain, a thiol-protease that modifies the platelet cytoskeleton and some signal transduction enzymes. We now report how the detection of platelet tyrosine phosphorylation by western-blotting clearly shows that when platelet activation and aminophospholipid exposure are accompanied by microparticle formation there is a decrease in the tyrosine phosphorylation of proteins.

Mechanism of inhibition of protein-tyrosine phosphatases by disodium aurothiomalate

Disodium aurothiomalate (AuTM) has been used successfully in the treatment of various autoimmune and inflammatory disorders; however, the molecular target(s) for this agent remains unknown. The aim of this study was to investigate whether the activity of CD45, a protein-tyrosine phosphatase (PTP, EC 3.1.3.48) essential for antigen-receptor-mediated lymphocyte signaling, was modified by AuTM exposure. The effects of AuTM on the activities of CD45 and other PTPs were monitored in vitro by a continuous assay using the substrate fluorescein diphosphate. In addition, the inhibition of PTP1B by AuTM was determined using a novel binding assay that employed an optical biosensor (BIAcore). The experimental results are summarized here: AuTM inhibited CD45 activity with an IC50 of 1.2 +/- 0.1 microM, and inhibition was competitive with substrate. The effect of AuTM, however, was not restricted to CD45, as the cytoplasmic PTP (PTP1B) was also inhibited, with an IC50 of 3.6 +/- 0.2 microM. AuTM also blocked the binding of GST-PTP1B to an immobilized active site inhibitor: a non-hydrolyzable difluorophosphonomethyl phenylalanine-containing biotinylated hexapeptide. AuTM-inhibited CD45 could be reactivated by the addition of excess dithiothreitol. These findings indicate that AuTM may interact with the essential active site cysteine residue involved in the catalytic mechanism of PTPs. Thus, it is possible that some of the cellular effects of gold result from the inhibition of these important cell signaling molecules.

P-TEN, the tumor suppressor from human chromosome 10q23, is a dual-specificity phosphatase

Protein tyrosine phosphatases (PTPs) have long been thought to play a role in tumor suppression due to their ability to antagonize the growth promoting protein tyrosine kinases. Recently, a candidate tumor suppressor from 10q23, termed P-TEN, was isolated, and sequence homology was demonstrated with members of the PTP family, as well as the cytoskeletal protein tensin. Here we show that recombinant P-TEN dephosphorylated protein and peptide substrates phosphorylated on serine, threonine, and tyrosine residues, indicating that P-TEN is a dual-specificity phosphatase. In addition, P-TEN exhibited a high degree of substrate specificity, showing selectivity for extremely acidic substrates in vitro. Furthermore, we demonstrate that mutations in P-TEN, identified from primary tumors, tumor cells lines, and a patient with Bannayan-Zonana syndrome, resulted in the ablation of phosphatase activity, demonstrating that enzymatic activity of P-TEN is necessary for its ability to function as a tumor suppressor.

The MBP fusion protein restores the activity of the first phosphatase domain of CD45

CD45 is a receptor-like protein tyrosine phosphatase critically involved in the regulation of initial effector functions in B- and T-cells. The protein comprises two phosphatase (PTP) domains in its cytoplasmic region. However, whether each PTP domain has enzyme activity by itself or whether both domains are required to build up a functional enzyme is unclear. We have studied different constructions of human CD45 comprising the two PTP domains, both separately and as a single protein, fused to maltose-binding protein (MBP). In apparent contrast with previous studies, we show that the first PTP domain of CD45 (when fused to MBP) may be a viable phosphatase in the absence of the second domain. Phosphatase activity resides in the monomeric form of the protein and is lost after proteolytic cleavage of the fusion partner, indicating that MBP specifically activates the first PTP domain. Furthermore, changes in the optimal pH for activity with respect to wild-type CD45 suggest that protein-protein interactions involving residues in the neighbourhood of the catalytic site mediate enzyme activation.

CD45 regulates Src family member kinase activity associated with macrophage integrin-mediated adhesion

BACKGROUND

Adhesion of leukocytes to the extracellular matrix and to other cells is mediated by members of the integrin family of adhesion molecules. Src family kinases are activated upon integrin-mediated adhesion. In lymphocytes, CD45 is a leukocyte-specific transmembrane protein tyrosine phosphatase that activates Src family kinases associated with B-cell and T-cell antigen receptor signaling by constitutive dephosphorylation of the inhibitory carboxy-terminal tyrosine phosphorylation site. Here, we show that CD45 is also important in downregulating the kinase activity of Src family members during integrin-mediated adhesion in macrophages.

RESULTS

We found that CD45 colocalized with beta2 integrin and the Src family kinase p53/56(lyn) to adhesion sites in bone marrow-derived macrophages. Macrophages from CD45(-/-) mice were unable to maintain integrin-mediated adhesion. In adherent macrophages, absence of CD45 led to the hyperphosphorylation and hyperactivation of p56/59(hck) and p53/56(lyn), but not of p58(c-fgr). CD45 directly inactivated p59(hck) but not p56(lck) in transient transfection assays. Furthermore, coexpression of CD45 with p59(hck) or p56(lyn) containing a tyrosine to phenylalanine mutation at the carboxy-terminal negative regulatory site resulted in decreased tyrosine phosphorylation of the Src family member kinases due to dephosphorylation of the potentiating tyrosine phosphorylation site within the kinase domain.

CONCLUSIONS

Using primary bone marrow macrophages, these studies demonstrate that CD45 regulates Src family kinases and is required to maintain macrophage adhesion. CD45 decreases Src family kinase activity by dephosphorylating the tyrosine residue located within the kinase domain.

A family of proteins that inhibit signalling through tyrosine kinase receptors

Phosphotyrosine phosphatases are critical negative or positive regulators in the intracellular signalling pathways that result in growth-factor-specific cell responses such as mitosis, differentiation, migration, survival, transformation or death. The SH2-domain-containing phosphotyrosine phosphatase SHP-2 is a positive signal transducer for several receptor tyrosine kinases (RTKs) and cytokine receptors. To investigate its mechanism of action we purified a tyrosine-phosphorylated glycoprotein which in different cell types associates tightly with SHP-2 and appears to serve as its substrate. Peptide sequencing in conjunction with complementary DNA cloning revealed a new gene family of at least fifteen members designated signal-regulatory proteins (SIRPs). They consist of two subtypes distinguished by the presence or absence of a cytoplasmic SHP-2-binding domain. The transmembrane polypeptide SIRP alpha1 is a substrate of activated RTKs and in its tyrosine-phosphorylated form binds SHP-2 through SH2 interactions and acts as its substrate. It also binds SHP-1 and Grb2 in vitro and has negative regulatory effects on cellular responses induced by growth factors, oncogenes or insulin. Our findings indicate that proteins belonging to the SIRP family generally regulate signals defining different physiological and pathological processes.

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

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

Identification of a receptor-type protein tyrosine phosphatase expressed in postmitotic maturing neurons: its structure and expression in the central nervous system

We have isolated a rat cDNA encoding a receptor-type protein-tyrosine-phosphatase (RTP) expressed in brain and kidney (RPTP-BK) and characterized its expression in the developing central nervous system. RPTP-BK has seven fibronectin type III-like repeats in the extracellular region and a unique catalytic phosphatase domain in the cytoplasmic region. Bacterial expression of its phosphatase domain showed that the dephosphorylation of phosphotyrosine residues was mediated by the cytoplasmic catalytic domain. Sequence comparison revealed that RPTP-BK is homologous with GLEPP1, a rabbit PTP expressed in renal glomerular epithelia, and has the same phosphatase domain as murine PTPphi expressed in macrophages. RPTP-BK has also significant homology with Drosophila DPTP10D in the phosphatase domain, whose expression is localized exclusively in growth cones of the embryonal brains. The gene for RPTP-BK is well conserved among other species, and the expression in the brain but not in the kidney is developmentally regulated during the neonatal stage. Hybridization in situ showed that RPTP-BK is highly expressed in the postmitotic maturing neurons of the olfactory bulb, developing neocortex, hippocampus and thalamus. Because the expression of RPTP-BK in the developing neocortex is correlated with the stage of axonogenesis in cortical neurons, RPTP-BK might be crucial in neural cell development of the mammalian central nervous system.

Energetics of nucleophile activation in a protein tyrosine phosphatase

The nucleophilic attack by cysteine 12 in the low-molecular-weight protein tyrosine phosphatase is believed to be carried out by the thiolate anion form of this residue. We here study the energetics of proton transfer between the thiol group of cysteine 12 and a substrate phosphate oxygen atom, to examine the effects of the enzymic environment on the stability of the thiolate nucleophile. This is done by molecular dynamics and free energy perturbation simulations, utilizing the empirical valence bond method to describe the potential surface of the system. The calculations show that the protein environment significantly stabilizes the thiolate ion, thereby setting the stage for the nucleophilic attack. We compare these results with those from further simulations of a mutant enzyme, and demonstrate the importance of serine 19 in thiolate stabilization.

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

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

The N-terminal domains of tensin and auxilin are phosphatase homologues

Tensin, an actin filament capping protein, and auxilin, a component of receptor-mediated endocytosis, are known to have 350 residue regions of significant sequence similarity near their N-termini (Schröder et al., 1995, Eur J Biochem 228:297-304). Here we demonstrate that these regions are homologous, not only to each other, but also to the catalytic domain of a putative protein tyrosine phosphatase (PTP) from Saccharomyces cerevisiae and to other PTPs. We propose that the PTP-like portion of the homology region of tensin and auxilin represents a distinct domain. A detailed sequence comparison indicates that the PTP-like domain in tensin is unlikely to exhibit phosphatase activity, whereas in auxilin it may possess a different phosphatase specificity from tyrosine phosphatases. It is probable that the PTP-like domains in tensin and auxilin mediate binding interactions with phosphorylated polypeptides; they may therefore represent members of a distinct class of phosphopeptide recognition domain.

Identification of p130(cas) as a substrate for the cytosolic protein tyrosine phosphatase PTP-PEST

PTP-PEST is a ubiquitously expressed, cytosolic, mammalian protein tyrosine phosphatase (PTP) which exhibits high specific activity in vitro. We have investigated the substrate specificity of PTP-PEST by a novel substrate-trapping approach in combination with in vitro dephosphorylation experiments. We initially identified a prominent 130-kDa tyrosine-phosphorylated protein in pervanadate-treated HeLa cell lysates which was preferentially dephosphorylated by PTP-PEST in vitro. In order to identify this potential substrate, mutant (substrate-trapping) forms of PTP-PEST were generated which lack catalytic activity but retain the ability to bind substrates. These mutant proteins associated in stable complexes exclusively with the same 130-kDa protein, which was identified as p130(cas) by immunoblotting. This exclusive association was observed in lysates from several cell lines and in transfected COS cells, but was not observed with other members of the PTP family, strongly suggesting that p130(cas) represents a major physiologically relevant substrate for PTP-PEST. Our studies suggest potential roles for PTP-PEST in regulation of p130(cas) function. These functions include mitogen- and cell adhesion-induced signalling events and probable roles in transformation by various oncogenes. These results provide the first demonstration of a PTP having an inherently restricted substrate specificity in vitro and in vivo. The methods used to identify p130(cas) as a specific substrate for PTP-PEST are potentially applicable to any PTP and should therefore prove useful in determining the physiological substrates of other members of the PTP family.