Use of fluorinated tyrosine phosphates to probe the substrate specificity of the low molecular weight phosphatase activity of calcineurin

Structural Basis for the Catalytic Activity of Human Serine/Threonine Protein Phosphatase-5

Serine/threonine protein phosphatase-5 (PP5) affects many signaling networks that regulate cell growth and cellular responses to stress. Here we report the crystal structure of the PP5 catalytic domain (PP5c) at a resolution of 1.6 A. From this structure we propose a mechanism for PP5-mediated hydrolysis of phosphoprotein substrates, which requires the precise positioning of two metal ions within a conserved Asp271-M1:M2-W1-His427-His304-Asp274 catalytic motif (where M1 and M2 are metals and W1 is a water molecule). The structure of PP5c provides a structural basis for explaining the exceptional catalytic proficiency of protein phosphatases, which are among the most powerful known catalysts. Resolution of the entire C terminus revealed a novel subdomain, and the structure of the PP5c should also aid development of type-specific inhibitors.

c-Fos phosphorylation induced by H2O2 prevents proteasomal degradation of c-Fos in cardiomyocytes

Oxidants cause activation of the AP-1 transcription factor in cardiomyocytes. c-Fos, a component of the AP-1 transcription factor, is transiently induced by H2O2 and the induction is sensitive to the protein synthesis inhibitor cycloheximide. With high percentage gel electrophoresis, multiple c-Fos bands were resolved by Western blot analyses, indicating post-translational modification of newly synthesized c-Fos protein after H2O2 exposure. Treatment of immunoprecipitated c-Fos protein with the type 2 serine/threonine phosphatase A (PP2A) and immunoblotting of c-Fos protein with antibodies against phosphorylated serine or threonine demonstrated that c-Fos was phosphorylated at serine residues. A pharmacological inhibitor of JNKs inhibited the formation of multiple c-Fos bands without affecting c-fos transcription. The proteasomal inhibitor MG132 and Proteasome Inhibitor I extended the time course of c-Fos protein elevation. An increase in ubiquitin was detectable in c-Fos protein from H2O2-treated cells. Interestingly, treating the whole cell lysates with PP2A, but not calcineurin (i.e. PP2B), resulted in disappearance of c-Fos protein and MG132 was able to prevent this loss. H2O2 caused an elevation of PP2B and total phosphatase activity. The phosphatase inhibitor okadaic acid, but not PP2B inhibiter cypermethrin, extended the time course of c-Fos protein elevation after H2O2 exposure. These data suggest that JNK-mediated phosphorylation of newly synthesized c-Fos protects the protein from being degraded by the proteasome. PP2B independent dephosphorylation contributes to degradation of c-Fos protein during oxidative stress response of cardiomyocytes.

Mechanism for the paradoxical inhibition and stimulation of calcineurin by the immunosuppresive drug tacrolimus (FK506)

We examined the paradoxical inhibition and stimulation of calcineurin, the calcium-activated protein phosphatase, using the drug FK506 (tacrolimus) which acts as a complex together with its binding protein; the complex is designated here as FKC. We reproduced FKC inhibition with RIIp, a phosphorylated peptide substrate, and FKC stimulation with p-nitrophenylphosphate (pNPP) as substrate. The presence of RIIp in the pNPP assay caused inhibition. Yet, under these conditions, FKC still stimulated pNPP dephosphorylation to the same extent. The effects of Mn2+ were strikingly different for the two substrates when calcineurin was measured under otherwise identical conditions: Mn2+ stimulated pNPP dephosphorylation several fold, but only stimulated RIIp dephosphorylation by about 50%. When Pi was used as product inhibitor, FKC stimulation, but not calmodulin stimulation, was attenuated. We conclude that FKC enhances substrate binding to the enzyme. This would lead to inhibition with RIIp, known to bind calcineurin tightly, but stimulation with pNPP, known to bind calcineurin weakly. The result not only resolves the paradox but also elucidates the mechanism of action for this class of immunosuppressive drugs.

Roles of calcineurin and calcium/calmodulin-dependent protein kinase II in pressure overload-induced cardiac hypertrophy

Calcineurin and calcium/calmodulin-dependent protein kinase (CaMK) II have been suggested to be the signaling molecules in cardiac hypertrophy. It was not known, however, whether these mechanisms are involved in cardiac hypertrophy induced by pressure overload without the influences of blood-derived humoral factors, such as angiotensin II. To elucidate the roles of calcineurin and CaMK II in this situation, we examined the effects of calcineurin and CaMK II inhibitors on pressure overload-induced expression of c-fos, an immediate-early gene, and protein synthesis using heart perfusion model. The hearts isolated from Sprague-Dawley rats were perfused according to the Langendorff technique, and then subjected to the acute pressure overload by raising the perfusion pressure. The activation of calcineurin was evaluated by its complex formation with calmodulin and by its R-II phosphopeptide dephosphorylation. CaMK II activation was evaluated by its autophosphorylation. Expression of c-fos mRNA and rates of protein synthesis were measured by northern blot analysis and by 14C-phenylalanine incorporation, respectively. Acute pressure overload significantly increased calcineurin activity, CaMK II activity, c-fos expression and protein synthesis. Cyclosporin A and FK506, the calcineurin inhibitors, significantly inhibited the increases in both c-fos expression and protein synthesis. KN62, a CaMK II inhibitor, also significantly prevented the increase in protein synthesis, whereas it failed to affect the expression of c-fos. These results suggest that both calcineurin and CaMK II pathways are critical in the pressure overload-induced acceleration of protein synthesis, and that transcription of c-fos gene is regulated by calcineurin pathway but not by CaMK II pathway.

The N- and C-terminal fragments of ubiquitin are important for the antimicrobial activities

Secretory granules of chromaffin cells contain catecholamines and several antimicrobial peptides derived from chromogranins and proenkephalin-A. These peptides are secreted in the extracellular medium following exocytosis. Here, we show that ubiquitin is stored in secretory chromaffin granules and released into the circulation upon stimulation of chromaffin cells. We also show that the C-terminal fragment (residues 65-76) of ubiquitin displays, at the micromolar range, a lytic antifungal activity. Using confocal laser scan microscopy and rhodamine-labeled synthetic peptides, we could demonstrate that the C-terminal peptide (residues 65-76) is able to cross the cell wall and the plasma membrane of fungi and to accumulate in fungi, whereas the N-terminal peptide (residues 1-34) is stopped at the fungal wall level. Furthermore, these two peptides act synergistically to kill filamentous fungi. Because of the interaction of the C-terminal sequence of ubiquitin with calmodulin, the synthetic peptide (residues 65-76) was tested in vitro against calmodulin-dependent calcineurin, an enzyme crucial for fungal growth. This peptide was found to inhibit the phosphatase activity of calcineurin. Our data show a new property of ubiquitin C-terminal-derived peptide (65-76) that could be used with N-terminal peptide (1-34) as a new potent antifungal agent.

Catalytic domains of the LAR and CD45 protein tyrosine phosphatases from Escherichia coli expression systems: purification and characterization for specificity and mechanism

The cytoplasmic domains of two human transmembrane protein tyrosine phosphatases (PTPases), LAR and CD45, have been expressed in Escherichia coli, purified to near-homogeneity, and compared for catalytic efficiency toward several phosphotyrosine-containing peptide substrates. A 615-residue LAR fragment (LAR-D1D2) containing both tandemly repeated PTPase domains shows almost identical specific activity and high catalytic efficiency as the 40-kDa single-domain LAR-D1 fragment, consistent with a single functional active site in the 70-kDa LAR-D1D2 enzyme. A 90-kDa fragment of the human leukocyte CD45 PTPase, containing two similar tandemly repeated PTPase domains, shows parallel specificity to LAR-D1 and LAR-D1D2 with a high kcat/Km value for a phosphotyrosyl undecapeptide. Sufficient purified LAR-D1 and LAR-D1D2 PTPases were available to demonstrate enzymatic exchange of 18O from 18O4 inorganic phosphate into H2(16)O at rates of approximately 1 x 10(-2) s-1. The oxygen-18 exchange probably proceeds via a phosphoenzyme intermediate. Brief incubation of all three PTPase fragments with a [32P]phosphotyrosyl peptide substrate prior to quench with SDS sample buffer and gel electrophoresis led to autoradiographic detection of 32P-labeled enzymes. Pulse/chase studies on the LAR 32P-enzyme showed turnover of the labeled phosphoryl group.

Structural and biological characterization of chromofungin, the antifungal chromogranin A-(47-66)-derived peptide

Vasostatin-I, the natural fragment of chromogranin A-(1-76), is a neuropeptide able to kill a large variety of fungi and yeast cells in the micromolar range. We have examined the antifungal properties of synthetic vasostatin-I-related peptides. The most active shortest peptide, named chromofungin, corresponds to the sequence Arg(47)-Leu(66). Extensive (1)H NMR analysis revealed that it adopts a helical structure. The biophysical mechanism implicated in the interaction of chromofungin with fungi and yeast cells was studied, showing the penetration of this peptide with different lipid monolayers. In order to examine thoroughly the antifungal activity of chromofungin, confocal laser microscopy was used to demonstrate the ability of the rhodamine-labeled peptide to interact with the fungal cell wall, to cross the plasma membrane, and to accumulate in Aspergillus fumigatus, Alternaria brassicola, and Candida albicans. Our present data reveal that chromofungin inhibits calcineurin activity, extending a previous observation that the N-terminal region of chromogranin A interacts with calmodulin in the presence of calcium. Therefore, the destabilization of fungal wall and plasma membrane, together with the possible intracellular inhibition of calmodulin-dependent enzymes, is likely to represent the mechanism by which vasostatin-I and chromofungin exert antifungal activity.

Role of calcineurin in insulin-like growth factor-1-induced hypertrophy of cultured adult rat ventricular myocytes

The present study examined the role of calcineurin in insulin-like growth factor (IGF)-1-induced hypertrophy in primary cultures of adult rat ventricular myocytes (ARVM), prepared from the ventricles of 14-16-week-old male Sprague-Dawley rats. The effects of several humoral factors, including phenylephrine, angiotensin II, endothelin-1, IGF-1 and interleukin-6, on the morphology of ARVM were studied. Myocyte surface area was significantly increased by IGF-1 (2,268 +/- 571 to 3,018 +/- 836 microm2, p < 0.01), but not by other humoral factors. This hypertrophic effect of IGF-1 was blocked by genistein (tyrosine kinase inhibitor), PD98059 (MEK inhibitor). These findings suggest that IGF-1 produces ARVM hypertrophy by a tyrosine kinase-MEK mediated pathway as has been reported in neonatal cardiomyocytes. IGF-1-mediated ARVM hypertrophy was also attenuated by cyclosporine A (calcineurin inhibitor), and staurosporine and chelerythrine (protein kinase C inhibitors). IGF-1 markedly increased calcineurin activity (8.7 +/- 1.2 to 98.0 +/- 54.3 pmol x h(-1) mg(-1), p < 0.01), and this activation was completely blocked by pre-treatment with cyclosporine A (8.5 +/- 11.4pmol x h(-1) x mg(-1), p < 0.01) and chelerythrine (2.3 +/- 2.7 pmol x h(-1) mg(-1), p < 0.01). It appears that IGF-1 activates calcineurin by a protein kinase C-dependent pathway. Increased mRNA expression of atrial natriuretic factor by IGF-1 was inhibited by cyclosporine A (p < 0.01). The findings indicate that IGF-1 induces ARVM hypertrophy by protein kinase C and calcineurin-related mechanisms. The fact that elevated calcineurin activity and induced atrial natriuretic factor mRNA expression by IGF-1 were blocked by cyclosporine A further supports the hypothesis that calcineurin is critically involved in IGF-1-induced ARVM hypertrophy.

Serine protease inhibitor causes F-actin redistribution and inhibition of calcium-mediated secretion in pancreatic acini

BACKGROUND & AIMS

The present study was undertaken to evaluate the role of serine proteases in regulating digestive enzyme secretion in pancreatic acinar cells.

METHODS

Isolated acini were stimulated by various secretagogues in the presence or absence of cell-permeant serine protease inhibitors 4-(2-aminoethyl)-benzenesulfonyl fluoride and N(alpha)-p-tosyl-L-phenylalanine chloromethyl ketone. F-actin distribution was studied after staining with rhodamine phalloidin.

RESULTS

Both cell-permeant serine protease inhibitors blocked amylase secretion in response to secretagogues that use calcium as a second messenger (e.g., cerulein, carbamylcholine, and bombesin) but not to those that use adenosine 3',5'-cyclic monophosphate (cAMP) as a second messenger (e.g., secretin and vasoactive intestinal polypeptide). Incubation of the acini with these inhibitors also resulted in a dramatic redistribution of the F-actin cytoskeleton. This redistribution was energy dependent. Similar redistribution of F-actin from the apical to the basolateral region was also observed when acini were incubated with a supramaximally stimulating concentration of cerulein, which is known to inhibit secretion.

CONCLUSIONS

These results suggest that a serine protease activity is essential for maintaining the normal apical F-actin distribution; its inhibition redistributes F-actin from the apical to the basolateral region and blocks secretion induced by secretagogues that act via calcium. cAMP reverses the F-actin redistribution and hence cAMP-mediated secretion is not affected.

Calcineurin: form and function

Calcineurin is a eukaryotic Ca(2+)- and calmodulin-dependent serine/threonine protein phosphatase. It is a heterodimeric protein consisting of a catalytic subunit calcineurin A, which contains an active site dinuclear metal center, and a tightly associated, myristoylated, Ca(2+)-binding subunit, calcineurin B. The primary sequence of both subunits and heterodimeric quaternary structure is highly conserved from yeast to mammals. As a serine/threonine protein phosphatase, calcineurin participates in a number of cellular processes and Ca(2+)-dependent signal transduction pathways. Calcineurin is potently inhibited by immunosuppressant drugs, cyclosporin A and FK506, in the presence of their respective cytoplasmic immunophilin proteins, cyclophilin and FK506-binding protein. Many studies have used these immunosuppressant drugs and/or modern genetic techniques to disrupt calcineurin in model organisms such as yeast, filamentous fungi, plants, vertebrates, and mammals to explore its biological function. Recent advances regarding calcineurin structure include the determination of its three-dimensional structure. In addition, biochemical and spectroscopic studies are beginning to unravel aspects of the mechanism of phosphate ester hydrolysis including the importance of the dinuclear metal ion cofactor and metal ion redox chemistry, studies which may lead to new calcineurin inhibitors. This review provides a comprehensive examination of the biological roles of calcineurin and reviews aspects related to its structure and catalytic mechanism.

In vitro effect of retinoids on calcineurin activity

Calcineurin was shown previously to be inhibited by members of the tyrphostin family of tyrosine kinase inhibitors, with the most effective inhibition suggested to be caused by the presence of a conjugated side chain (Martin BL, Biochem Pharmacol 56: 483-488, 1998). Retinoids are a family of naturally occurring biomolecules having non-aromatic ring structures and conjugated side chains as substituents on the ring. Three oxidation states of the all-trans configuration of retinoids (retinol, retinal, and retinoic acid) were tested as effectors of calcineurin. Only retinoic acid was found to inhibit calcineurin effectively, with an IC(50) value of approximately 50 microM. Retinol and retinal caused less than 30% inhibition at concentrations up to 100 microM. All three retinoids caused some precipitation of reaction components: retinoic acid and retinal above 50 microM, and retinol above 250 microM. Bacterial alkaline phosphatase was not inhibited by the retinoids, indicating that metal centers alone are insufficient for significant inhibition by retinoic acid. An aromatic ring was not required for inhibition and may not provide additional inhibition, inasmuch as an aromatic analog of retinoic acid (acitretin) showed less effective inhibition. These data are consistent with the presence of conjugated, unsaturated groups enhancing the inhibition of calcineurin.

Differences between Mg(2+) and transition metal ions in the activation of calcineurin

Exogenous metal ion activation of calcineurin catalyzed hydrolysis of para-nitrophenyl phosphate was kinetically characterized at 20, 25, 30, and 37 degrees C. Analysis yielded estimates for thermodynamic parameters for the activation of calcineurin by each of the metal ions. Values for DeltaG(Me)( degrees ) were varied with the best activators resulting in more stable enzyme-metal ion complexes and with DeltaG(Me)( degrees ) dominated by the entropic component. Mg(2+) was the only nontransition metal ion which supported significant activity and showed some distinct characteristics including a negative DeltaS(Me)( degrees ), suggesting that activation by Mg(2+) may have resulted in a unique enzyme-metal ion form. Circular dichroism showed that metal ions increased the alpha-helical content of calcineurin, but little significant differences in the spectra were identified between using activating and nonactivating metal ions. Activating Mg(2+), but not nonactivating Ca(2+), did cause changes in the Fourier transform infrared photoacoustic spectrum of calcineurin compared to the spectrum of calcineurin with Mn(2+). Other metal ions, Co(2+) and Ni(2+), also caused no changes in the infrared spectrum. Possible explanations for these differences between Mg(2+) and transition metal ions in the activation of calcineurin are discussed.

Characterization of diphosphonucleotide phosphatase/phosphodiesterase from yellow lupin (Lupinus luteus) seeds

A phosphatase cleaving the pyrophosphate bond in diphosphonucleotides and phosphodiester bond in various phosphodiesters (pH optimum at 6.25) was purified from yellow lupin (Lupinus luteus L.) seeds. The enzyme is 75 kDa monomeric glycoprotein (pI=6.4) with 4.4% of carbohydrate (mannose, N-acetylglucosamine, fucose and xylose). Analysis of its partial amino acid sequence (8 peptides, 101 amino acid residues) together with no divalent cation requirements for catalysis points out that the purified enzyme is different from known plant pyrophosphate cleaving enzymes (apyrases and inorganic pyrophosphatases). Its physiological role could be related to a regulation of diphosphonucleotides level in plant metabolism.

Effect of substitution inert metal complexes on calcineurin

As a substitute for M(H2O)2+6, Co(NH3)3+6 was found to activate calcineurin with para-nitrophenyl phosphate as substrate. Kinetics for calcineurin catalyzed hydrolysis of para-nitrophenyl phosphate at pH 7.0 with Mn2+, Mg2+, Co2+, and Co(NH3)3+6 were compared. Although kcat and Km were different with the metals, values of kcat/Km were nearly identical for Mn2+ and Mg2+, but lower for Co2+ and Co(NH3)3+6. The concentration of each metal providing half-maximal activation, designated Kact, was evaluated as 15.9 mM for Co(NH3)3+6, compared to Kact = 0.17 mM for Mn2+ and Co2+ and 6.3 mM for Mg2+, respectively. Comparing kcat/Kcat showed that Co(NH3)3+6 was a 170-fold poorer activator of calcineurin than was Mn2+, but only 1.5-fold poorer than Mg2+. Activation by Co(NH3)3+6 indicated that activation of calcineurin by exogenous metal ions can proceed via an outer coordination sphere reaction mechanism with no requirement for the direct coordination of substrate by metal. Because Co(NH3)3+6 was found to support calcineurin activity, the related compound [Co-(ethylenediamine)3]3+ (or Co(en)3+3) was tested as a possible activator. Co(en)3+3 did not support calcineurin activity but did inhibit calcineurin. Co(en)3+3 showed competitive inhibition kinetics with either Mn2+ or pNPP as the varied ligand and the other at a fixed, subsaturating concentration. Inorganic phosphate was used as a known competitive inhibitor to pNPP (B. L. Martin and D. J. Graves, J. Biol. Chem. 261, 14545-14550, 1986) and showed uncompetitive inhibition with Mn2+ as the varied ligand. These patterns are consistent with the mechanism of ligand binding to calcineurin being ordered with metal preceding substrate. Prior formation of a metal-substrate complex was not required for association with calcineurin.

Regulation of the gamma-aminobutyric acid transporter activity by protein phosphatases in synaptic plasma membranes

The influence of the phosphorylation dephosphorylation states on the gamma-aminobutyric acid (GABA) transporter activity of synaptic plasma membranes (SPM) was studied by using either specific phosphatase inhibitors or activators. Calyculin A and okadaic acid (phosphatase 1 and phosphatase 2A inhibitors) inhibited the GABA uptake by isolated SPM vesicles, whereas cyclosporin A (phosphatase 2B inhibitor) had a stimulatory effect (approximately 10%) which was higher (approximately 38%) when all these drugs were present in the reaction medium. On the other hand, intravesicular Ca2+, up to about 10 microM, inhibited the GABA uptake (approximately 50%) in a manner which appeared to be facilitated in the presence of PP1 and PP2A inhibitors and this inhibition was relieved by the calmodulin antagonist W-7. We also observed that isolated SPM vesicles contain both Ca(2+)-independent phosphatase activity that is significantly inhibited by PP1 and PP2A inhibitors, and Ca(2+)-dependent phosphatase activity that is abolished in the presence of the PP2B inhibitor, cyclosporin A. These results indicate that regulation of the SPM GABA transporter is determined by the internally localized Ca-calmodulin-dependent phosphatase activity (calcineurin), and that other phosphorylated sites, sensitive to PP1 and PP2A inhibitors, potentiate either the positive or negative effects exerted by those internal sites when they are in their phosphorylated or dephosphorylated states, respectively.

Inhibition of calcineurin by the tyrphostin class of tyrosine kinase inhibitors

Because of their similarity to tyrosine, members of the tyrphostin family of tyrosine kinase inhibitors were tested as possible inhibitors of the protein serine/threonine phosphatase calcineurin. Calcineurin was inhibited by tyrphostins A8 (also designated AG10), A23 (AG18), and A48 (AG112) with p-nitrophenyl phosphate as substrate. The IC50 values estimated with this substrate were 21, 62, and 30 microM for A8, A23, and A48, respectively. Two other tyrphostins, A46 (AG99) and A63 (AG13), did not inhibit calcineurin at concentrations up to 200 microM. Similar inhibition was observed with tyrphostins A8 and A23 using a phosphopeptide substrate (1.0 mM). Tyrphostin A8 showed competitive inhibition against p-nitrophenyl phosphate as the substrate, with an inhibition constant of 18 microM, comparable to the IC50 value. Possible chemical and structural features influencing inhibition are discussed based on a comparison of the structures of the tyrphostins tested.

Selective activation of calcineurin by dipicolinic acid

Calcineurin was activated at 30 degrees C by incubation with dipicolinic acid, a metal chelator, in the absence of activating, exogenous Mn2+. The activation reached a plateau after 90 min with 8- to 12-fold higher activity. Inclusion of the activating metal Mn2+ (1.0 mM) in the incubation mixture slightly lessened the activation induced by dipicolinic acid. The chelator 1,10-phenanthroline had no effect on the activity of calcineurin in concurrent experiments. Activation by dipicolinic acid was reversed by the addition of Zn2+ or Fe3+. The reversal occurred within 30 min after the addition of either metal and returned the activity of calcineurin to its initial level. Atomic absorption spectrometry analysis showed no loss of iron or zinc from calcineurin after activation (2 h) by dipicolinic acid. Because there seemed to be no interaction between dipicolinic acid and exogenous metal, the effect of dipicolinic acid was concluded to result from masking of at least one intrinsic metal. Calcineurin incubated with 1.0 mM Mn2+ (saturating levels) also did not show any loss of intrinsic metal by atomic absorption analysis. The consequences of these data concerning the role(s) of intrinsic metals in calcineurin catalysis are discussed.

Kinetic and spectroscopic analyses of mutants of a conserved histidine in the metallophosphatases calcineurin and lambda protein phosphatase

Calcineurin belongs to a family of serine/threonine protein phosphatases that contain active site dinuclear metal cofactors. Bacteriophage lambda protein phosphatase is also considered to be a member of this family based on sequence comparisons (Lohse, D. L., Denu, J. M., and Dixon, J. E. (1995) Structure 3, 987-990). Using EPR spectroscopy, we demonstrate that lambda protein phosphatase accommodates a dinuclear metal center. Calcineurin and lambda protein phosphatase likewise contain a conserved histidine that is not a metal ligand but is within 5 A of either metal in calcineurin. In this study the conserved histidine in calcineurin was mutated to glutamine and the mutant protein analyzed by EPR spectroscopy and kinetic methods. Parallel studies with an analogous lambda protein phosphatase mutant were also carried out. Kinetic studies using paranitrophenyl phosphate as substrate showed a decrease in kcat of 460- and 590-fold for the calcineurin and lambda protein phosphatase mutants, respectively, compared with the wild type enzymes. With a phosphopeptide substrate, mutagenesis of the conserved histidine resulted in a decrease in kcat of 1,300-fold for calcineurin. With the analogous lambda protein phosphatase mutant, kcat decreased 530-fold compared with wild type lambda protein phosphatase using phenyl phosphate as a substrate. EPR studies of the iron-reconstituted enzymes indicated that although both mutant enzymes can accommodate a dinuclear metal center, spectroscopic differences compared with wild type proteins suggest a perturbation of the ligand environment, possibly by disruption of a hydrogen bond between the histidine and a metal-coordinated solvent molecule.

Kinetic and mechanistic characterization of a mammalian protein-tyrosine phosphatase, PTP1

The kinetic mechanism of the hydrolysis of phosphate monoesters catalyzed by a soluble form of rat protein-tyrosine phosphatase (PTPase), PTP1, was probed with a variety of steady-state and pre-steady-state kinetic techniques. Product inhibition and 18O exchange experiments are consistent with the enzymatic reaction proceeding through two chemical steps, i.e. formation and breakdown of a covalent phosphoenzyme intermediate. The variation of kcat/Km with pH indicates that three ionizable groups are involved in enzyme substrate binding and catalysis. The first group must be deprotonated and is attributed to the second ionization of the substrate. The other two groups with pK alpha values of 5.1 and 5.5 correspond to two enzyme active site residues. The kcat-pH profiles for both p-nitrophenyl phosphate and beta-naphthyl phosphate are bell-shaped and are superimposable, with the apparent pK alpha values derived from the acidic limb and the basic limb of the profile being 4.4 and 6.8, respectively. This suggests that the rate-limiting step corresponds to the decomposition of the phosphoenzyme intermediate at all pH values. Results from leaving group dependence of kcat at two different pH values support the above conclusion. Furthermore, burst kinetics have been demonstrated with PTP1 using p-nitrophenyl phosphate as a substrate. The rate constants for the formation and the breakdown of the intermediate are 241 and 12 s-1, respectively, at pH 6.0 and 3.5 degrees C. A normal D2O solvent isotope effect (kcatH/kcatD = 1.5) is associated with the breakdown of the phosphoenzyme intermediate, indicating a solvent-derived proton in the transition state. The leaving group dependence of kcat/Km suggests that there is a strong electrophilic interaction between the enzyme and the leaving group oxygen in the transition state of the phosphorylation event. These results are compared with those of the Yersinia PTPase and suggest that the mechanism for PTPase-catalyzed phosphate monoester hydrolysis is conserved from bacterial to mammals.

Isotope effects on the mechanism of calcineurin catalysis: kinetic solvent isotope and isotope exchange studies

The reaction scheme of calcineurin was examined with kinetic and physical approaches. Proton inventory studies of the calcineurin-catalyzed hydrolysis of para-nitrophenyl phosphate were done to probe the role of proton transfer in the mechanism. Control experiments determined that the solvent did not cause the irreversible inactivation of the enzyme and had no effect on the dependence on metal ion or calmodulin. A solvent isotope effect was observed on the Vmax/Km term, but not the Vmax term. The isotope effect was modest with a value of 1.35. Proton inventory data could be fit by multiple parameter sets. The parameter sets yielded fractionation factors of 0.73 for a one-proton transfer or 0.85 for a two-proton transfer. These values compare to the value of 0.69 for reactions involving a water molecule or hydroxide coordinated to metal ion. A chemical mechanism consistent with the proton inventory data and other information about calcineurin catalysis is presented. The simplest model for catalysis involves a single proton transfer from water coordinated to metal that is reasoned to occur during association of the substrate with calcineurin. Questions about the reaction intermediate were also addressed. Attempts to monitor a phosphate-water exchange reaction with 31P nuclear magnetic resonance spectroscopy were unsuccessful. Failure to observe an exchange reaction suggests that no phosphoryl enzyme is formed during the progress of the reaction. Together these data are explained by a model in which cleavage of the phosphate ester bond is catalyzed by a water (hydroxide) molecule coordinated to a divalent metal ion without the formation of a covalent intermediate.

NMR analysis of regioselectivity in dephosphorylation of a triphosphotyrosyl dodecapeptide autophosphorylation site of the insulin receptor by a catalytic fragment of LAR phosphotyrosine phosphatase

An autophosphorylation site in the activated insulin receptor tyrosine kinase domain has three tyrosines phosphorylated when fully activated. To begin to examine recognition of triphosphotyrosyl sites by protein tyrosine phosphatases in possible control of signal transduction a triphosphotyrosyl dodecapeptide TRDIpYETDpYpYRK corresponding to residues 1,142-1,153 of the insulin receptor was prepared and incubated with the 40-kDa catalytic domain of the human PTPase LAR. To assess regioselectivity of recognition, the three diphosphotyrosyl regioisomers, and the three monophosphotyrosyl regioisomers were prepared and assayed. All seven peptides were PTPase substrates. To identify any preferences in dephosphorylation at pY5, pY9, or pY10, 1H-NMR analyses were conducted during enzyme incubations and distinguishing fingerprint regions determined for each of the seven phosphotyrosyl peptides. LAR PTPase shows strong preference for dephosphorylation first at pY5 (at tri-, di-, and monophosphotyrosyl levels). Initially this regioselectivity gives the Y5(pY9)(pY10) diphospho regioisomer, followed by equal dephosphorylation at pY9 or pY10 to give the corresponding monophosphoryl species on the way to fully dephosphorylated product. The NMR methodology is applicable to other peptides with multiple sites of phosphorylation that undergo attack by any phosphatase.

Specific inhibition of calcineurin by type II synthetic pyrethroid insecticides

The inhibitory action of synthetic pyrethroids and some chlorinated hydrocarbon insecticides on the neural calcium-calmodulin-dependent protein phosphatase, calcineurin, was studied using one radiotracer and two colorimetric methods. It was found that all insecticidal Type II pyrethroids (cypermethrin, deltamethrin and fenvalerate) are potent inhibitors of isolated calcineurin from bovine brain. Their IC50 values were approximately 10(-9) to 10(-11) M. By contrast, neither noninsecticidal chiral isomers of these pyrethroids, neuroactive Type I pyrethroids nor neuroactive chlorinated hydrocarbon insecticides showed comparable potencies against this enzyme. To confirm the action of Type II pyrethroid in situ, isolated intact rat brain synaptosomes were incubated with [32P]phosphoric acid and subsequently depolarized in the presence and absence of 0.1 microM deltamethrin. As expected, there was a sharp rise in protein phosphorylation due to the action of calcineurin. Deltamethrin caused a distinct delay in the dephosphorylation process. The results clearly indicate that calcineurin is specifically inhibited by Type II pyrethroids.

Leaving group dependence and proton inventory studies of the phosphorylation of a cytoplasmic phosphotyrosyl protein phosphatase from bovine heart

The kcat and Km values for the bovine heart low molecular weight phosphotyrosyl protein phosphatase catalyzed hydrolysis of 16 aryl phosphate monoesters and of five alkyl phosphate monoesters having the structure Ar(CH2)nOPO3H2 (n = 1-5) were measured at pH 5.0 and 37 degrees C. With the exception of alpha-naphthyl phosphate and 2-chlorophenyl phosphate, which are subject to steric effects, the values of kcat are effectively constant for the aryl phosphate monoesters. This is consistent with the catalysis being nucleophilic in nature, with the existence of a common covalent phosphoenzyme intermediate, and with the breakdown of this intermediate being rate-limiting. In contrast, kcat for the alkyl phosphate monoesters is much smaller and the rate-limiting step for these substrates is interpreted to be the phosphorylation of the enzyme. A single linear correlation is observed for a plot of log (kcat/Km) vs leaving group pKa for both classes of substrates at pH 5.0: log (kcat/Km) = -0.28pKa + 6.88 (n = 19, r = 0.89), indicating a uniform catalytic mechanism for the phosphorylation event. The small change in effective charge (-0.28) on the departing oxygen of the substrate is similar to that observed in the specific acid catalyzed hydrolysis of monophosphate monoanions (-0.27) and is consistent with a strong electrophilic interaction of the enzyme with this oxygen atom in the transition state. The D2O solvent isotope effect and proton inventory experiments indicate that only one proton is "in flight" in the transition state of the phosphorylation process and that this proton transfer is responsible for the reduction of effective charge on the leaving oxygen.

Purification and characterization of a soluble catalytic fragment of the human transmembrane leukocyte antigen related (LAR) protein tyrosine phosphatase from an Escherichia coli expression system

A 350 amino acid soluble fragment of the intracellular catalytic domain of the human transmembrane leukocyte antigen related (LAR) protein tyrosine phosphatase has been purified 17-fold to greater than 90% purity from an Escherichia coli expression vector in quantities sufficient for kinetic and structural characterization. To assess substrate specificity, phosphotyrosine peptides corresponding to autophosphorylation sites of the two major classes of tyrosine kinases have been synthesized. Thus 6-12-residue phosphotyrosine peptides of the insulin receptor and epidermal growth factor receptor kinase domains and of the autophosphorylation and C-terminal regulatory sites of p60src and p56lck have been analyzed for kcat and KM by using a nonradioactive chromogenic assay for Pi release. The catalytic domain of LAR PTPase shows kcat values of 20-70 s-1 for phosphotyrosine peptides and affinities that vary 150-fold from 27 microM to 4.1 mM.

Catalysis of the hydrolysis of phosphorylated pyridines by Mg(OH)+: a possible model for enzymatic phosphoryl transfer

The second-order rate constants for reaction of the Mg2+ complexes of phosphorylated pyridine monoanions with Mg(OH)+ are 10(4)-10(6)-fold larger than the second-order rate constants for their reaction with water (25 degrees C, ionic strength 1.5). Of the 10(6)-fold rate enhancement with the phosphorylated 4-morpholinopyridine/Mg2 complex, approximately 10(4)-fold is attributed to the greater nucleophilicity of Mg(OH)+ compared with water. The remaining catalysis of approximately 10(2)-fold is attributed to induced intramolecularity from positioning of the hydroxide ion and phosphoryl group by the Mg2+ ions. This reaction may provide a model for the role of a metal ion in increasing the concentration of the anions of enolpyruvate and serine and holding the nucleophile in the correct position for phosphoryl transfer in the reactions catalyzed by pyruvate kinase and alkaline phosphatase, for example. Some mechanisms that can provide catalysis of phosphoryl transfer through a metaphosphate-like transition state are reviewed briefly.

Stimulation of the ATP, Mg-dependent protein phosphatase by p-nitrophenyl phosphate

The phosphorylase phosphatase activity of the ATP,Mg-dependent protein phosphatase is stimulated by p-nitrophenyl phosphate (pNPP). All the active forms of this type of enzyme show this property, which seems to be unrelated to any pNPP-hydrolyzing activity. The increase in activity is due to an increase in Vm, the Km being unchanged. The possibility that pNPP acts as a deinhibitor is excluded. pNPP acts as a competitive inhibitor on the phosphorylase phosphatase activity of the different polycation-stimulated protein phosphatases. Stimulation by pNPP can be used as a differential criterion in a specific assay of the active forms of the ATP,Mg-dependent phosphatase.

Modulation of the substrate specificity of the polycation-stimulated protein phosphatase from Xenopus laevis oocytes

A polycation-stimulated (PCS) protein phosphatase was isolated in high yield (280 micrograms/100 g ovaries) from Xenopus laevis oocytes through a procedure involving a tyrosine-agarose hydrophobic chromatography. The 220-kDa enzyme contains a 35-kDa and a 62-kDa subunit. It was identified as the low-Mr polycation-stimulated (PCSL) protein phosphatase. The labile p-nitrophenyl phosphatase activity, copurifying with the phosphorylase phosphatase activity, can be increased severalfold by preincubating the purified enzyme with ATP, its analogues or PPi. This activation is time-dependent and accompanied by a parallel decrease of the phosphorylase phosphatase activity. Although the stimulation was antagonized by metal ions during the preincubation, the basal and ATP-stimulated p-nitrophenyl phosphatase requires Mg2+ or Mn2+ in the assay, with pH optima of 8.5-9 and 7.5 respectively.

Application of 19F nuclear magnetic resonance to examine covalent modification reactions of tyrosyl derivatives: a study of calcineurin catalysis

The hydrolysis of fluorotyrosine phosphate by the calmodulin-activated phosphatase calcineurin has been monitored by 19F nuclear magnetic resonance spectroscopy. Previous work had established that the 19F nuclear magnetic resonance shift of the fluorine nucleus was altered after the phosphorylation of the phenolic hydroxyl group (B. Martin, C.J. Pallen, J.H. Wang, and D.J. Graves (1985) J. Biol. Chem. 260, 14592-14597). The disappearance of substrate and the appearance of product can be measured simultaneously with this approach. Application of the integrated form of the Michaelis-Menten equation yields estimates of the kinetic parameter, KM, close to the values obtained by initial rate kinetics. The velocity term, VM, was also evaluated to be approximately the same value. Calcineurin was determined not to be inactivated over the time period of the reaction. The results demonstrate that 19F nuclear magnetic resonance spectroscopy can be applied to the examination of enzyme-catalyzed reactions.

Distinct specificities of repressible acid phosphatase from yeast toward phosphoseryl and phosphotyrosyl phosphopeptides

By using [32P]-labeled phosphoaminoacids it has been shown that, at mu molar range concentrations, Tyr-32P but neither Ser-32P nor Thr-32P can be significantly dephosphorylated by highly purified repressible acid phosphatase from Saccharomyces cerevisiae. The phosphopeptide Arg-Arg-Ala-Ser(32P)-Val-Ala however, reproducing the phosphorylation site of pyruvate kinase and previously phosphorylated by cAMP-dependent protein kinase, can be very readily dephosphorylated with favourable kinetic constants (Km 0.28 microM, Vmax = 62 units/micrograms) while its derivatives Ala-Ser(32P)-Val-Ala, Arg-Arg-Ala-Thr(32P)-Val-Ala, Arg-Arg-Pro-Ser(32P)-Pro-Ala as well as other peptides and protein substrates phosphorylated by either protein kinase-C or casein kinase-2 are either unaffected or very slowly dephosphorylated by the phosphatase. Conversely Tyr-32P containing angiotensin, poly (Glu, Tyr) 4:1 and the phosphopeptide Asp-Ala-Glu-Tyr(32P)-Ala-Ala-Arg-Arg-Arg-Gly are all dephosphorylated with kinetic constants comparable to those of free phosphotyrosine (Km 0.2-1 microM; Vmax = 4-10 units/micrograms). It is proposed that, while acid phosphatase exhibits a broad specificity toward phosphotyrosine and phosphotyrosyl polypeptides, it is highly selective toward phosphoseryl sites fulfilling definite structural requirements which are reminiscent of those determining phosphorylation by cAMP-dependent protein kinase.