Protein tyrosine phosphatases take off

Expression and physicochemical characterization of an extracellular segment of the receptor protein tyrosine phosphatase IA-2

The receptor protein tyrosine phosphatase superfamily (RPTP) includes proteins with a single transmembrane, one or more intracellular phosphatase, and a variety of extracellular domains. The 106-kDa insulinoma-associated protein (IA-2, ICA512) receptor is unique among RPTP members because: (a) it has a single, phosphatase-like intracellular domain identified as one of the most prominent self antigens in autoimmune diabetes; (b) its extracellular region bears no sequence similarity to known domains; (c) it is present in the membrane of secretory granules in neurons and pancreatic beta-cells where it suffers a complex processing; and (d) it has very poorly understood biological properties. In this work, we describe the expression, purification, and physicochemical characterization of residues 449-576 of IA-2 (IA-2ec(449-576)). Judging from CD, fluorescence, hydrodynamic, and thermal unfolding analyses, this fragment forms an autonomously folding unit with tight packing and well-defined secondary and tertiary structure. CD analysis suggests that about 25% of IA-2ec(449-576) residues are alpha-helical, whereas about the same amount are in beta-sheet structure. The availability of soluble and folded IA-2ec(449-576) is a step forward toward the characterization of a part of IA-2 at atomic detail, which may provide new insight in the biology of diabetes, the neurotransmission process, and the dynamic of secretory granules.

Oxidative Aging and Insulin Receptor Signaling

The life span of nematodes, fruit flies, and mice can be significantly increased (and aging-related changes decreased) by mutations affecting insulin receptor signaling. This effect involves several cellular functions which are negatively regulated by the insulin receptor and thus typically expressed under fasting conditions. This involvement raises the question of whether the insulin-independent basal receptor kinase activity in the postabsorptive state can be decreased without compromising the physiologically important response to insulin in the postprandial state. Recent studies have shown that (a) the basal human insulin receptor kinase activity is increased under oxidative conditions in the absence of insulin and (b) insulin signaling in the fasted state can be decreased by cysteine supplementation. Cysteine supplementation has also been shown to improve certain aging-related parameters, suggesting that the average dietary cysteine consumption in Western countries may be suboptimal. These findings provide a conceptual framework that extends the "free radical theory of aging."

Oxidative enhancement of insulin receptor signaling: experimental findings and clinical implications

Signaling through the insulin receptor and several other receptor tyrosine kinases is subject to redox regulation. Prolonged exposure to hydrogen peroxide impairs the action of insulin, and may account to some extent for the decreased insulin responsiveness in hyperglycemic diabetic patients. However, insulin receptor kinase (IRK) autophosphorylation and/or kinase activity were found to be markedly enhanced by a more limited exposure to hydrogen peroxide or by an oxidative shift in the thiol/disulfide redox status. Oxidative enhancement of IRK function may be mediated by two different mechanisms with similar effects, i.e., by direct oxidative activation of IRK activity or by oxidative inactivation of a protein tyrosine phosphatase, which otherwise down-regulates IRK-mediated signaling. As both mechanisms enhance IRK activity in the absence of insulin, there is a strong possibility that the background IRK activity in the postabsorptive period may be abnormally increased in certain oxidative conditions and thereby disturb the metabolism of glucose and other energy substrates. This remains to be tested. In line with the oxidative enhancement of IRK activity, clinical studies have shown that treatment with a thiol-containing antioxidant increases the postabsorptive glucose and/or insulin concentrations (i.e., the HOMA-R index) at least under certain conditions. This effect may have therapeutic implications.

A structural perspective of CTD function

The C-terminal domain (CTD) of RNA polymerase II (Pol II) integrates nuclear events by binding proteins involved in mRNA biogenesis. CTD-binding proteins recognize a specific CTD phosphorylation pattern, which changes during the transcription cycle, due to the action of CTD-modifying enzymes. Structural and functional studies of CTD-binding and -modifying proteins now reveal some of the mechanisms underlying CTD function. Proteins recognize CTD phosphorylation patterns either directly, by contacting phosphorylated residues, or indirectly, without contact to the phosphate. The catalytic mechanisms of CTD kinases and phosphatases are known, but the basis for CTD specificity of these enzymes remains to be understood.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

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.

Mitochondrial Function Is Required for Hydrogen Peroxide-induced Growth Factor Receptor Transactivation and Downstream Signaling

The transactivation of growth factor receptors is an early event in H(2)O(2)-induced signaling, although proximal targets in this process remain unclear. We found that inhibition of flavin- or heme-containing proteins eliminated H(2)O(2)-induced transactivation of the epidermal growth factor receptor and stimulation of its downstream targets, JNK and Akt. Inhibition of mitochondrial function with rotenone, antimycin A, KCN, carbonylcyanide-m-chlorophenylhydrazone, or oligomycin reproduced this effect, as did generation of mitochondrial DNA-deficient (pseudo-rho(0)) cells. Mitochondrial function had no role in JNK activation in response to UV irradiation or tumor necrosis factor-alpha. The impact of mitochondrial function on H(2)O(2)-induced growth factor transactivation was ubiquitous and applied to both the vascular endothelial growth factor (VEGF)-2 receptor and the platelet-derived growth factor-beta receptor in endothelium and fibroblasts, respectively. In contrast, ligand-induced growth factor activation was unrelated to mitochondrial function. Growth factor receptor transactivation and its downstream signaling in response to H(2)O(2) appeared to involve redox-sensitive mitochondrial events as they were abrogated by a mitochondrial-targeted antioxidants but not their nontargeted counterparts. Functionally, we found that mitochondrial-targeted antioxidants inhibited H(2)O(2)-induced apoptosis and cell death but had no effect with UV irradiation. These data establish a novel role for the mitochondrion as a proximal target specific to H(2)O(2)-induced signaling and growth factor transactivation.

The tyrosine phosphatase inhibitor orthovanadate mimics NGF-induced neuroprotective signaling in rat hippocampal neurons

Activation of the high affinity neurotrophin receptor tropomyosin-related kinase A (TrkA) by nerve growth factor (NGF) leads to phosphorylation of intracellular tyrosine residues of the receptor with subsequent activation of signaling pathways involved in neuronal survival such as the phosphoinositide-3-kinase (PI3-K)/protein kinase B (PKB/Akt) pathway and the mitogen-activated protein kinase (MAPK) cascade. In the present study, we tested whether inhibition of protein-tyrosine phosphatases (PTP) by orthovanadate could enhance tyrosine phosphorylation of TrkA thereby stimulating NGF-like survival signaling in embryonic hippocampal neurons. We found that the PTP inhibitor orthovanadate (1 microM) enhanced TrkA phosphorylation and protected neurons against staurosporine (STS)-induced apoptosis in a time-and concentration-dependent manner. Inhibition of PTP enhanced TrkA phosphorylation also in the presence of NGF antibodies indicating that NGF binding to TrkA was not required for the effects of orthovanadate. Moreover, orthovanadate enhanced phosphorylation of Akt and the MAPK Erk1/2 suggesting that the signaling pathways involved in the protective effect were similar to those activated by NGF. Accordingly, inhibition of PI3-K by wortmannin and MAPK-kinase (MEK) inhibition by UO126 abolished the neuroprotective effects. In conclusion, the results indicate that orthovanadate mimics the effect of NGF on survival signaling pathways in hippocampal neurons. Thus, PTP inhibition appears to be an appropriate strategy to trigger neuroprotective signaling pathways downstream of neurotrophin receptors.

Oxidative stress and aging

Free radical-derived reactive oxygen species (ROS) are constantly generated in most living tissue and can potentially damage DNA, proteins and lipids. "Oxidative stress" occurs if ROS reach abnormally high concentrations. Harman was the first to propose that the damaging effects of ROS may play a key role in the mechanism of aging. Genetic studies of such distantly related species as C. elegans, Drosophila melanogaster, and mice support this hypothesis. However, ROS are not only a cause of structural damage, but also physiologically important mediators in biological signaling processes. Abnormally high levels of ROS may therefore lead to dysregulation of redox-sensitive signaling pathways. The redox-sensitive targets in these pathways are often signaling proteins with redox-sensitive cysteine residues which are oxidized to sulfenic acid moieties and mixed disulfides, thereby altering the signaling function of the protein. Because the formation of these mixed disulfides can also occur through a prooxidative shift in the intracellular thiol/disulfide redox status (REDST), the respective signaling pathways respond not only to ROS but also to changes in REDST. Information about the concentration of ROS in living tissue is scarce, but aging-related changes in REDST are well documented. Several studies with cell cultures or experimental animals have shown that the oxidative shift in the intracellular glutathione REDST is typically associated with cellular dysfunction. Complementary studies in humans have shown that oxidative changes in the plasma (i.e., extracellular) REDST are correlated with aging-related pathophysiological processes. The available evidence suggests that these changes play a key role in various conditions which limit the human life span. Several attempts have been made to ameliorate the consequences of aging by thiol-containing antioxidants, but this approach requires a detailed knowledge of the effects of thiol-containing antioxidants on cysteine homeostasis, REDST, and redox-sensitive signaling pathways of the host.

Subdomain X of the kinase domain of Lck binds CD45 and facilitates dephosphorylation

CD45 is a transmembrane, two-domain protein-tyrosine phosphatase expressed exclusively in nucleated hematopoietic cells. The Src family kinase, Lck, is a major CD45 substrate in T cells and CD45 dephosphorylation of Lck is important for both T cell development and activation. However, how the substrate specificity of phosphatases such as CD45 is achieved is not well understood. Analysis of the interaction between the cytoplasmic domain of CD45 and its substrate, Lck, revealed that the active, membrane-proximal phosphatase domain of CD45 (CD45-D1) bound to the phosphorylated Lck kinase domain, the SH2 domain, and the unique N-terminal region of Lck. The second, inactive phosphatase domain (CD45-D2) bound only to the kinase domain of Lck. CD45-D2 was unable to bind phosphotyrosine, and its interaction with the kinase domain of Lck was independent of tyrosine phosphorylation. The binding of CD45-D2 was localized to subdomain X (SD10) of Lck. CD45-D2 bound similarly to Src family kinases but bound Csk to a lesser extent and did not bind significantly to the less related kinase, Erk1. CD45 dephosphorylated Lck and Src at similar rates but dephosphorylated Csk and Erk1 at lower rates. Replacement of Erk1 SD10 with that of Lck resulted in the binding of CD45-D2 and the conversion of Erk1 to a more efficient CD45 substrate. This demonstrates a role for CD45-D2 in binding substrate and identifies the SD10 region in Lck as a novel site involved in substrate recognition.

The mRNA transcription/processing factor Ssu72 is a potential tyrosine phosphatase

Ssu72 is an essential and highly conserved protein involved in mRNA transcription and 3'-end processing. The biochemical function of Ssu72 was so far unknown. We report here evidence that Ssu72 is a phosphatase that resembles protein tyrosine phosphatases (PTPases). First, recombinant Ssu72 cleaves the phosphotyrosine analogue p-nitrophenylphosphate, and this catalytic activity is impaired by PTPase-inhibiting agents. Second, the Ssu72 sequence contains the CX(5)R signature motif of PTPases; mutation of the catalytic cysteine in this motif abolishes Ssu72 activity in vitro and has been shown to confer lethality in vivo. Third, secondary structure prediction and site-directed mutagenesis predict that Ssu72 adopts the fold of PTPases of the low molecular weight family. Distinguishing features, such as a short "aspartate loop" at the active site, suggest however that Ssu72 is the founding member of a new phosphatase subfamily. The novel Ssu72 activity may regulate coupling events during mRNA biogenesis.

Enzyme kinetic characterization of protein tyrosine phosphatases

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

Development of an efficient "substrate-trapping" mutant of Src homology phosphotyrosine phosphatase 2 and identification of the epidermal growth factor receptor, Gab1, and three other proteins as target substrates

Src homology containing phosphotyrosine phosphatase 2 (SHP2) is a positive effector of growth factor, cytokine, and integrin signaling. However, neither its physiological substrate nor its mechanism of action in tyrosine kinase signaling has been demonstrated. We reasoned that the identification of physiological substrates of SHP2 would be a stepping stone in elucidating its mechanism of action, and, thus, we constructed a potent trapping mutant of SHP2. Surprisingly, the frequently used Asp to Ala substitution did not give rise to a trapping mutant. However, we were able to develop an efficient trapping mutant of SHP2 by introducing Asp to Ala and Cys to Ser double mutations. The double mutant (DM) protein identified the epidermal growth factor receptor (EGFR), the Grb2 binder 1, and three other, as yet unidentified, phosphotyrosyl proteins as candidate physiological substrates. Given that substrate trapping occurred in intact cells and that the interaction was very specific, it is highly likely that EGFR and Gab1 represent physiological SHP2 substrates. Therefore, the DM protein would serve as an important tool in future SHP2 studies, including identification of p190, p150, and p90.

Archaeal protein kinases and protein phosphatases: insights from genomics and biochemistry

Protein phosphorylation/dephosphorylation has long been considered a recent addition to Nature's regulatory arsenal. Early studies indicated that this molecular regulatory mechanism existed only in higher eukaryotes, suggesting that protein phosphorylation/dephosphorylation had emerged to meet the particular signal-transduction requirements of multicellular organisms. Although it has since become apparent that simple eukaryotes and even bacteria are sites of protein phosphorylation/dephosphorylation, the perception widely persists that this molecular regulatory mechanism emerged late in evolution, i.e. after the divergence of the contemporary phylogenetic domains. Only highly developed cells, it was reasoned, could afford the high 'overhead' costs inherent in the acquisition of dedicated protein kinases and protein phosphatases. The advent of genome sequencing has provided an opportunity to exploit Nature's phylogenetic diversity as a vehicle for critically examining this hypothesis. In tracing the origins and evolution of protein phosphorylation/dephosphorylation, the members of the Archaea, the so-called 'third domain of life', will play a critical role. Whereas several studies have demonstrated that archaeal proteins are subject to modification by covalent phosphorylation, relatively little is known concerning the identities of the proteins affected, the impact on their functional properties, or the enzymes that catalyse these events. However, examination of several archaeal genomes has revealed the widespread presence of several ostensibly 'eukaryotic' and 'bacterial' protein kinase and protein phosphatase paradigms. Similar findings of 'phylogenetic trespass' in members of the Eucarya (eukaryotes) and the Bacteria suggest that this versatile molecular regulatory mechanism emerged at an unexpectedly early point in development of 'life as we know it'.

Regulation of insulin receptor signaling by the protein tyrosine phosphatase TCPTP

The human protein tyrosine phosphatase TCPTP exists as two forms: an endoplasmic reticulum-targeted 48-kDa form (TC48) and a nuclear 45-kDa form (TC45). Although targeted to the nucleus, TC45 can exit in response to specific stimuli to dephosphorylate cytoplasmic substrates. In this study, we investigated the downregulation of insulin receptor (IR) signaling by TCPTP. In response to insulin stimulation, the TC48-D182A and TC45-D182A "substrate-trapping" mutants formed stable complexes with the endogenous tyrosine-phosphorylated IR beta-subunit in 293 cells. Moreover, in response to insulin stimulation, the TC45-D182A mutant accumulated in the cytoplasm of cells overexpressing the IR and in part colocalized with the IR beta-subunit at the cell periphery. These results indicate that the IR may serve as a cellular substrate for both TC48 and TC45. In immortalized TCPTP(-/-) murine embryo fibroblasts, insulin-induced IR beta-subunit tyrosine phosphorylation and protein kinase PKB/Akt activation were enhanced relative to the values in TCPTP(+/+) cells. Importantly, the expression of TC45 or TC48 to physiological levels suppressed the enhanced insulin-induced signaling in TCPTP(-/-) cells. These results indicate that the differentially localized variants of TCPTP may dephosphorylate the IR and downregulate insulin-induced signaling in vivo.

Intramolecular dynamics of low molecular weight protein tyrosine phosphatase in monomer-dimer equilibrium studied by NMR: a model for changes in dynamics upon target binding

Low molecular weight protein tyrosine phosphatase (LMW-PTP) dimerizes in the phosphate-bound state in solution with a dissociation constant of K(d)=1.5(+/-0.1)mM and an off-rate on the order of 10(4)s(-1). 1H and 15N NMR chemical shifts identify the dimer interface, which is in excellent agreement with that observed in the crystal structure of the dimeric S19A mutant. Two tyrosine residues of each molecule interact with the active site of the other molecule, implying that the dimer may be taken as a model for a complex between LMW-PTP and a target protein. 15N relaxation rates for the monomeric and dimeric states were extrapolated from relaxation data acquired at four different protein concentrations. Relaxation data of satisfactory precision were extracted for the monomer, enabling model-free analyses of backbone fluctuations on pico- to nanosecond time scales. The dimer relaxation data are of lower quality due to extrapolation errors and the possible presence of higher-order oligomers at higher concentrations. A qualitative comparison of order parameters in the monomeric and apparent dimeric states shows that loops forming the dimer interface become rigidified upon dimerization. Qualitative information on monomer-dimer exchange and intramolecular conformational exchange was obtained from the concentration dependence of auto- and cross-correlated relaxation rates. The loop containing the catalytically important Asp129 fluctuates between different conformations in both the monomeric and dimeric (target bound) states. The exchange rate compares rather well with that of the catalyzed reaction step, supporting existing hypotheses that catalysis and enzyme dynamics may be coupled. The side-chain of Trp49, which is important for substrate specificity, exhibits conformational dynamics in the monomer that are largely quenched upon formation of the dimer, suggesting that binding is associated with the selection of a single side-chain conformer.

Synthesis and PTP1B inhibition of 1,2-naphthoquinone derivatives as potent anti-diabetic agents

A new series of 1,2-naphthoquinone derivatives was synthesized by various synthetic methods and evaluated for their ability to inhibit protein tyrosine phosphatase 1B (PTP1B). 1,2-Naphthoquinone derivatives with substituent at R(4) position showed submicromolar inhibitory activity, and compound 24 demonstrated 10- to 60-fold selectivity against the tested phosphatases. Also, several 4-aryl-1,2-naphthoquinone derivatives with substituents at R(3), R(6), R(7), or/and R(8) showed submicromolar inhibitory activity and good plasma stability.

Tk-PTP, protein tyrosine/serine phosphatase from hyperthermophilic archaeon Thermococcus kodakaraensis KOD1: enzymatic characteristics and identification of its substrate proteins

The Tk-ptp gene encoding a protein tyrosine phosphatase from the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1 was cloned and biochemical characteristics of the recombinant protein (Tk-PTP) were examined. A series of mutants, D63A (replacing Asp-63 with Ala), C93S, C93A, R99K, and R99M, were also constructed and analyzed. Two unique features were found. First, the Tk-PTP showed the phosphatase activity not only toward phosphotyrosine but also toward phosphoserine. Second, the conserved Asp-63, which corresponds to a critical residue among other known PTPs, was not essential for catalysis. Cys-93 and Arg-99 residues played a crucial role in substrate binding and catalysis. To know a specific substrate for Tk-PTP, C93S mutant was used to trap substrate proteins from cell extract of KOD1. Phenylalanyl-tRNA synthetase subunit beta-chain, one of the gene products of RNA terminal phosphate cyclase operon and phosphomannomutase, was identified, suggesting that they functioned for phosphate donation.

Reversible oxidation and inactivation of protein tyrosine phosphatases in vivo

We have investigated the regulation of protein tyrosine phosphatases (PTPs) by reactive oxygen species (ROS) in a cellular environment. We demonstrate that multiple PTPs were reversibly oxidized and inactivated following treatment of Rat-1 cells with H(2)O(2) and that inhibition of PTP function was important for ROS-induced mitogenesis. Furthermore, we show transient oxidation of the SH2 domain containing PTP, SHP-2, in response to PDGF that requires association with the PDGFR. Our results indicate that SHP-2 inhibits PDGFR signaling and suggest a mechanism by which autophosphorylation of the PDGFR occurs despite its association with SHP-2. The data suggest that several PTPs may be regulated by oxidation and that characterization of this process may define novel links between specific PTPs and particular signaling pathways in vivo.

Free radicals in the physiological control of cell function

At high concentrations, free radicals and radical-derived, nonradical reactive species are hazardous for living organisms and damage all major cellular constituents. At moderate concentrations, however, nitric oxide (NO), superoxide anion, and related reactive oxygen species (ROS) play an important role as regulatory mediators in signaling processes. Many of the ROS-mediated responses actually protect the cells against oxidative stress and reestablish "redox homeostasis." Higher organisms, however, have evolved the use of NO and ROS also as signaling molecules for other physiological functions. These include regulation of vascular tone, monitoring of oxygen tension in the control of ventilation and erythropoietin production, and signal transduction from membrane receptors in various physiological processes. NO and ROS are typically generated in these cases by tightly regulated enzymes such as NO synthase (NOS) and NAD(P)H oxidase isoforms, respectively. In a given signaling protein, oxidative attack induces either a loss of function, a gain of function, or a switch to a different function. Excessive amounts of ROS may arise either from excessive stimulation of NAD(P)H oxidases or from less well-regulated sources such as the mitochondrial electron-transport chain. In mitochondria, ROS are generated as undesirable side products of the oxidative energy metabolism. An excessive and/or sustained increase in ROS production has been implicated in the pathogenesis of cancer, diabetes mellitus, atherosclerosis, neurodegenerative diseases, rheumatoid arthritis, ischemia/reperfusion injury, obstructive sleep apnea, and other diseases. In addition, free radicals have been implicated in the mechanism of senescence. That the process of aging may result, at least in part, from radical-mediated oxidative damage was proposed more than 40 years ago by Harman (J Gerontol 11: 298-300, 1956). There is growing evidence that aging involves, in addition, progressive changes in free radical-mediated regulatory processes that result in altered gene expression.

The YRD Motif Is a Major Determinant of Substrate and Inhibitor Specificity in T-cell Protein-tyrosine Phosphatase

We have studied T-cell protein-tyrosine phosphatase (TCPTP) as a model phosphatase in an attempt to unravel amino acid residues that may influence the design of specific inhibitors. Residues 48--50, termed the YRD motif, a region that is found in protein-tyrosine phosphatases, but absent in dual-specificity phosphatases was targeted. YRD derivatives of TCPTP were characterized by steady-state kinetics and by inhibition studies with BzN-EJJ-amide, a potent inhibitor of TCPTP. Substitution of Asp(50) to alanine or Arg(49) to lysine, methionine, or alanine significantly affected substrate hydrolysis and led to a substantial decrease in affinity for BzN-EJJ-amide. The influence of residue 49 on substrate/inhibitor selectivity was further investigated by comparing subsite amino acid preferences of TCPTP and its R49K derivative by affinity selection coupled with mass spectrometry. The greatest effect on selectivity was observed on the residue that precedes the phosphorylated tyrosine. Unlike wild-type TCPTP, the R49K derivative preferred tyrosine to aspartic or glutamic acid. BzN-EJJ-amide which retains the preferred specificity requirements of TCPTP and PTP1B was equipotent on both enzymes but greater than 30-fold selective over other phosphatases. These results suggest that Arg(49) and Asp(50) may be targeted for the design of potent and selective inhibitors of TCPTP and PTP1B.

The catalytic mechanism of protein tyrosine phosphatases revisited

Experimental and theoretical studies of the catalytic mechanism in protein tyrosine phosphatases and dual specific phosphatases are reviewed. The structural properties of these enzymes contributing to the efficient rate enhancement of phosphate monoester hydrolysis have been established during the last decade. There are, however, uncertainties in the interpretation of available experimental data that make the commonly assumed reaction mechanism somewhat doubtful. Theoretical calculations as well as analysis of crystal structures point towards an alternative interpretation of the ionisation state in the reactive complex.

Inhibition of cell growth and spreading by stomach cancer-associated protein-tyrosine phosphatase-1 (SAP-1) through dephosphorylation of p130cas

SAP-1 (stomach cancer-associated protein-tyrosine phosphatase-1) is a transmembrane-type protein-tyrosine phosphatase that is abundant in the brain and certain cancer cell lines. With the use of a "substrate-trapping" approach, p130(cas), a major focal adhesion-associated phosphotyrosyl protein, has now been identified as a likely physiological substrate of SAP-1. Expression of recombinant SAP-1 induced the dephosphorylation of p130(cas) as well as that of two other components of the integrin-signaling pathway (focal adhesion kinase and p62(dok)) in intact cells. In contrast, expression of a substrate-trapping mutant of SAP-1 induced the hyperphosphorylation of these proteins, indicating a dominant negative effect of this mutant. Overexpression of SAP-1 induced disruption of the actin-based cytoskeleton as well as inhibited various cellular responses promoted by integrin-mediated cell adhesion, including cell spreading on fibronectin, growth factor-induced activation of extracellular signal-regulated kinase 2, and colony formation. Finally, the enzymatic activity of SAP-1, measured with an immunocomplex phosphatase assay, was substantially increased by cell-cell adhesion. These results suggest that SAP-1, by mediating the dephosphorylation of focal adhesion-associated substrates, negatively regulates integrin-promoted signaling processes and, thus, may contribute to contact inhibition of cell growth and motility.

Combinatorial control of the specificity of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs), the enzymes that dephosphorylate tyrosyl phosphoproteins, were initially believed to be few in number and serve a 'housekeeping' role in signal transduction. Recent work indicates that this is totally incorrect. Instead, PTPs comprise a large superfamily whose members play critical roles in a wide variety of cellular processes. Moreover, PTPs exhibit exquisite substrate specificity in vivo. Recent evidence has led us to propose that members of the PTP family achieve selectivity through different combinations of specific targeting strategies and intrinsic catalytic domain specificity.

HD-PTP: A novel protein tyrosine phosphatase gene on human chromosome 3p21.3

A human cDNA encoding a novel protein tyrosine phosphatase has been isolated. The phosphatase has unique features in its domain structure: a "Zn-hand" domain containing several SH3-binding motifs, a tyrosine phosphatase domain, a C-terminal PEST motif, and an N-terminal domain similar to yeast BRO1, an apoptosis-related mammalian AIP1 and to a RHO-binding protein, Rhophilin. The gene is located at chromosome 3p21.3, an area frequently deleted in many types of cancer, especially within the functionally defined narrow region. The gene may be a human homolog of the rat PTP-TD14 gene reported by others, which can suppress H-ras-mediated transformation. We identified a hemizygous missense mutation in a lung cancer cell line. Thus, the phosphatase gene may be a candidate for one of the tumor suppressor genes located on 3p21.3.

Deficiency of Src homology 2-containing phosphatase 1 results in abnormalities in murine neutrophil function: studies in motheaten mice

Neutrophils, an essential component of the innate immune system, are regulated in part by signaling pathways involving protein tyrosine phosphorylation. While protein tyrosine kinase functions in regulating neutrophil behavior have been extensively investigated, little is known about the role for specific protein tyrosine phosphatases (PTP) in modulating neutrophil signaling cascades. A key role for Src homology 2 domain-containing phosphatase 1 (SHP-1), a PTP, in neutrophil physiology is, however, implied by the overexpansion and inappropriate activation of granulocyte populations in SHP-1-deficient motheaten (me/me) and motheaten viable (me(v)/me(v)) mice. To directly investigate the importance of SHP-1 to phagocytic cell function, bone marrow neutrophils were isolated from both me/me and me(v)/me(v) mice and examined with respect to their responses to various stimuli. The results of these studies revealed that both quiescent and activated neutrophils from motheaten mice manifested enhanced tyrosine phosphorylation of cellular proteins in the 60- to 80-kDa range relative to that detected in wild-type congenic control neutrophils. MOTHEATEN: neutrophils also demonstrated increased oxidant production, surface expression of CD18, and adhesion to protein-coated plastic. Chemotaxis, however, was severely diminished in the SHP-deficient neutrophils relative to control neutrophils, which was possibly attributable to a combination of defective deadhesion and altered actin assembly. Taken together, these results indicate a significant role for SHP-1 in modulating the tyrosine phosphorylation-dependent signaling pathways that regulate neutrophil microbicidal functions.

Mapping and identification of protein-protein interactions by two-dimensional far-Western immunoblotting

Studies of protein-protein interactions have proved to be a useful approach to link proteins of unknown function to known cellular processes. In this study we have combined several existing methods to attempt the comprehensive identification of substrates for poorly characterized human protein tyrosine phosphatases (PTPs). We took advantage of so-called "substrate trapping" mutants, a procedure originally described by Flint et al. (Proc. Natl. Acad. Sci. USA 1997, 94, 1680-1685) to identify binding partners of cloned PTPs. This procedure was adapted to a proteome-wide approach to probe for candidate substrates in cellular extracts that were separated by two-dimensional (2-D) gel electrophoresis and blotted onto membranes. Protein-protein interactions were revealed by far-Western immunoblotting and positive binding proteins were subsequently identified from silver-stained gels using tandem mass spectrometry. With this method we were able to identify possible substrates for PTPs without using any radio-labeled cDNA or protein probes and showed that they corresponded to tyrosine phosphorylated proteins. We believe that this method could be generally applied to identify possible protein-protein interactions.

Acid phosphatase locus 1 (ACP1): Possible relationship of allelic variation to body size and human population adaptation to thermal stress-A theoretical perspective

The acid phosphatase locus 1 (ACP1) codes for a low molecular weight phosphotyrosine protein phosphatase that has the important action of dephosphorylating tyrosine phosphorylated proteins and peptides and a second important role in modulating flavin cofactor levels and the activity of flavo-enzymes. These functions significantly influence cell division, differentiation, and growth. Two alleles (ACP1*A and ACP1*B) reach polymorphic frequencies at the ACP1 locus in all human populations, while the ACP1*C and ACP1*R alleles reach polymorphic frequencies in restricted geographical regions. The worldwide distribution of these alleles, and data from several clinical studies, strongly suggest that the ACP1 locus functions to modulate growth and that selection at this locus is a component of the selective processes influencing body mass and human population adaptation to thermal stress. The ACP1*A allele reaches highest frequencies at extreme latitudes and appears to be associated with maximizing body mass and adaptation to cold stress, whereas the ACP1*B allele reaches highest frequencies in tropical and subtropical environments and appears to be associated with minimizing body mass and adaptation to heat stress. The high frequency of the ACP1*C allele at northern latitudes, where ACP1*A allele frequencies are elevated, may be a mechanism for limiting fetal and maternal complications associated with fetal macrosomia and adult obesity in populations where protein and calorie intake are relatively high. Am. J. Hum. Biol. 12:688-701, 2000. Copyright 2000 Wiley-Liss, Inc.

Protein-tyrosine phosphatase reduces the number of apical small conductance K+ channels in the rat cortical collecting duct

Previous studies have demonstrated that an increase in the activity of protein-tyrosine kinase (PTK) is involved in the down-regulation of the activity of apical small conductance K(+) (SK) channels in the cortical collecting duct (CCD) from rats on a K(+)-deficient diet (). We used the patch clamp technique to investigate the role of protein-tyrosine phosphatase (PTP) in the regulation of the activity of SK channels in the CCD from rats on a high K(+) diet. Western blot analysis indicated that PTP-1D is expressed in the renal cortex. Application of 1 microm phenylarsine oxide (PAO) or 1 mm benzylphosphonic acid, agents that inhibit PTP, reversibly reduced channel activity by 95%. Pretreatment of CCDs with PAO for 30 min decreased the mean NP(o) reversibly from control value 3.20 to 0.40. Addition of 1 microm herbimycin A, an inhibitor of PTK, had no significant effect on channel activity in the CCDs from rats on a high K(+) diet. However, herbimycin A abolished the inhibitory effect of PAO, indicating that the effect of PAO is the result of interaction between PTK and PTP. Addition of brefeldin A, an agent that blocks protein trafficking from Golgi complex to the membrane, had no effect on channel activity. Moreover, application of colchicine, a microtubule inhibitor, or paclitaxel, a microtubule stabilizer, had no effect on channel activity. In contrast, PAO still reduced channel activity in the presence of brefeldin A, colchicine, or paclitaxel. Furthermore, the effect of PAO on channel activity was absent when the tubules were bathed in 16% sucrose-containing bath solution or treated with concanavalin A. We conclude that PTP is involved in the regulation of the activity of SK channels and that inhibition of PTP may facilitate the internalization of the SK channels.

Stable interdomain interaction within the cytoplasmic domain of CD45 increases enzyme stability

CD45 is a leukocyte-specific, two domain transmembrane tyrosine phosphatase. Co-purification of a recombinant protein containing the first phosphatase domain of CD45 (6His-D1) with a recombinant protein containing the second phosphatase domain (GST-D2) from E. coli indicated a stable interaction which resulted in increased stability of the active phosphatase domain present in 6His-D1. This interaction was not dependent on the acidic region unique to CD45 domain 2, but was affected by a destabilizing point mutation (Q1180G) in GST-D2. CD45 domain 2 enhanced phosphatase activity of the first domain in the full length cytoplasmic domain protein, whereas a chimeric protein with the SH2 domain of p56(lck) in place of the CD45 C-terminal region did not. Thus the C-terminal domain of CD45 associates with the N-terminal domain and this stabilizes the active phosphatase domain. A single destabilizing point mutation in the second domain is sufficient to attenuate this effect.

Hyperoxia induces S-phase cell-cycle arrest and p21(Cip1/Waf1)-independent Cdk2 inhibition in human carcinoma T47D-H3 cells

Little is known about cell-cycle checkpoint activation by oxidative stress in mammalian cells. The effects of hyperoxia on cell-cycle progression were investigated in asynchronous human T47D-H3 cells, which contain mutated p53 and fail to arrest at G1/S in response to DNA damage. Hyperoxic exposure (95% O(2), 40-64 h) induced an S-phase arrest associated with acute inhibition of Cdk2 activity and DNA synthesis. In contrast, exit from G2/M was not inhibited in these cells. After 40 h of hyperoxia, these effects were partially reversible during recovery under normoxic conditions. The inhibition of Cdk2 activity was not due to degradation of Cdk2, cyclin E or A, nor impairment of Cdk2 complex formation with cyclin A or E and p21(Cip1). The loss of Cdk2 activity occurred in the absence of induction and recruitment of cdk inhibitor p21(Cip1) or p27(Kip1) in cyclin A/Cdk2 or cyclin E/Cdk2 complexes. In contrast, Cdk2 inhibition was associated with increased Cdk2-Tyr15 phosphorylation, increased E2F-1 recruitment, and decreased PCNA contents in Cdk2 complexes. The latter results indicate a p21(Cip1)/p27(Kip1)-independent mechanism of S-phase checkpoint activation in the hyperoxic T47D cell model investigated.

N-(cyclohexanecarboxyl)-O-phospho-l-serine, a minimal substrate for the dual-specificity protein phosphatase IphP

Three dual-specific phosphatases [DSPs], IphP, VHR, and Cdc14, and three protein-tyrosine phosphatases [PTPs], PTP-1B, PTP-H1, and Tc-PTPa, were challenged with a set of low molecular weight phosphoesters to probe the factors underlying the distinct substrate specificities displayed by these two mechanistically homologous families of protein phosphatases. It was observed that beta-naphthyl phosphate represented an excellent general substrate for both PTPs and DSPs. While DSPs tended to hydrolyze alpha-naphthyl phosphate at rates comparable to that of the beta-isomer, the PTPs PTP-1B and Tc-PTPa did not. PTP-H1, however, displayed high alpha-naphthyl phosphatase activity. Intriguingly, PTP-H1 also displayed much higher protein-serine phosphatase activity in vitro, 0.2-0.3% that toward equivalent tyrosine phosphorylated proteins, than did PTP-1B or Tc-PTPa. The latter two PTPs discriminated between the serine- and tyrosine-phosphorylated forms of two test proteins by factors of >/=10(4)-10(6). While free phosphoserine represented an extremely poor substrate for all of the DSPs examined, the addition of a hydrophobic "handle" to form N-(cyclohexanecarboxyl)-O-phospho-l-serine produced a compound that was hydrolyzed by IphP with high efficiency, i.e., at a rate comparable to that of free phosphotyrosine or p-nitrophenyl phosphate. VHR also hydrolyzed N-(cyclohexanecarboxyl)-O-phospho-l-serine (1 mM) at a rate approximately one-tenth that of beta-naphthyl phosphate. None of the PTPs tested exhibited significant activity against this compound. However, N-(cyclohexanecarboxyl)-O-phospho-l-serine did not prove to be a universal substrate for DSPs as Cdc14 displayed little propensity to hydrolyze it.

2-(oxalylamino)-benzoic acid is a general, competitive inhibitor of protein-tyrosine phosphatases

Protein-tyrosine phosphatases (PTPs) are critically involved in regulation of signal transduction processes. Members of this class of enzymes are considered attractive therapeutic targets in several disease states, e.g. diabetes, cancer, and inflammation. However, most reported PTP inhibitors have been phosphorus-containing compounds, tight binding inhibitors, and/or inhibitors that covalently modify the enzymes. We therefore embarked on identifying a general, reversible, competitive PTP inhibitor that could be used as a common scaffold for lead optimization for specific PTPs. We here report the identification of 2-(oxalylamino)-benzoic acid (OBA) as a classical competitive inhibitor of several PTPs. X-ray crystallography of PTP1B complexed with OBA and related non-phosphate low molecular weight derivatives reveals that the binding mode of these molecules to a large extent mimics that of the natural substrate including hydrogen bonding to the PTP signature motif. In addition, binding of OBA to the active site of PTP1B creates a unique arrangement involving Asp(181), Lys(120), and Tyr(46). PTP inhibitors are essential tools in elucidating the biological function of specific PTPs and they may eventually be developed into selective drug candidates. The unique enzyme kinetic features and the low molecular weight of OBA makes it an ideal starting point for further optimization.

Mutation analysis of the putative tumor suppressor gene PTEN/MMAC1 in cervical cancer

OBJECTIVE

PTEN/MMAC1, a candidate tumor suppressor gene located at chromosome 10q23.3, was recently identified and found to be homozygously deleted or mutated in several different types of human tumors. The aim of this study is to determine whether PTEN/MMAC1 is a target for 10q loss of heterozygosity in cervical cancer.

METHOD

We examined 50 primary cervical carcinoma specimens using a PCR-based assay followed by SSCP and direct sequencing. The genomic DNA was also confirmed by Southern blot analysis.

RESULTS

All specimens except one, which has a 7-base deletion, showed a negative result. Among them, 30 randomly selected cases and their paired noncancerous tissue were further screened using nested RT-PCR. Six of 30 cervical cancerous tissues had aberrant transcripts. However, 4 of the matched noncancerous tissues also had aberrant transcripts. Southern blot analysis of the entire genomic DNA did not reveal any evidence of gene alteration.

CONCLUSIONS

Sequence abnormalities in the PTEN/MMAC1 gene were only detected in 1 of 50 cervical cancers analyzed indicating that aberrant PTEN/MMAC1 function is an uncommon event in the development of cervix cancers. However, similar to studies with the TSG101 gene, screening for aberrant transcripts of PTEN/MMAC1 with nested RT-PCR may detect transcripts, which, although they vary from the normal size, may not be related to oncogenesis as they are also frequently found in normal tissues of the same patient.

Redox regulation of signal transduction in mammalian cells

This mini-review addresses the mechanism of ultraviolet-light-induced activation of receptor tyrosine kinases. The experimental approach into this mechanism revealed the existence of redox regulation of signal transduction in mammalian cells. It is postulated that, in addition to responsiveness to oxidative attacks from outside, redox regulation of specific redox-sensitive proteins likely represents an important physiological mechanism.

Protein tyrosine phosphatase epsilon increases the risk of mammary hyperplasia and mammary tumors in transgenic mice

Accurate phosphorylation of tyrosine residues in proteins plays a central role in regulation of cellular function. Although connections between aberrant tyrosine kinase activity and malignancy are well-established, significantly less is known about the roles of protein tyrosine phosphatases (PTPases) in tumorigenesis. We have previously shown that the transmembranal form of PTPase Epsilon (PTPepsilon) is upregulated in mouse mammary tumors initiated specifically by ras or neu, suggesting that PTPepsilon may play a role in transformation by these two oncogenes. In order to test this notion in vivo, we created transgenic mice that express elevated levels of PTPepsilon in their mammary epithelium by use of the MMTV promoter/enhancer. Following several cycles of pregnancy female MMTV-PTPepsilon mice uniformly developed pronounced and persistent mammary hyperplasia which was accompanied by residual milk production. Solitary mammary tumors were often detected secondary to mammary hyperplasia. The sporadic nature of the tumors, the long latency period prior to their development, and low levels of transgene expression in the tumors indicate that PTPepsilon provides a necessary, but insufficient, signal for oncogenesis. The results provide genetic evidence that PTPepsilon plays an accessory role in production of mammary tumors in a manner consistent with its upregulation in mammary tumors induced by ras or neu.

Role of protein phosphatases in the regulation of human mast cell and basophil function

Many extracellular stimuli mediate physiological change in target cells by altering the phosphorylation state of proteins. These alterations result from the dynamic interplay of protein kinases, which mediate phosphorylations, and protein phosphatases, which catalyse dephosphorylations. The antigen-mediated aggregation of high-affinity receptors for IgE on mast cells and basophils triggers rapid changes in the phosphorylation of many proteins and culminates in the generation of inflammatory mediators involved in allergic inflammatory diseases such as asthma. Although protein kinases have an established role in this process, less is known about the involvement of protein phosphatases. This imbalance has been redressed in recent years by the availability of phosphatase inhibitors, such as okadaic acid, that facilitate investigations of the role of protein phosphatases in intact cells. Here we review a number of studies in which inhibitors of protein phosphatases have been used to shed light on the potential importance of these enzymes in the regulation of human mast cell and human basophil function.

Structure, modelling, and molecular dynamics studies of the inhibition of protein tyrosine phosphatase 1B by sulfotyrosine peptides

The protein tyrosine phosphatases comprise a class of enzymes that are crucial for the regulation of a number of cellular processes. Because of this, they are attracting increasing attention, not only as legitimate therapeutic targets, but also because of their relationship to many fundamental cellular processes. Certain sulfotyrosine peptides derived from casein are known to be good inhibitors of the protein tyrosine phosphatase, PTP1B. In this study, NMR transfer nuclear Overhauser effect studies have been used to ascertain the bound-state conformation adopted by the 12-amino acid residue casein-derived peptide, CAS200 (NANEEE(sY)SIGSA) and N-terminal truncated forms of this peptide, CAS203 and CAS205. Each of the peptides were found to bind in an extended beta-strand conformation. Extensive molecular modelling and molecular dynamics simulations of the PTP1B/peptide complexes, in a fully hydrated model, allowed a detailed description of the potential sources of the binding interactions to be developed. In agreement with the NMR studies, the modelling provided a picture of binding of CAS200 in which only the central (E203- I208) residues contributed significantly to the binding while the 3 N-terminal and 3 C-terminal residues were quite fluxional. Critical cationic surface residues, lying near to, but outside the active site pocket were the source of strong stabilizing forces that complemented the stabilizing interactions of the active site pocket. Electrostatic, hydrophobic, and hydrogen bonding interactions, in a residue specific manner, were all found to make significant contributions to the binding of these inhibitors.Key words: protein tyrosine phosphatase, PTP1B, casein peptide, inhibitor, NMR structure, molecular modelling, molecular dynamics.

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

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

Post-translational modifications of ICA512, a receptor tyrosine phosphatase-like protein of secretory granules

The autoantigen of type I diabetes ICA512 is a receptor tyrosine phosphatase-like protein enriched in the secretory granule membranes of neurons and peptide secreting endocrine cells. While the function of ICA512 remains unknown, it is thought to link regulated neuropeptide and peptide hormone secretion with signal transduction pathways involving tyrosine phosphorylation/dephosphorylation. To characterize further its biochemical properties, we conducted studies in the bovine pituitary, an abundant source of native ICA512, as well as in fibroblasts transfected with various human ICA512 cDNA constructs. Based on these studies we have established that the signal peptide of ICA512 encompasses residues 1-34 and that the ectodomain of ICA512 undergoes multiple post-translation modifications, including N-glycosylation. Newly synthesized ICA512 appears first as a pro-protein of 110 kDa that is then converted by post-translational modifications into a 130-kDa species. Cleavage of pro-ICA512 at a consensus for furin-like convertases generates a 60-66-kDa ICA512 transmembrane fragment (amino acids 449-979). Such processing ICA512 is not restricted to neuroendocrine cells, as it can also occur in transfected fibroblasts. Finally, the predicted N-terminal fragment of ICA512 resulting from this cleavage (amino acids 35-448) or parts thereof are present in the neurosecretosomes of posterior pituitary, raising the possibility that they may be secreted upon exocytosis of secretory granules.

Identification of the cell cycle regulator VCP (p97/CDC48) as a substrate of the band 4.1-related protein-tyrosine phosphatase PTPH1

The human band 4.1-related protein-tyrosine phosphatase PTPH1 was introduced into NIH3T3 cells under the control of a tetracycline-repressible promoter. Ectopic expression of wild type PTPH1 dramatically inhibited cell growth, whereas a catalytically impaired mutant showed no effect. To identify the direct target of PTPH1 in the cell, we generated a substrate-trapping mutant, in which an invariant aspartate residue was changed to alanine (D811A in PTPH1). The PTPH1-D811A mutant trapped primarily a 97-kDa tyrosine-phosphorylated protein, which was determined to be VCP (also named p97 or yeast CDC48), from various cell lysates in vitro. However, when expressed in mammalian cells, the D811A mutant was observed to contain high levels of phosphotyrosine and did not trap substrates. Mutation of tyrosine 676 to phenylalanine (Y676F) in the PTPH1-D811A mutant led to a marked reduction in phosphotyrosine content. Furthermore, this double mutant specifically trapped VCP in vivo and recognized the C-terminal tyrosines of VCP, whose phosphorylation is important for cell cycle progression in yeast. Like wild type PTPH1, this double mutant also inhibited cell proliferation. Moreover, induction of wild type PTPH1 resulted in specific dephosphorylation of VCP without changing the overall phosphotyrosine profile of the cells. VCP has been implicated in control of a variety of membrane functions, including membrane fusions, and is a regulator of the cell cycle. Our results suggest that PTPH1 may exert its effects on cell growth through dephosphorylation of VCP, thus implicating tyrosine phosphorylation as an important regulator of VCP function.

Zinc(II) complexes of tripodal peptides mimicking the zinc(II)-coordination structure of carbonic anhydrase

Two new tripodal peptide ligands with histidine side chains have been synthesized and were shown to form stable zinc(II) complexes. Their NMR and mass spectra indicate a structure that is analogous to the active center of carbonic anhydrase. Both the ligands and the zinc complexes were titrated potentiometrically in order to obtain the pKa values for the coordinated water of the zinc complexes; due to the low solubility of the complexes only estimates could be obtained.

Negative Regulation of Myeloid Cell Proliferation and Function by the SH2 Domain-Containing Tyrosine Phosphatase-1

The SH2 domain containing tyrosine phosphatase SHP-1 has been implicated in the regulation of a multiplicity of signaling pathways involved in hemopoietic cell growth, differentiation, and activation. A pivotal contribution of SHP-1 in the modulation of myeloid cell signaling cascades has been revealed by the demonstration that SHP-1 gene mutation is responsible for the overexpansion and inappropriate activation of myelomonocytic populations in motheaten mice. To investigate the role of SHP-1 in regulation of myeloid leukocytes, an HA epitope-tagged dominant negative (interfering) SHP-1 (SHP-1C453S) was expressed in the myelo-monocytic cell line U937 using the pcDNA3 vector. Overexpression of this protein in SHP-1C453S transfectants was demonstrated by Western blot analysis and by detection of decreased specific activity. Growth, proliferation, and IL-3-induced proliferative responses were substantially increased in the SHP-1C453S-overexpressing cells relative to those in control cells. The results of cell cycle analysis also revealed that the proportion of cells overexpressing SHP-1C453S in S phase was greater than that of control cells. The SHP-1C453S-expressing cells also displayed diminished rates of apoptosis as detected by flow cytometric analysis of propidium iodide-stained cells and terminal deoxynucleotidyltransferase-mediated fluorescein-dUTP nick end-labeling assay. While motility and phagocytosis were not affected by SHP-1C453S overexpression, adhesion and the oxidative burst in response to PMA were enhanced in the SHP-1C453S compared with those in the vector alone transfectants. Taken together, these results suggest that SHP-1 exerts an important negative regulatory influence on cell proliferation and activation while promoting spontaneous cell death in myeloid cells.

Periodinates: a new class of protein tyrosine phosphatase inhibitors

A series of periodinates has been synthesized and tested as protein tyrosine phosphatase substrates. Their potency is comparable to or higher than that of vanadates but much lower than that of peroxovanadates.

Protein tyrosine phosphatases: counting the trees in the forest

The recent identification of many different protein tyrosine phosphatases (PTPs) has led to the recognition that these enzymes match protein tyrosine kinases (PTKs) in importance for intracellular signalling. The total number of PTPs encoded by the mammalian genome has been estimated at between 500 and approx. 2000. These estimates are imprecise due to the large number of sequence database entries that represent different splice forms, or duplicates of the same PTP sequence. A careful analysis of these entries, grouped by identical catalytic domain shows that no more than 48 full-length PTP sequences are currently known, and that their total number in the human genome may not exceed 100. An alignment of all catalytic domains also suggests that during evolution intragenic catalytic domain duplication, as seen in most membrane-bound PTPs, preceded gene duplication.

Serine kinase activity of a Bacillus subtilis switch protein is required to transduce environmental stress signals but not to activate its target PP2C phosphatase

The RsbT serine kinase has two known functions in the signal transduction pathway that activates the general stress factor sigmaB of Bacillus subtilis. First, RsbT can phosphorylate and inactivate its specific antagonist protein, RsbS. Second, upon phosphorylation of RsbS, RsbT is released to stimulate RsbU, a PP2C phosphatase, thereby initiating a signalling cascade that ultimately activates sigmaB. Here we describe a mutation that separates these two functions of RsbT. Although the mutant RsbT protein had essentially no kinase activity, it still retained the capacity to stimulate the RsbU phosphatase in vitro and to activate sigmaB when overexpressed in vivo. These results support the hypothesis that phosphatase activation is accomplished via a long-lived interaction between RsbT and RsbU. In contrast, RsbT kinase activity was found to be integral for the transmission of external stimuli to sigmaB. Thus, one route by which environmental stress signals could enter the sigmaB network is by modulation of the RsbT kinase activity, thereby controlling the magnitude of the partner switch between the RsbS-RsbT complex and the RsbT-RsbU complex.