Substitution studies of the second divalent metal cation requirement of protein tyrosine kinase CSK

Inhibition of mutationally activated HER2

Human epidermal growth factor receptor 2 (HER2) is an oncogenic driver and key therapeutic target for human cancers. Current therapies targeting HER2 are primarily based on overexpression of the wild-type form of HER2. However, kinase domain mutations have been identified that can increase the activity of HER2 even when expressed at basal levels. Using purified enzymes, we confirmed the hyperactivity of two HER2 mutants (D769Y and P780insGSP). To identify small molecule inhibitors against these cancer-associated variants, we used a combined approach consisting of biochemical testing, similarity-based searching, and in silico modeling. These approaches resulted in the identification of a candidate molecule that inhibits mutant forms of HER2 in vitro and in cell-based assays. Our structural model predicts that the compound takes advantage of water-mediated interactions in the HER2 kinase binding pocket.

Dissection of the catalytic and regulatory structure-function relationships of Csk protein tyrosine kinase

Protein tyrosine kinases (PTKs) are a large enzyme family that regulates many cellular processes. The key to their broad role in signaling is their tunable substrate specificity and regulatory mechanisms that allow each to respond to appropriate regulatory signals and phosphorylate the correct physiological protein substrates. Thus, in addition to the general PTK catalytic platform, each PTK acquires unique structural motifs that confer a unique combination of catalytic and regulatory properties. Understanding the structural basis for these properties is essential for understanding and manipulating the PTK-based signaling networks in normal and cancer cells. C-terminal Src kinase (Csk) and its homolog, Csk-homologous kinase (Chk), phosphorylate Src family kinases on a C-terminal Tyr residue and negatively regulate their kinase activity. While this regulatory function is biologically essential, Csk and Chk have also been excellent model PTKs for dissecting the structural basis of PTK catalysis and regulation. In this article, we review the structure-function studies of Csk and Chk that shed light on the regulatory and catalytic mechanisms of protein tyrosine kinases in general.

Zap70 Regulates TCR-Mediated Zip6 Activation at the Immunological Synapse

The essential microelement zinc plays immunoregulatory roles via its ability to influence signaling pathways. Zinc deficiency impairs overall immune function and resultantly increases susceptibility to infection. Thus, zinc is considered as an immune-boosting supplement for populations with hypozincemia at high-risk for infection. Besides its role as a structural cofactor of many proteins, zinc also acts as an intracellular messenger in immune cell signaling. T-cell activation instructs zinc influx from extracellular and subcellular sources through the Zip6 and Zip8 zinc transporters, respectively. Increased cytoplasmic zinc participates in the regulation of T-cell responses by modifying activation signaling. However, the mechanism underlying the activation-dependent movement of zinc ions by Zip transporters in T cells remains elusive. Here, we demonstrate that Zip6, one of the most abundantly expressed Zip transporters in T cells, is mainly localized to lipid rafts in human T cells and is recruited into the immunological synapse in response to TCR stimulation. This was demonstrated through confocal imaging of the interaction between CD4⁺ T cells and antigen-presenting cells. Further, immunoprecipitation assays show that TCR triggering induces tyrosine phosphorylation of Zip6, which has at least three putative tyrosine motifs in its long cytoplasmic region, and this phosphorylation is coupled with its physical interaction with Zap70. Silencing Zip6 reduces zinc influx from extracellular sources and suppresses T-cell responses, suggesting an interaction between Zip6-mediated zinc influx and TCR activation. These results provide new insights into the mechanism through which Zip6-mediated zinc influx occurs in a TCR activation-dependent manner in human CD4⁺ T cells.

Checkpoint-mediated DNA polymerase ε exonuclease activity curbing counteracts resection-driven fork collapse

DNA polymerase ε (Polε) carries out high-fidelity leading strand synthesis owing to its exonuclease activity. Polε polymerase and exonuclease activities are balanced, because of partitioning of nascent DNA strands between catalytic sites, so that net resection occurs when synthesis is impaired. In vivo, DNA synthesis stalling activates replication checkpoint kinases, which act to preserve the functional integrity of replication forks. We show that stalled Polε drives nascent strand resection causing fork functional collapse, averted via checkpoint-dependent phosphorylation. Polε catalytic subunit Pol2 is phosphorylated on serine 430, influencing partitioning between polymerase and exonuclease active sites. A phosphormimetic S430D change reduces exonucleolysis in vitro and counteracts fork collapse. Conversely, non-phosphorylatable pol2-S430A expression causes resection-driven stressed fork defects. Our findings reveal that checkpoint kinases switch Polε to an exonuclease-safe mode preventing nascent strand resection and stabilizing stalled replication forks. Elective partitioning suppression has implications for the diverse Polε roles in genome integrity maintenance.

Regulatory Role of Zinc in Immune Cell Signaling

Zinc is an essential micronutrient with crucial roles in multiple facets of biological processes. Dysregulated zinc homeostasis impairs overall immune function and resultantly increases susceptibility to infection. Clinically, zinc supplementation is practiced for treatment of several infectious diseases, such as diarrhea and malaria. Recent focus on zinc as a beneficial element for immune system support has resulted in investigation of the immunomodulatory roles of zinc in a variety of immune cells. Besides its classical role as a cofactor that regulates the structural function of thousands of proteins, accumulating evidence suggests that zinc also acts, in a manner similar to calcium, as an ionic regulator of immune responses via participation as an intracellular messenger in signaling pathways. In this review, we focus on the role of zinc as a signaling molecule in major pathways such as those downstream of Toll-like receptors-, T cell receptor-, and cytokine-mediated signal transduction that regulate the activity and function of monocytes/macrophages and T cells, principal players in the innate and adaptive immune systems.

High-throughput investigation of transglutaminase 2 kinase regulation using a novel cysteine-modified peptide array

Transglutaminase 2 (TGase2) kinase has emerged as an important regulator of apoptosis as well as chromatin structure and function; however, details about the pathophysiological functions of TGase2 kinase have been limited because of the lack of a suitable activity assay for systematic investigation of TGase2 kinase regulation in a high-throughput manner. Thus, we developed a novel on-chip TGase2 kinase activity assay using a cysteine-modified insulin-like growth factor-binding protein-3-derived peptide (CMI peptide) on an array platform. This peptide array-based activity assay was reproducible, with a detection limit of 2.127 μg/ml. We successfully applied this assay to investigate the effects of thiol-reactive compounds and divalent cations on TGase2 kinase by determining the half maximal inhibitory concentrations (IC₅₀). Thiol-reactive compounds inhibited TGase2 kinase activity in a concentration-dependent manner, with IC₅₀ values ranging from 0.125 to 5.550 mM. Divalent metal cations also showed a concentration-dependent inhibition, with IC₅₀ values ranging from 0.005 to 1.937 mM; however, Ca²⁺ had no effect on TGase2 kinase activity. Thus, this novel kinase activity assay using the CMI peptide array described here is suitable for systematic investigation of TGase2 kinase regulation and may be useful for investigating the roles of TGase2 kinase in pathogenesis of kinase-mediated diseases.

Identification of a Water-Coordinating HER2 Inhibitor by Virtual Screening Using Similarity-Based Scoring

Human epidermal growth factor receptor 2 (HER2) is a validated breast cancer drug target for small molecule inhibitors that target the ATP-binding pocket of the kinase domain. In this work, a large-scale virtual screen was performed to a novel homology model of HER2, in a hypothesized “fully active” state, that considered water-mediated interactions during the prioritization of compounds for experimental testing. This screen led to the identification of a new inhibitor with micro molar affinity and potency (K_d = 7.0 μM, IC₅₀ = 4.6 μM). Accompanying molecular dynamics simulations showed that inhibitor binding likely involves water coordination through an important water-mediated network previously identified in our laboratory. The predicted binding geometry also showed a remarkable overlap with the crystallographic poses for two previously reported inhibitors of the related Chk1 kinase. Concurrent with the HER2 studies, we developed formalized computational protocols that leverage solvated footprints (per-residue interaction maps that include bridging waters) to identify ligands that can “coordinate” or “displace” key binding site waters. Proof-of-concept screens targeting HIVPR and PARP1 demonstrate that molecules with high footprint overlap can be effectively identified in terms of their coordination or displacement patterns relative to a known reference. Overall, the procedures developed as a result of this study should be useful for researchers targeting HER2 and, more generally, for any protein in which the identification of compounds that exploit binding site waters is desirable.

Hydroxamic acid derivatives as histone deacetylase inhibitors: a DFT study of their tautomerism and metal affinities/selectivities

Hydroxamic acids are regarded as potent inhibitors of histone deacetylases (HDAC), and can therefore be used to reduce malignancy growth and size in affected organisms. Although there is a substantial body of information on the structures, syntheses, and biological activities of HDAC inhibitors, several important questions regarding their physicochemical properties and metal affinities/selectivities remain answered. First, how do the conformation and ionization of the hydroxamic group depend on its chemical composition and the dielectric properties of the medium? Second, how do these factors affect the affinities and selectivities of HDAC inhibitors for essential biogenic metal cations? Third, what is the preferred deprotonation site of the hydroxamic moiety and its mode of binding to the metal cation? The present work addressed these questions by performing density functional calculations combined with polarizable continuum model computations. The geometry, deprotonation pattern, metal-binding mode, and metal affinity/selectivity of SAHA, a typical HDAC inhibitor, were examined, and key factors affecting its ligation properties were elucidated. Sulfur- and selenium-containing analogs of SAHA were also modeled for the first time, and their potential as efficient metal-binding entities (to Mg²⁺, Fe²⁺, and Zn²⁺ cations) was assessed. The present calculations shed light on the thermodynamics of the binding of HDAC inhibitors to metal ions, and suggest techniques for enhancing their metal-ligating properties.

Spectroscopic and calorimetric assays reveal dependence on dCTP and two metals (Zn2++Mg2+) for enzymatic activity of Schistosoma mansoni deoxycytidylate (dCMP) deaminase

The parasite Schistosoma mansoni possess all pathways for pyrimidine biosynthesis, whereby deaminases play an essential role in the thymidylate cycle, a crucial step to controlling the ratio between cytidine and uridine nucleotides. In this study, we heterologously expressed and purified the deoxycytidylate (dCMP) deaminase from S. mansoni to obtain structural, biochemical and kinetic information. Small-angle X-ray scattering of this enzyme showed that it is organized as a hexamer in solution. Isothermal titration calorimetry was used to determine the kinetic constants for dCMP-dUMP conversion and the role of dCTP and dTTP in enzymatic regulation. We evaluated the metals involved in activating the enzyme and show for the first time the dependence of correct folding on the interaction of two metals. This study provides information that may be useful for understanding the regulatory mechanisms involved in the metabolic pathways of S. mansoni. Thus, improving our understanding of the function of these essential pathways for parasite metabolism and showing for the first time the hitherto unknown deaminase function in this parasite.

Roles of Zinc Signaling in the Immune System

Zinc (Zn) is an essential micronutrient for basic cell activities such as cell growth, differentiation, and survival. Zn deficiency depresses both innate and adaptive immune responses. However, the precise physiological mechanisms of the Zn-mediated regulation of the immune system have been largely unclear. Zn homeostasis is tightly controlled by the coordinated activity of Zn transporters and metallothioneins, which regulate the transport, distribution, and storage of Zn. There is growing evidence that Zn behaves like a signaling molecule, facilitating the transduction of a variety of signaling cascades in response to extracellular stimuli. In this review, we highlight the emerging functional roles of Zn and Zn transporters in immunity, focusing on how crosstalk between Zn and immune-related signaling guides the normal development and function of immune cells.

Metal preferences and metallation

The metal binding preferences of most metalloproteins do not match their metal requirements. Thus, metallation of an estimated 30% of metalloenzymes is aided by metal delivery systems, with ∼ 25% acquiring preassembled metal cofactors. The remaining ∼ 70% are presumed to compete for metals from buffered metal pools. Metallation is further aided by maintaining the relative concentrations of these pools as an inverse function of the stabilities of the respective metal complexes. For example, magnesium enzymes always prefer to bind zinc, and these metals dominate the metalloenzymes without metal delivery systems. Therefore, the buffered concentration of zinc is held at least a million-fold below magnesium inside most cells.

Zinc ions as effectors of environmental oxidative lung injury

The redox-inert transition metal Zn is a micronutrient that plays essential roles in protein structure, catalysis, and regulation of function. Inhalational exposure to ZnO or to soluble Zn salts in occupational and environmental settings leads to adverse health effects, the severity of which appears dependent on the flux of Zn(2+) presented to the airway and alveolar cells. The cellular toxicity of exogenous Zn(2+) exposure is characterized by cellular responses that include mitochondrial dysfunction, elevated production of reactive oxygen species, and loss of signaling quiescence leading to cell death and increased expression of adaptive and inflammatory genes. Central to the molecular effects of Zn(2+) are its interactions with cysteinyl thiols, which alters their functionality by modulating their reactivity and participation in redox reactions. Ongoing studies aimed at elucidating the molecular toxicology of Zn(2+) in the lung are contributing valuable information about its role in redox biology and cellular homeostasis in normal and pathophysiology.

Regulation of T cell receptor signaling by activation-induced zinc influx

Zinc is a trace element that is essential for innate and adaptive immune responses. In addition to being a structural element of many proteins, zinc also functions as a neurotransmitter and an intracellular messenger. Temporal or spatial changes in bioavailable zinc may influence the activity of several enzymes, including kinases and phosphatases. We provide evidence that zinc functions as an ionic signaling molecule after T cell activation. Cytoplasmic zinc concentrations increased within 1 min after T cell receptor (TCR) triggering, in particular in the subsynaptic compartment. The increase depended on the extracellular zinc concentrations and was inhibited by silencing zinc transporter Zip6. Increased zinc influx reduced the recruitment of SHP-1 to the TCR activation complex, augmented ZAP70 phosphorylation and sustained calcium influx. By calibrating TCR activation thresholds, increased extracellular zinc bioavailability facilitated the induction of T cell proliferative responses to suboptimal stimuli.

Zinc differentially regulates mitogen-activated protein kinases in human T cells

Zinc is an essential nutrient with remarkable importance for immunity, in particular for T-cell function. This is, at least in part, based on an involvement of zinc ions in immune cell signal transduction; dynamic changes of the intracellular free zinc concentration have recently been recognized as signaling events. Because the molecular targets of zinc signals remain incompletely understood, we investigated the impact of elevated intracellular free zinc on mitogen-activated protein kinase (MAPK) activity and MAPK-dependent cytokine production in human T-cells. p38 was activated by treatment with zinc and the ionophore pyrithione, whereas ERK1/2 and c-Jun N-terminal kinases were unaffected. In contrast, after T-cell receptor stimulation with antibodies against CD3, ERK1/2-phosphorylation was selectively suppressed by intracellular zinc. Mechanisms that had been shown to mediate zinc-effects in other cells, such as activation of the Src kinase Lck, inhibition of the protein tyrosine phosphatase CD45 or MAPK phosphatases and cyclic nucleotide/protein kinase A signaling were not involved. This indicates that the differential impact of zinc on the MAPK families in T-cells is mediated by mechanisms that differ from the ones observed in other cell types. Further investigation of the activation of p38 by zinc demonstrated that this MAPK is responsible for the zinc-mediated activation of CREB and mRNA expression of the Th1 cytokines interferon-gamma and interleukin-2. In conclusion, regulation of MAPK activity contributes to the impact of zinc on T-cell function.

Steady-state kinetic characterization of kinase activity and requirements for Mg2+ of interleukin-1 receptor-associated kinase-4

Interleukin-1 receptor-associated kinase-4 (IRAK-4) is a Ser/Thr-specific protein kinase that plays a critical role in intracellular signaling cascades mediated by Toll-like and interleukin-1 (IL-1) receptors. Despite a growing body of information on the physiological functions of IRAK-4, its kinase activity remains poorly studied. The present study entails characterization of the steady-state kinetic properties and Mg(2+) requirements of full-length, recombinant human IRAK-4 preactivated by incubation with MgATP. In the presence of 20 mM Mg(2+), activated IRAK-4 herein is demonstrated to phosphorylate a peptide substrate (IRAK-1 peptide), derived from the activation loop of IRAK-1, with a k(cat) of 30 +/- 2.9 s(-1) and K(m) values of 668 +/- 120 and 852 +/- 273 microM for ATP and the peptide, respectively. Two-substrate, dead-end and product inhibition data, using analogues of ATP, are consistent with both a sequential ordered kinetic mechanism with ATP binding to the enzyme prior to the peptide and a sequential random mechanism. Investigation of the Mg(2+) requirements for phosphoryl transfer activity of IRAK-4 revealed that more than one Mg(2+) ion interacts with the enzyme and that the enzyme is maximally active in the presence of 5-10 mM free Mg(2+). While one divalent metal, as part of a chelate complex with ATP, is essential for catalysis, kinetic evidence is provided to show that uncomplexed Mg(2+) further enhances the catalytic activity of IRAK-4 by bringing about an approximately 3-fold increase in k(cat) and an approximately 6-fold reduction in the K(m) for ATP and by rendering the interaction between the nucleotide and peptide substrate binding sites less antagonistic.

Her4 and Her2/neu tyrosine kinase domains dimerize and activate in a reconstituted in vitro system

Her4 (ErbB-4) and Her2/neu (ErbB-2) are receptor-tyrosine kinases belonging to the epidermal growth factor receptor (EGFR) family. Crystal structures of EGFR and Her4 kinase domains demonstrate kinase dimerization and activation through an allosteric mechanism. The kinase domains form an asymmetric dimer, where the C-lobe surface of one monomer contacts the N-lobe of the other monomer. EGFR kinase dimerization and activation in vitro was previously reported using a nickel-chelating lipid-liposome system, and we now apply this system to all other members of the EGFR family. Polyhistidine-tagged Her4, Her2/neu, and Her3 kinase domains are bound to these nickel-liposomes and are brought to high local concentration, mimicking what happens to full-length receptors in vivo following ligand binding. Addition of nickel-liposomes to Her4 kinase domain results in 40-fold activation in kinase activity and marked enhancement of C-terminal tail autophosphorylation. Activation of Her4 shows a sigmoidal dependence on kinase concentration, consistent with a cooperative process requiring kinase dimerization. Her2/neu kinase activity is also activated by nickel-liposomes, and is increased further by heterodimerization with Her3 or Her4. The ability of Her3 and Her4 to heterodimerize and activate other family members is studied in vitro. Her3 kinase domain readily activates Her2/neu but is a poor activator of Her4, which differs from the prediction made by the asymmetric dimer model. Mutation of Her3 residues (952)ENI(954) to the corresponding sequence in Her4 enhanced the ability of Her3 to activate Her4, demonstrating that sequence differences on the C-lobe surface influence the heterodimerization and activation of ErbB kinase domains.

Functional significance of zinc-related signaling pathways in immune cells

Recent years have brought a paradigm shift for the role of the essential trace element zinc in immunity. Although its function as a structural component of many enzymes has been known for decades, current experimental evidence points to an additional function of the concentration of free or loosely bound zinc ions as an intracellular signal. The activity of virtually all immune cells is modulated by zinc in vitro and in vivo. In this review, we discuss the interactions of zinc with major signaling pathways that regulate immune cell activity, and the implications of zinc deficiency or supplementation on zinc signaling as the molecular basis for an effect of zinc on immune cell function.

Force fields including charge transfer and local polarization effects: Application to proteins containing multi/heavy metal ions

The question whether molecular dynamics (MD) simulations can yield reliable structural and dynamical properties of metalloproteins depend on the accuracy of the force field, i.e., the potential energy function (PEF) and associated parameters modeling the interactions of the metal ion of interest with water and protein ligands. Previously, we had developed a CTPOL PEF for protein simulations of Zn(2+) bound to Cys(-) and/or His(0) that includes charge transfer and local polarization effects as well as metal van der Waals parameters that reproduce the structural and thermodynamical properties of 22 dications. Here, we evaluate if the CTPOL PEF and the new metal parameters (referred to as the CTPOLa force field) can be applied to proteins containing polynuclear metal-binding sites and heavy toxic metal ions, using the CdZn(2)-Cys(9) beta-domain of rat liver metallothionein-2 and the Hg(2+)-bound 18-residue peptide from MerP as test systems. Using the CTPOLa force field, simulations of the beta-domain of rat liver metallothionein-2 totaling 19 ns could preserve the experimentally observed CdZn(2)-Cys(9) complex geometry and overall protein structure, whereas simulations neglecting charge transfer and local polarization effects could not. However, the CTPOLa force field cannot reproduce the experimentally observed linear bicoordination of Hg(2+) in the MerP peptide without adding an angular restraint to the CTPOL PEF to correct the angle distribution about Hg(2+). Thus, the force fields presented herein for the group IIB metal ions can be applied to simulation studies of proteins containing polynuclear metal-binding sites and heavy metal ions in aqueous solution. PEF neglecting charge transfer and local polarization effects in conjunction with vdW parameters adjusted to reproduce the structural and thermodynamical properties of only the metal ion in question could not yield an accurate representation of the metal-binding site and overall protein structure.

Zinc-mediated transactivation of TrkB potentiates the hippocampal mossy fiber-CA3 pyramid synapse

The receptor tyrosine kinase, TrkB, is critical to diverse functions of the mammalian nervous system in health and disease. Evidence of TrkB activation during epileptogenesis in vivo despite genetic deletion of its prototypic neurotrophin ligands led us to hypothesize that a non-neurotrophin, the divalent cation zinc, can transactivate TrkB. We found that zinc activates TrkB through increasing Src family kinase activity by an activity-regulated mechanism independent of neurotrophins. One subcellular locale at which zinc activates TrkB is the postsynaptic density of excitatory synapses. Exogenous zinc potentiates the efficacy of the hippocampal mossy fiber (mf)-CA3 pyramid synapse by a TrkB-requiring mechanism. Long-term potentiation of this synapse is impaired by deletion of TrkB, inhibition of TrkB kinase activity, and by CaEDTA, a selective chelator of zinc. The activity-dependent activation of synaptic TrkB in a neurotrophin-independent manner provides a mechanism by which this receptor can regulate synaptic plasticity.

Unique Ligand-Based Oxidative DNA Cleavage by Zinc(II) Complexes of Hpyramol and Hpyrimol

The zinc(II) complexes reported here have been synthesised from the ligand 4-methyl-2-N-(2-pyridylmethyl)aminophenol (Hpyramol) with chloride or acetate counterions. All the five complexes have been structurally characterised, and the crystal structures reveal that the ligand Hpyramol gradually undergoes an oxidative dehydrogenation to form the ligand 4-methyl-2-N-(2-pyridylmethylene)aminophenol (Hpyrimol), upon coordination to Zn(II). All the five complexes cleave the phiX174 phage DNA oxidatively and the complexes with fully dehydrogenated pyrimol ligands were found to be more efficient than the complexes with non-dehydrogenated Hpyramol ligands. The DNA cleavage is suggested to be ligand-based, whereas the pure ligands alone do not cleave DNA. The DNA cleavage is strongly suggested to be oxidative, possibly due to the involvement of a non-diffusible phenoxyl radical mechanism. The enzymatic religation experiments and DNA cleavage in the presence of different radical scavengers further support the oxidative DNA cleavage by the zinc(II) complexes.

Metals in membranes

In this critical review we discuss recent advances in understanding the modes of interaction of metal ions with membrane proteins, including channels, pumps, transporters, ATP-binding cassette proteins, G-protein coupled receptors, kinases and respiratory enzymes. Such knowledge provides a basis for elucidating the mechanism of action of some classes of metallodrugs, and a stimulus for the further exploration of the coordination chemistry of metal ions in membranes. Such research offers promise for the discovery of new drugs with unusual modes of action. The article will be of interest to bioinorganic chemists, chemical biologists, biochemists, pharmacologists and medicinal chemists. (247 references).

Role of threonine residues in the regulation of manganese-dependent arabidopsis serine/threonine/tyrosine protein kinase activity

Tyrosine phosphorylation in plants could be performed only by dual-specificity kinases. Arabidopsis thaliana dual-specificity protein kinase (AtSTYPK) exhibited strong preference for manganese over magnesium for its kinase activity. The kinase autophosphorylated on serine, threonine and tyrosine residues and phosphorylated myelin basic protein on threonine and tyrosine residues. The AtSTYPK harbors manganese dependent serine/threonine kinase domain, COG3642. His248 and Ser265 on COG3642 are conserved in AtSTYPK and the site-directed mutant, H248A showed loss of serine/threonine kinase activity. The protein kinase activity was abolished when Thr208 in the TEY motif and Thr293 of the activation loop were converted to alanine. The conversion of Thr284 in the activation loop to alanine resulted in an increased phosphorylation. This study reports the first identification of a manganese dependent dual-specificity kinase and the importance of Thr208, Thr284, and Thr293 residues in the regulation of kinase activity.

Inhibition of protein tyrosine phosphatase activity mediates epidermal growth factor receptor signaling in human airway epithelial cells exposed to Zn2+

Epidemiological studies have implicated zinc (Zn2+) in the toxicity of ambient particulate matter (PM) inhalation. We previously showed that exposure to metal-laden PM inhibits protein tyrosine phosphatase (PTP) activity in human primary bronchial epithelial cells (HAEC) and leads to Src-dependent activation of EGFR signaling in B82 and A431 cells. In order to elucidate the mechanism of Zn2+-induced EGFR activation in HAEC, we treated HAEC with 500 microM ZnSO4 for 5-20 min and measured the state of activation of EGFR, c-Src and PTPs. Western blots revealed that exposure to Zn2+ results in increased phosphorylation at both trans- and autophosphorylation sites in the EGFR. Zn2+-mediated EGFR phosphorylation did not require ligand binding and was ablated by the EGFR kinase inhibitor PD153035, but not by the Src kinase inhibitor PP2. Src activity was inhibited by Zn2+ treatment of HAEC, consistent with Src-independent EGFR transactivation in HAEC exposed to Zn2+. The rate of exogenous EGFR dephosphorylation in lysates of HAEC exposed to Zn2+ or V4+ was significantly diminished. Moreover, exposure of HAEC to Zn2+ also resulted in a significant impairment of dephosphorylation of endogenous EGFR. These data show that Zn2+-induced activation of EGFR in HAEC involves a loss of PTP activities whose function is to dephosphorylate EGFR in opposition to baseline EGFR kinase activity. These findings also suggest that there are marked cell-type-specific differences in the mechanism of EGFR activation induced by Zn2+ exposure.

Characterization of the interactions between the active site of a protein tyrosine kinase and a divalent metal activator

BACKGROUND

Protein tyrosine kinases are important enzymes for cell signalling and key targets for anticancer drug discovery. The catalytic mechanisms of protein tyrosine kinase-catalysed phosphorylation are not fully understood. Protein tyrosine kinase Csk requires two Mg2+ cations for activity: one (M1) binds to ATP, and the other (M2) acts as an essential activator.

RESULTS

Experiments in this communication characterize the interaction between M2 and Csk. Csk activity is sensitive to pH in the range of 6 to 7. Kinetic characterization indicates that the sensitivity is not due to altered substrate binding, but caused by the sensitivity of M2 binding to pH. Several residues in the active site with potential of binding M2 are mutated and the effect on metal activation studied. An active mutant of Asn319 is generated, and this mutation does not alter the metal binding characteristics. Mutations of Glu236 or Asp332 abolish the kinase activity, precluding a positive or negative conclusion on their role in M2 coordination. Finally, the ability of divalent metal cations to activate Csk correlates to a combination of ionic radius and the coordination number.

CONCLUSION

These studies demonstrate that M2 binding to Csk is sensitive to pH, which is mainly responsible for Csk activity change in the acidic arm of the pH response curve. They also demonstrate critical differences in the metal activator coordination sphere in protein tyrosine kinase Csk and a protein Ser/Thr kinase, the cAMP-dependent protein kinase. They shed light on the physical interactions between a protein tyrosine kinase and a divalent metal activator.

Characterization of the protein kinase activity of TRPM7/ChaK1, a protein kinase fused to the transient receptor potential ion channel

Channel-kinase TRPM7/ChaK1 is a member of a recently discovered family of protein kinases called alpha-kinases that display no sequence homology to conventional protein kinases. It is an unusual bifunctional protein that contains an alpha-kinase domain fused to an ion channel. The TRPM7/ChaK1 channel has been characterized using electrophysiological techniques, and recent evidence suggests that it may play a key role in the regulation of magnesium homeostasis. However, little is known about its protein kinase activity. To characterize the kinase activity of TRPM7/ChaK1, we expressed the kinase catalytic domain in bacteria. ChaK1-cat is able to undergo autophosphorylation and to phosphorylate myelin basic protein and histone H3 on serine and threonine residues. The kinase is specific for ATP and cannot use GTP as a substrate. ChaK1-cat is insensitive to staurosporine (up to 0.1 mM) but can be inhibited by rottlerin. Because the kinase domain is physically linked to an ion channel, we investigated the effect of ions on ChaK1-cat activity. The kinase requires Mg(2+) (optimum at 4-10 mM) or Mn(2+) (optimum at 3-5 mM), with activity in the presence of Mn(2+) being 2 orders of magnitude higher than in the presence of Mg(2+). Zn(2+) and Co(2+) inhibited ChaK1-cat kinase activity. Ca(2+) at concentrations up to 1 mM did not affect kinase activity. Considering intracellular ion concentrations, our results suggest that, among divalent metal ions, only Mg(2+) can directly modulate TRPM7/ChaK1 kinase activity in vivo.

Expression, purification, and biochemical characterization of Chk, a soluble protein tyrosine kinase

CSK family contains two protein tyrosine kinases: Csk (C-terminal Src kinase) and Chk (Csk homologous kinase). They are responsible for phosphorylating Src family protein tyrosine kinases on a C-terminal Tyr (Tyr527) and negatively regulating their activities. However, Chk and Csk have different expression patterns, mechanisms of regulation, and different biological functions, and appear to play different roles in the development of breast cancer. To obtain pure human Chk for biochemical characterization, its coding region was amplified by polymerase chain reaction and expressed as a fusion protein with glutathione S-transferase in Escherichia coli. The enzyme was highly expressed but unusually prone to proteolytic degradation during purification. Expression of the enzyme as a dual fusion protein with glutathione S-transferase on N-terminus and streptag, a 10 amino acid peptide, on C-terminus allowed purification of the full-length fusion protein. The purified enzyme was able to phosphorylate and inactivate Src. Chk (no inhibition up to 18.5 microM) and Csk (IC(50)= 1 microM) were differentially inhibited by PP2, probably due to the size difference of one residue (Thr265 in Csk versus Met304 in Chk) in the ATP-binding domain. The expression, purification, and initial characterizations of Chk provided an important step toward full characterization of Chk and Csk, two important enzymes in cellular regulation.

Effect of autophosphorylation on the catalytic and regulatory properties of protein tyrosine kinase Src

The function of autophosphorylation in Src family protein tyrosine kinases is not fully understood. In this paper we compared the catalytic and ligand-binding properties of autophosphorylated and nonautophosphorylated (control) Src. The following are the main differences we found. First, while both forms had the same K(m) for ATP-Mg, autophosphorylated Src had significantly higher K(m) values for the phosphate-accepting substrates, polyE(4)Y, and RCM-lysozyme. The autophosphorylated form also had significantly higher V(max) values than the control. The substrate specificity, as measured by V(max)/K(m) ratio, was altered by autophosphorylation and was dependent on the phosphate-accepting substrate. Second, while autophosphorylation did not affect Src activation by free Mg(2+), Zn(2+), which inhibited Src by competing against an essential Mg(2+) activator, inhibited the control threefold more potently than the autophosphorylated form. Third, autophosphorylation significantly reduced the ability of its SH2 domain to bind phosphotyrosine. Fourth, a Pro-rich Src SH3 domain binding peptide activated the control, but not the autophosphorylated Src even though the apparent binding affinity was not significantly affected by autophosphorylation. These differences indicated that autophosphorylation induced significant and widespread changes in the catalytic and regulatory properties of Src. The implications of these findings relative to Src biological regulation are discussed.

Phosphorylation down-regulates the RNA binding function of the coat protein of potato virus A

Plant viruses encode movement proteins (MPs) to facilitate transport of their genomes from infected into neighboring healthy cells through plasmodesmata. Growing evidence suggests that specific phosphorylation events can regulate MP functions. The coat protein (CP) of potato virus A (PVA; genus Potyvirus) is a multifunctional protein involved both in virion assembly and virus movement. Labeling of PVA-infected tobacco leaves with [(33)P]orthophosphate demonstrated that PVA CP is phosphorylated in vivo. Competition assays established that PVA CP and the well characterized 30-kDa MP of tobacco mosaic virus (genus Tobamovirus) are phosphorylated in vitro by the same Ser/Thr kinase activity from tobacco leaves. This activity exhibits a strong preference for Mn(2+) over Mg(2+), can be inhibited by micromolar concentrations of Zn(2+) and Cd(2+), and is not Ca(2+)-dependent. Tryptic phosphopeptide mapping revealed that PVA CP was phosphorylated by this protein kinase activity on multiple sites. In contrast, PVA CP was not phosphorylated when packaged into virions, suggesting that the phosphorylation sites are located within the RNA binding domain and not exposed on the surface of the virion. Furthermore, two independent experimental approaches demonstrated that the RNA binding function of PVA CP is strongly inhibited by phosphorylation. From these findings, we suggest that protein phosphorylation represents a possible mechanism regulating formation and/or stability of viral ribonucleoproteins in planta.

Affinity purification of Csk protein tyrosine kinase based on its catalytic requirement for divalent metal cations

Protein tyrosine kinase Csk requires two Mg2+ ions for activity: one magnesium is part of the ATP-Mg complex, and the second free Mg2+ ion is required as an essential activator. Zn2+ can bind to this site to replace Mg2+, which inhibits Csk kinase activity. The binding is reversible and removal of Zn2+ results in an active Csk apoenzyme. In this communication, we report that this tight binding can be used as a mechanism for affinity purification of Csk. When bacterial cell lysate containing overexpressed GST-Csk was applied to a column of Zn2+-iminodiacetic acid immobilized to agarose, Csk was specifically retained by the column. Since the binding of Csk to Zn2+ is not affected by up to 200 mM NaCl, high ionic strength conditions were used in the purification procedure, minimizing nonspecific binding due to ionic interactions. Washing the column with 200 mM NaCl and 50 mM imidazole removed virtually all other proteins from the column while Csk remained bound. The retained Csk enzyme was eluted with 1 M imidazole. The 1 M imidazole-eluted fraction contained pure Csk that had a specific activity similar to the enzyme purified by a glutathione-agarose affinity column.