Activation of the Purified Protein Tyrosine Kinase Domain of the Epidermal Growth Factor Receptor

Autophosphorylation at Thr279 of Entamoeba histolytica atypical kinase EhAK1 is required for activity and regulation of erythrophagocytosis

Phagocytosis plays a key role in survival and pathogenicity of Entamoeba histolytica. We have recently demonstrated that an atypical kinase EhAK1 is involved in phagocytosis in this parasite. It is recruited to the phagocytic cups through interaction with EhCaBP1. EhAK1 manipulates actin dynamics by multiple mechanisms including phosphorylation of G-actin. Biochemical analysis showed that EhAK1 is a serine/threonine kinase with broad ion specificity and undergoes multiple trans-autophosphorylation. Three autophosphorylation sites were identified by mass spectrometry. Out of these Thr279 appears to be involved in both autophosphorylation as well as substrate phosphorylation. Over expression of the mutant Thr279A inhibited erythrophagocytosis showing dominant negative phenotype. Multiple alignments of different kinases including alpha kinases displayed conserved binding sites that are thought to be important for function of the protein. Mutation studies demonstrated the importance of some of these binding sites in kinase activity. Binding studies with fluorescent-ATP analogs supported our prediction regarding ATP binding site based on sequence alignment. In conclusion, EhAK1 has multiple regulatory features and enrichment of EhAK1 at the site of phagocytosis stimulates trans-autophosphorylation reaction that increases kinase activity resulting in enhanced actin dynamics and phagocytosis. Some of the properties of EhAK1 are similar to that seen in alpha kinases.

Autophosphorylation of Ser428 of EhC2PK plays a critical role in regulating erythrophagocytosis in the parasite Entamoeba histolytica

The protozoan parasite Entamoeba histolytica can invade both intestinal and extra intestinal tissues resulting in amoebiasis. During the process of invasion E. histolytica ingests red blood and host cells using phagocytic processes. Though phagocytosis is considered to be a key virulence determinant, the mechanism is not very well understood in E. histolytica. We have recently demonstrated that a novel C2 domain-containing protein kinase, EhC2PK is involved in the initiation of erythrophagocytosis. Because cells overexpressing the kinase-dead mutant of EhC2PK displayed a reduction in erythrophagocytosis, it appears that kinase activity is necessary for initiation. Biochemical analysis showed that EhC2PK is an unusual Mn(2+)-dependent serine kinase. It has a trans-autophosphorylated site at Ser(428) as revealed by mass spectrometric and biochemical analysis. The autophosphorylation defective mutants (S428A, KDΔC) showed a reduction in auto and substrate phosphorylation. Time kinetics of in vitro kinase activity suggested two phases, an initial short slow phase followed by a rapid phase for wild type protein, whereas mutations in the autophosphorylation sites that cause defect (S428A) or conferred phosphomimetic property (S428E) displayed no distinct phases, suggesting that autophosphorylation may be controlling kinase activity through an autocatalytic mechanism. A reduction and delay in erythrophagocytosis was observed in E. histolytica cells overexpressing S428A and KDΔC proteins. These results indicate that enrichment of EhC2PK at the site of phagocytosis enhances the rate of trans-autophosphorylation, thereby increasing kinase activity and regulating the initiation of erythrophagocytosis in E. histolytica.

The ErbB kinase domain: structural perspectives into kinase activation and inhibition

Epidermal growth factor receptor (EGFR) and its family members, ErbB2, ErbB3 and ErbB4, are receptor tyrosine kinases which send signals into the cell to regulate many critical processes including development, tissue homeostasis, and tumorigenesis. Central to the signaling of these receptors is their intracellular kinase domain, which is activated by ligand-induced dimerization of the receptor and phosphorylates several tyrosine residues in the C-terminal tail. The phosphorylated tail then recruits other signaling molecules and relays the signal to downstream pathways. A model of the autoinhibition, activation and feedback inhibition mechanisms for the ErbB kinase domain has emerged from a number of recent structural studies. Meanwhile, recent clinical studies have revealed the relationship between specific ErbB kinase mutations and the responsiveness to kinase inhibitor drugs. We will review these regulation mechanisms of the ErbB kinase domain, and discuss the binding specificity of kinase inhibitors and the effects of kinase domain mutations found in cancer patients from a structural perspective.

Inhibition of the EGF receptor by binding of MIG6 to an activating kinase domain interface

Members of the epidermal growth factor receptor family (EGFR/Erb1, Erb2/HER2, ErbB3/HER3 and ErbB4/HER4) are key targets for inhibition in cancer therapy¹. Critical for activation is the formation of an asymmetric dimer by the intracellular kinase domains, in which the C-terminal lobe (C-lobe) of one kinase domain induces an active conformation in the other². The cytoplasmic protein Mig6 (Mitogen-induced gene 6) interacts with and inhibits the kinase domains of EGFR and ErbB2^3–5. Crystal structures of complexes between the EGFR kinase domain and a fragment of Mig6 show that a ~25-residue epitope (segment 1) from Mig6 binds to the distal surface of the C-lobe of the kinase domain. Biochemical and cell-based analyses confirm that this interaction contributes to EGFR inhibition by blocking the formation of the activating dimer interface. A longer Mig6 peptide that is extended C-terminal to segment 1 has increased potency as an inhibitor of the activated EGFR kinase domain, while retaining a critical dependence on segment 1. We show that signaling by EGFR molecules that contain constitutively active kinase domains still requires formation of the asymmetric dimer, underscoring the importance of dimer interface blockage in Mig6-mediated inhibition.

Syntheses of 4-(indole-3-yl)quinazolines: a new class of epidermal growth factor receptor tyrosine kinase inhibitors

The epidermal growth factor (EGF) family of membrane receptors has been identified as a key element in the complex signaling network that is utilized by various classes of cell-surface receptors. The synthesis and pharmacological results of 4-(indole-3-yl)quinazolines are described. The synthesized compounds are new high potent EGFR-tyrosine kinase inhibitors with excellent cytotoxic properties at different cell lines. Furthermore the 4-(indole-3-yl)quinazolines show some tendencies to inhibit the HER-2 TK, too. Moreover this substance class has remarkable strong fluorescence properties.

Protein-protein interactions in the allosteric regulation of protein kinases

Protein-protein interactions involving the catalytic domain of protein kinases are likely to be generally important in the regulation of signal transduction pathways, but are rather sparsely represented in crystal structures. Recently determined structures of the kinase domains of the mitogen-activated protein kinase Fus3, the RNA-dependent kinase PKR, the epidermal growth factor receptor and Ca(2+)/calmodulin-dependent protein kinase II have revealed unexpected and distinct mechanisms by which interactions with the catalytic domain can modulate kinase activity.

An Allosteric Mechanism for Activation of the Kinase Domain of Epidermal Growth Factor Receptor

The mechanism by which the epidermal growth factor receptor (EGFR) is activated upon dimerization has eluded definition. We find that the EGFR kinase domain can be activated by increasing its local concentration or by mutating a leucine (L834R) in the activation loop, the phosphorylation of which is not required for activation. This suggests that the kinase domain is intrinsically autoinhibited, and an intermolecular interaction promotes its activation. Using further mutational analysis and crystallography we demonstrate that the autoinhibited conformation of the EGFR kinase domain resembles that of Src and cyclin-dependent kinases (CDKs). EGFR activation results from the formation of an asymmetric dimer in which the C-terminal lobe of one kinase domain plays a role analogous to that of cyclin in activated CDK/cyclin complexes. The CDK/cyclin-like complex formed by two kinase domains thus explains the activation of EGFR-family receptors by homo- or heterodimerization.

4-Anilinoquinazolines with Lavendustin A subunit as inhibitors of epidermal growth factor receptor tyrosine kinase: syntheses, chemical and pharmacological properties

4-Anilinoquinazoline derivatives are widely investigated due to their potent epidermal growth factor receptor (EGFR) tyrosine kinase inhibitory activity. Two 4-anilinoquinazolines with Lavendustin A subunit (10a,b) were synthesized and examined for their EGFR tyrosine kinase inhibitory activity as well as their antiproliferative properties on variant human cancer cell lines. Both compounds maintained their EGFR tyrosine kinase inhibitory activity at the 10-7 M level and led to significant growth inhibition in certain leukemia, non-small cell lung cancer (NSCLC), ovarian cancer, renal cancer and breast cancer cell lines with GI50 values at the 10-6 M level. There could not be observed any notable difference between 10a and 10b regarding to their antiproliferative activity. Interestingly, we observed the high tendency of 10a and 10b to include certain solvents, e.g. water, DMF, DMSO, which may be due to the remarkable number of hydrogen accepting/donating groups in 10a and b. An X-ray analysis of 10a including water and DMF illustrates a possible hydrogen bond pattern and could serve as information for preferred receptor (e.g. EGFR tyrosine kinase) binding sites. Finally, we aimed for irreversible EGFR tyrosine kinase inhibitors. The p-quinone derivatives 11a and 11b, which contain a Michael acceptor position according to the irreversible inhibitor CI-1033, could be derived from the p-hydroquinone derivatives 10a or 10b, respectively, by oxidation. However, due to their instability 11a and 11b could not be obtained in a pure form.

SHP-2-Dependent Mitogen-Activated Protein Kinase Activation Regulates EGFRvIII but not Wild-Type Epidermal Growth Factor Receptor Phosphorylation and Glioblastoma Cell Survival

In human glioblastomas, the most common mutation of epidermal growth factor receptor (EGFR) is an in-frame deletion of an 801-bp sequence in the extracellular domain of EGFR termed EGFRvIII. The EGFRvIII does not bind ligand EGF but has constitutive tyrosine phosphorylation (pTyr) content and kinase activity that result in enhanced transformation, reduced apoptosis, and resistance to therapy. Here we report that the protein tyrosine phosphatase SHP-2 modulates a mitogen-activated protein kinase (MAPK) kinase (MEK)-mediated signaling pathway that regulates EGFRvIII pTyr and cell survival in U87MG.EGFRvIII cells. Overexpression of the phosphatase-inactive form of SHP-2 inhibited EGFRvIII pTyr by decreasing MAPK phosphorylation. Consistent with this, we observed that the MEK inhibitor PD98059, but not the phosphatidylinositol 3'-kinase inhibitor LY294002, inhibited EGFRvIII pTyr. Furthermore, constitutive EGFRvIII pTyr content observed in U87MG, LN229, and U373MG glioblastoma cells, but not in NR6.EGFRvIII fibroblasts, correlated with elevated MAPK levels in these cells. Interestingly, LY294002, but not PD98059, inhibited wild-type EGFR pTyr in response to EGF treatment in U87MG parental cells and in wild-type EGFR-overexpressing U87MG cells. Inhibition of EGFRvIII pTyr by PD98059 was not observed to be phosphorylation site specific. However, LY294002 more specifically inhibited wild-type EGFR pTyr at residues Tyr(992) and Tyr(1068) in the COOH terminus. Treatment of U87MG.EGFRvIII cells with PD98059, but not LY294002, also resulted in increased cell death in response to cisplatin. Collectively, a distinct MEK-mediated pathway in human glioblastoma cells appears to differentially modulate EGFRvIII and wild-type EGFR pTyr, and inhibition of the MAPK pathway sensitizes EGFRvIII-containing human glioblastoma cells to cisplatin-induced cell death.

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.

4-hydroxynonenal triggers multistep signal transduction cascades for suppression of cellular functions

4-hydroxynonenal (HNE), an aldehyde product of membrane lipid peroxidation, has been suggested to mediate a number of oxidative stress-linked pathological events in humans, including cellular growth inhibition and apoptosis induction. Because HNE is potentially reactive to a number of both cell surface and intracellular proteins bearing sulfhydryl, amino and imidazole groups, it seems that there are multiple signal transduction cascades. Here we briefly review the HNE-triggered signal transduction cascades that lead to suppression of cellular functions and to cell death, based mainly on our own recent study results. We first showed that formation of HNE-cell surface protein adducts, which mimicked ligand-cell surface receptor binding, induced activation of receptor-type protein tyrosine kinases such as epithelial growth factor receptor (EGFR) and that this caused growth inhibition through a cascade of activation of EGFR, Shc and ERK. Next, we showed that HNE-mediated scavenging of cellular glutathione led to activation of caspases and to DNA fragmentation through a Fas-independent and mitochondria-linked pro-apoptotic signal pathway. More recently, we have obtained evidence that the HNE-triggered signal cascade for caspase activation encounters complex positive feedback regulatory mechanisms that are linked to the inhibition of anti-apoptotic signals and are dependent on caspase activity. Underlying multiple regulatory mechanisms, including mechanisms of activation of Akt-dephosphorylating PP2A activity, activities of protein tyrosine kinases have been shown to be biphasically controlled by HNE. In addition, we have obtained results suggesting that HNE inhibits phosphorylation of IkappaB, possibly by targeting some elements upstream of IkappaB, which might downregulate the NF-kappaB-mediated cellular responses, including serum deprivation-induced iNOS expression and generation of anti-apoptotic signals. These results suggest that HNE reacts with multiple cell surface and intracellular sites for triggering a network of signal transduction that is ultimately focused on suppression of cellular functions.

Comparison of the biochemical and kinetic properties of the type 1 receptor tyrosine kinase intracellular domains. Demonstration of differential sensitivity to kinase inhibitors

Epidermal growth factor receptor (EGFR), ErbB-2, and ErbB-4 are members of the type 1 receptor tyrosine kinase family. Overexpression of these receptors, especially ErbB-2 and EGFR, has been implicated in multiple forms of cancer. Inhibitors of EGFR tyrosine kinase activity are being evaluated clinically for cancer therapy. The potency and selectivity of these inhibitors may affect the efficacy and toxicity of therapy. Here we describe the expression, purification, and biochemical comparison of EGFR, ErbB-2, and ErbB-4 intracellular domains. Despite their high degree of sequence homology, the three enzymes have significantly different catalytic properties and substrate kinetics. For example, the catalytic activity of ErbB-2 is less stable than that of EGFR. ErbB-2 uses ATP-Mg as a substrate inefficiently compared with EGFR and ErbB-4. The three enzymes have very similar substrate preferences for three optimized peptide substrates, but differences in substrate synergies were observed. We have used the biochemical and kinetic parameters determined from these studies to develop an assay system that accurately measures inhibitor potency and selectivity between the type 1 receptor family. We report that the selectivity profile of molecules in the 4-anilinoquinazoline series can be modified through specific aniline substitutions. Moreover, these compounds have activity in whole cells that reflect the potency and selectivity of target inhibition determined with this assay system.

Controlling beta-sheet assembly in genetically engineered silk by enzymatic phosphorylation/dephosphorylation

Enzymatic phosphorylation and dephosphorylation reactions were used to modify a genetically engineered variant of spider dragline silk. The approximately 25 kDa protein was phosphorylated with cyclic AMP-dependent kinase and dephosphorylated with calf intestinal alkaline phosphatase. Phosphorylation inhibited beta-sheet assembly of the protein and enhanced solubility to about 5 mg/mL in water, compared to about 20% of this level upon enzymatic dephosphorylation. The cyclability of the phosphorylation-dephosphorylation system was confirmed by MALDI with a model peptide. Kinetic studies conducted with [gamma-(32)P]ATP illustrate that the phosphorylation reaction proceeds over 6 h. Secondary structure of the phosphorylated and dephosphorylated proteins was determined by CD and FTIR. The results illustrate that an enzymatic phosphorylation event can be used to control the solution structure of a protein like silk, which has a tendency to prematurely precipitate due to the formation of beta-sheets.

Insights into the HER-2 receptor tyrosine kinase mechanism and substrate specificity using a transient kinetic analysis

The HER-2/erbB-2/c-neu proto-oncogene encodes for an EGF receptor-like protein which has been implicated in the pathogenesis of several human malignancies. Although much has been learned about the physiological significance of this receptor tyrosine kinase, its catalytic mechanism remains poorly understood. We have expressed, purified, and characterized two recombinant proteins corresponding to a full-length (HCD) and truncated (HKD) construct of the HER-2 intracellular tyrosine kinase domain and have identified an optimal substrate (GGMEDIYFEFMGGKKK; HER2Peptide) through screening of a degenerate peptide library. We have conducted a transient kinetic analysis of the HER-2 proteins (HCD and HKD) to illuminate mechanistic details of the HER-2 pathway. In particular, stopped-flow fluorescence studies with mant (N-methylanthraniloyl)-nucleotide derivatives provided direct measurements of the association and dissociation rate constants for these nucleotide interactions with the HER-2 recombinant proteins, thereby enabling the determination of nucleotide K(d) values. Moreover, the actual step of chemical catalysis was isolated using rapid chemical quench techniques and shown to occur approximately 3-fold faster than the steady-state rate which corresponds to product release. Evidence is also provided that suggests a conformational change that is partially rate-limiting at least in HCD. Furthermore, the role that the phosphorylation state of the protein may play on catalysis was examined. Studies carried out with pre-phosphorylated recombinant HER-2 proteins suggest that while autophosphorylation is not a prerequisite for enzymatic activity, this protein modification actually directly affects the catalytic mechanism by enhancing the rate of ADP release and that of the rate-limiting step. While a pre-steady-state kinetic analysis has been carried out on the catalytic subunit of cAMP-dependent serine/threonine kinase, to our knowledge, this study represents the first reported transient kinetic investigation of a receptor tyrosine kinase. This work serves as a basis for comparison of these two important protein kinase families and in this report we highlight these similarities and differences.

Dimerization of the extracellular domain of the receptor for epidermal growth factor containing the membrane-spanning segment in response to treatment with epidermal growth factor

A recombinant fragment of the human receptor for epidermal growth factor containing both its extracellular domain and its membrane-spanning segment, when dissolved with Triton X-100, was observed to dimerize in response to addition of epidermal growth factor (EGF) even at the lowest concentration of this fragment that could be assayed (4 nM). Consequently, the dissociation constant for the dimer of this fragment is at least 10,000-fold smaller than that for dimers of soluble, recombinant forms of the extracellular domain lacking the membrane-spanning segment. The second-order rate constant for dimerization of the fragment containing the extracellular domain and the membrane-spanning segment was estimated to be greater than 0.3 nM(-1) min(-1), more than 10-fold that of the native enzyme under the same conditions. This result suggests that the cytoplasmic domain of the intact enzyme sterically hinders its dimerization. When EGF is removed from the dimer of the fragment, the rate constant for its dissociation is greater than 0.2 min(-1), more than 40-fold that of the native enzyme. This result suggests that interfaces between cytoplasmic domains of intact EGF receptor impart significant stabilization to the dimer of the enzyme.

4-hydroxynonenal triggers an epidermal growth factor receptor-linked signal pathway for growth inhibition

Lipid peroxidation has been implicated in the pathogenesis of various diseases. As a major product of membrane lipid peroxidation, 4-hydroxynonenal (HNE) appears after various kinds of oxidative stress, and is known to induce cell growth inhibition. We here analysed the HNE-mediated signal transduction cascade for the growth inhibition of human epidermoid carcinoma A431 cells. HNE dose-dependently induced phosphorylation of multiple cellular proteins including epidermal growth factor receptor (EGFR) in A431 cells, and rapidly upregulated the catalytic actions of EGFR for autophosphorylation and for phosphorylation of casein as an exogenous substrate. Immunoblot analysis by use of HNE-specific antibody demonstrated the binding of HNE to EGFR along with its activation. This binding, which did not induce cross-linking of EGFR, caused a capping of the receptor on the cell surface which mimicked the capping induced by EGF. Phosphorylation and activation of EGFR were followed by phosphorylation of adaptor protein Shc and activation of MAP kinase. Both genistein as a wide spectrum protein tyrosine kinase inhibitor and AG1478 as a specific EGFR tyrosine phosphorylation blocker inhibited activation of EGFR and MAP kinase by HNE. The same inhibitors prevented HNE-mediated growth inhibition, suggesting a close linkage between EGFR/MAP kinase activation and growth inhibition after exposure to HNE. Our results suggest that EGFR may be one of the primary targets of HNE for an oxidative stress-linked cell growth inhibition.

Kinetics of activation of the tyrosine kinase of a deletion mutant of epidermal growth factor receptor missing the sites of self-phosphorylation

The addition of epidermal growth factor (EGF) to epidermal growth factor receptor (EGF receptor) dissolved in a solution of the detergent Triton X-100 results in the activation of its protein tyrosine kinase. To investigate the importance of the sites for self-phosphorylation on the enzyme in this process, the kinetics of activation of a deletion mutant missing the last 195 amino acids of the protein, including all of the sites for self-phosphorylation, were followed by monitoring the initial velocity at which the enzyme catalyzes the phosphorylation of the exogenous substrate RRKGSTAENAEYLRV. The activation of the enzymatic activity of this deletion mutant of EGF receptor displays kinetics that are second-order with respect to the concentration of the enzyme as does wild-type EGF receptor. The second-order rate constant for its activation is 36 +/- 10 microM-1 min-1, which is only 3-fold greater than the second-order rate constant for the activation of wild-type EGF receptor under the same conditions (13 +/- 2 microM-1 min-1). These results suggest that the mechanism by which the protein tyrosine kinase of the deletion mutant is activated is the same as that for the activation of the wild-type receptor and that the sites of self-phosphorylation in the wild-type EGF receptor do not participate in the mechanism of activation of the enzyme.

A basic residue, Lys 782, composes part of the ATP-binding site on the epidermal growth factor receptor tyrosine kinase

To identify amino acids specific for tyrosine kinase activity, the role of several conserved basic residues in kinase function was tested. Modeling of the epidermal growth factor receptor tyrosine kinase domain based on the crystal structure of cyclic AMP-dependent protein kinase and insulin receptor revealed several basic residues present on the surface of epidermal growth factor receptor. Using the molecular modeling program, GRASP, the basic residues Arg 779, Lys 782, and Lys 855 were shown to provide an area of positive charge to the surface of the molecule. To deduce the role of these residues in ATP and substrate binding, site-directed mutants were prepared and kinetic constants were measured. Mutation of Lys 855 to Ala destabilized the enzyme and caused partial inactivation. Mutation of either Arg 779 or Lys 782 had little effect on the Km value for peptide substrate. However, alteration of Lys 782 increased the Km value for ATP 28-fold, indicating a role for Lys 782 in binding ATP. Because residues similar to Lys 782 in the sequences of mitogen-activated protein kinase and insulin receptor make contact with a ribose hydroxyl of ATP, it is proposed that Lys 782 may be one of the residues composing the ribose-binding site of epidermal growth factor receptor.

Tyrosine kinase activity of purified recombinant cytoplasmic domain of platelet-derived growth factor beta-receptor (beta-PDGFR) and discovery of a novel inhibitor of receptor tyrosine kinases

Aberrant expression of platelet-derived growth factor and its receptor (PDGFR) has been implicated in various human disorders, including cardiovascular disease and certain types of cancer. Inhibitors of the tyrosine kinase activity of PDGFR are leads in the development of novel agents to combat these diseases. We describe here a novel, potent inhibitor of PDGFR tyrosine kinase, 3-(4-dimethylamino-benzylidenyl)-2-indolinone (DMBI). The compound also inhibits signal transduction through fibroblast growth factor receptor 1 (FGFR1), but is not active towards epidermal growth factor receptor (EGFR) or c-Src tyrosine kinase. The activity of DMBI and other tyrosine kinase inhibitors was compared in a cell-based assay as well as in an assay based on purified recombinant platelet-derived growth factor beta-receptor (beta-PDGFR) lacking the transmembrane and ligand-binding domain. We showed that this truncated beta-PDGFR could dimerize, and that dimerization was required for tyrosine kinase activity. Tyrosine kinase activity was modulated by inhibitors of beta-PDGFR autophosphorylation in cells, but not by specific inhibitors of EGFR or c-Src tyrosine kinase. We conclude that beta-PDGFR lacking the transmembrane and ligand-binding domain retains the essential properties of the full-length receptor tyrosine kinase.

Persistence of dimerization and enzymatic activity of epidermal growth factor receptor in the absence of epidermal growth factor

Dimerized and enzymatically active epidermal growth factor receptor, in the presence of saturating epidermal growth factor (EGF), was passed over a column containing an immunoadsorbent for EGF. The immunoadsorbent removed both EGF free in solution and EGF bound to EGF receptor, while EGF receptor passed through unimpeded. Both the dimerization of EGF receptor and the activation of its tyrosine kinase were arrested in samples containing EGF receptor that had been mixed with saturating EGF and then immediately passed over the immunoadsorbent for EGF. The EGF receptor in these samples, however, dimerized completely, and its tyrosine kinase became fully active upon readdition of EGF. Samples of EGF receptor that were fully dimerized and enzymatically active prior to being passed over the immunoadsorbent for EGF remained dimerized and enzymatically active even in the absence of bound EGF. The first-order rate constant for the inactivation of the tyrosine kinase of EGF receptor depleted of EGF was estimated by subtracting the rate constant of inactivation of the tyrosine kinase in samples replenished with EGF from the rate constant of inactivation of the tyrosine kinase in samples that had been depleted of EGF. The rate constant of inactivation was found to be 0.26 +/- 0.06 h-1.

Self-phosphorylation of epidermal growth factor receptor is an intermolecular reaction

The binding of epidermal growth factor (EGF) to epidermal growth factor receptor (EGF receptor) results in the dimerization and self-phosphorylation of the receptor. Both of these responses were followed as a function of time and the concentration of EGF receptor. Dimerization of EGF receptor was monitored by immunoblotting the protein after it had been cross-linked with glutaraldehyde. The capacity for self-phosphorylation was followed by measuring the relative level of incorporation of [32P]phosphate into EGF receptor on autoradiograms of the same immunoblots used for the assay of its dimerization. When these two properties were followed as a function of time, it was found that dimerization preceded the appearance of the capacity for self-phosphorylation. Both dimeric and monomeric forms of EGF receptor were self-phosphorylated in the presence of EGF, but the dimeric form was phosphorylated preferentially to the monomeric form. When the dimerization and the capacity for self-phosphorylation were followed as a function of the concentration of dimeric EGF receptor, it was observed that the self-phosphorylation of dimeric EGF receptor increased as the concentration of dimeric EGF receptor increased. An equation including terms representing both intramolecular and intermolecular rates of self-phosphorylation was fit to the plots of self-phosphorylation as a function of concentration of EGF receptor. These fits demonstrate that intramolecular self-phosphorylation within dimers of EGF receptor is insignificant and that self-phosphorylation is an intermolecular process between dimers of EGF receptor.

Molecular approaches to receptors as targets for drug discovery

The cloning of a great number of receptors and channels has revealed that many of these targets for drug discovery can be grouped into superfamilies based on sequence and structural similarities. This review presents an overview of how molecular biological approaches have revealed a plethora of receptor subtypes, led to new definitions of subtypes and isoforms, and played a role in the development of high selective drugs. Moreover, the diversity of subtypes has molded current views of the structure and function of receptor families. Practical difficulties and limitations inherent in the characterization of the ligand binding and signaling properties of expressed recombinant receptors are discussed. The importance of evaluating drug-receptor interactions that differ with temporally transient and distinct receptor conformational states is emphasized. Structural motifs and signal transduction features are presented for the following major receptor superfamilies: ligand-gated ion channel, voltage-dependent ion channel, G-protein coupled, receptor tyrosine-kinase, receptor protein tyrosine-phosphatase, cytokine and nuclear hormone. In addition, a prototypic receptor is analyzed to illustrate functional properties of a given family. The review concludes with a discussion of future directions in receptor research that will impact drug discovery, with a specific focus on orphan receptors as targets for drug discovery. Methods for classifying orphan receptors based upon homologies with members of existing superfamilies are presented together with molecular approaches to the greater challenge of defining their physiological roles. Besides revealing new orphan receptors, the human genome sequencing project will result in the identification of an abundance of novel receptors that will be molecular targets for the development of highly selective drugs. These findings will spur the discovery and development of an exciting new generation of receptor-subtype specific drugs with enhanced therapeutic specificity.

Insufficiency of self-phosphorylation for the activation of epidermal growth factor receptor

Polyclonal immunoglobulins were produced against the carboxy terminus, -SEFIGA, of the receptor for epidermal growth factor (EGF). The addition of these immunoglobulins to a solution containing EGF receptor resulted in the activation of its protein tyrosine kinase. The levels of activation were assessed by measuring the initial velocities of the phosphorylation of the tyrosine in angiotensin II. The enzymatic activity induced by the immunoglobulins was significant, usually 50-70% of the maximum activity induced by EGF, and the induction occurred over a narrow range of concentration of the immunoglobulins. In order to achieve the activation, the immunoglobulins had to be bivalent; the addition of monovalent Fab fragments to EGF receptor did not produce any activation of the protein tyrosine kinase. The activation produced by the immunoglobulins was found to be reversible upon the addition of the synthetic peptide SEFIGA against which the immunoglobulins had been produced. Self-phosphorylation of the EGF receptor also occurred as the enzyme was activated by the immunoglobulins. Tryptic peptide maps demonstrated that the self-phosphorylation caused by the immunoglobulins had the same signature as that produced by EGF. When the synthetic peptide that had been used as the hapten was added to EGF receptor that had been self-phosphorylated in the presence of the immunoglobulins, the stimulated enzymatic activity was lost even though the protein remained phosphorylated. It follows from the results of deletion mutation [Walton, G. M., Chen, W. S., Rosenfeld, M. G., & Gill, G. N. (1990) J. Biol. Chem. 265, 1750-1754] and the results reported here that self-phosphorylation is neither necessary nor sufficient for the activation of EGF receptor.

Sequential cis/trans autophosphorylation in TrkB tyrosine kinase

TrkB, a member of the tyrosine kinase family of growth factor receptors, is activated by binding of brain-derived neurotrophic factor or neurotrophin 4/5. The intracellular kinase domain of TrkB (ICD-TrkB) was prepared by an insect cell expression system and characterized to identify the mechanism of autophosphorylation. The time course of autophosphorylation, which shows a biphasic progression with a slow nonlinear phase followed by a fast linear phase, indicates the existence of autophosphorylation-induced activation in ICD-TrkB. This is also supported by the finding that phosphorylated ICD-TrkB shows significantly higher activity than control naive ICD-TrkB. Interestingly, the autophosphorylation rate in the linear phase clearly depends on the ICD-TrkB concentration, whereas the rate of initial autophosphorylation is independent of the concentration of ICD-TrkB in the reaction mixture. This observation suggests a two-step autophosphorylation, first an intramolecular activating step and then an intermolecular step. This mechanism is confirmed by the result that only the later phase of autophos-phorylation is inhibited by addition of glycerol which interferes with intermolecular interactions. Therefore, we propose the mechanism of ICD-TrkB autophosphorylation as a sequential cis/trans phosphorylation.

The Schistosoma mansoni epidermal growth factor receptor homologue, SER, has tyrosine kinase activity and is localized in adult muscle

DNA encoding the full-length Schistosoma mansoni epidermal growth factor receptor, SER, was obtained by combining partial cDNA clones. Three different anti-SER antibody preparations, specific either to the SER amino-terminus or the predicted ligand binding domain were generated with recombinant or synthetic peptides and purified by antigen affinity. Recombinant SER was expressed within insect cells using the baculovirus expression system and detected by each of the anti-SER antibodies as well as anti-phosphotyrosine antibodies. By in vitro phosphorylation of immunoprecipitated recombinant SER, the protein has been shown to be capable of tyrosine autophosphorylation but this activity is not affected by human epidermal growth factor. Native SER is detected as a 170 kDa protein in Western blots of S. mansoni adult worm membrane preparations. Adult worm sections, labeled with anti-SER antibodies, localize SER predominantly to the muscle of adult male and female worms. These results confirm a place for SER in the EGFR family of tyrosine kinases and strongly suggest that it participates in schistosome signal transduction, perhaps related to muscle development or function.

In vitro activation of Stat3 by epidermal growth factor receptor kinase

Stat proteins are SH2 domain-containing transcription factors that are activated in cells by various cytokines and growth factors. In the case of cytokines whose receptors lack protein kinase activity, phosphorylation-activation is mediated by members of the JAK family of tyrosine protein kinases. In the case of growth factors whose receptors have intrinsic tyrosine protein kinase activity, it is thought that Stat proteins can be activated either directly by the receptor or indirectly through JAK proteins. To test the possibility of direct activation, we have used purified Stat3 alpha, Stat3 beta, and epidermal growth factor receptor kinase produced in recombinant baculovirus-infected Sf9 insect cells. The Stat proteins formed a stable complex with the receptor kinase, and they were phosphorylated on tyrosine by the receptor kinase and activated for binding to DNA, properties shared with Stat proteins purified from Sf9 cells coexpressing JAK1 or JAK2. Both JAK-phosphorylated Stat3 beta and Stat3 beta phosphorylated in vitro by the receptor kinase were 20-50 times more active on a molar basis for DNA binding than phosphorylated Stat3 alpha. We conclude that Stat3 isoforms can be directly phosphorylated and thereby activated in vitro by the epidermal growth factor receptor kinase.

Mutational analysis of the nucleotide binding site of the epidermal growth factor receptor and v-Src protein-tyrosine kinases

Tyrosine kinases differ from serine/threonine kinases in sequences located at the active site where ATP and substrate bind. In the structure of cyclic AMP-dependent protein kinase, the catalytic loop contains the sequence Lys-Pro-Glu where the Lys residue contacts the gamma-phosphate of ATP and the Glu residue contacts a basic residue located in the peptide substrate. In tyrosine kinases, the analogous sequence is Ala-Ala-Arg in the receptor tyrosine kinase subfamily and Arg-Ala-Ala in the Src tyrosine kinase subfamily. To deduce the role of these residues in tyrosine kinase function, site-directed mutations were prepared in the epidermal growth factor receptor (EGFR) and in v-Src and effects on ATP binding and kinase activity were determined. Changing Arg to either Lys or Ala dramatically reduced activity of both tyrosine kinases and this correlated with loss of ATP binding. Changing the orientation of this sequence impaired activity of EGFR to a greater extent than that of v-Src but did not change substrate specificity of the two enzymes. These results support the hypothesis that Arg functions to coordinate the gamma-phosphate of ATP. Analysis of sequence inversions in the catalytic loop indicate that the active site of v-Src exhibits greater flexibility than that of EGFR.

Activation of epidermal growth factor receptor by epidermal growth factor

The binding of epidermal growth factor (EGF) to epidermal growth factor receptor (EGF receptor) induces dimerization of the receptor and activation of its protein tyrosine kinase. Each of these three steps was followed as a function of the concentrations of EGF and of EGF receptor. Binding of EGF was followed by sedimentation of the complex between [3H]EGF and EGF receptor, dimerization was measured by quantitative cross-linking with glutaraldehyde, and the activation of the protein tyrosine kinase was monitored under the same conditions by following the initial velocity of the phosphorylation of peptides containing tyrosine. The binding of epidermal growth factor to its receptor was measured as a function of the concentration of epidermal growth factor, and the relationship was sigmoid with an average value of 1.7 for the Hill coefficient. Both dimerization and the activation of the tyrosine kinase displayed saturation as a function of the concentration of EGF. The ranges of the concentrations of EGF where dimerization and activation of the tyrosine kinase activity were half-maximal were 15-30 and 50-200 nM, respectively, but the value for the concentration of EGF at the half-maximum for the activation of the tyrosine kinase was a complex function of the concentration of EGF receptor. The observed behavior of the binding of EGF, the dimerization of EGF receptor, and the activation of the tyrosine kinase were used as criteria against which to test mechanisms for the process of activation. Equations were derived for various reversible and irreversible mechanisms and used to calculate the theoretical behaviors of the three properties. In direct comparisons of the experimental and the theoretical data, several of the previously proposed reversible and irreversible mechanisms for the activation of EGF receptor were found to be inadequate, but a reasonable mechanism was formulated that was compatible with the experimental data. In this mechanism, dimeric EGF receptor must be occupied by two molecules of EGF for enzymatic activity to be expressed.

Role of the regulatory domain of the EGF-receptor cytoplasmic tail in selective binding of the clathrin-associated complex AP-2

BACKGROUND

After stimulation of a cell by the mitogenic epidermal growth factor (EGF), the EGF receptor (EGF-R) is cleared from the cell surface in order to turn off receptor signaling. This internalization is mediated via clathrin-coated pits and coated vesicles, and ultimately the receptors are delivered to the lysosome and destroyed. It is believed that clathrin-associated protein complexes or adaptors (APs) link the entrapment of EGF-R and other nutrient and growth-factor receptors to the formation of the clathrin-coated pit. Two classes of APs are known--AP-2, found at the plasma membrane, and AP-1, found in the trans-Golgi network. Activated EGF-R associates with AP-2s at the plasma membrane, but the mechanism responsible for this association is not known. Here, we investigate, in vivo and in vitro, three aspects of the interaction between APs and EGF-R: firstly, we ask whether EGF-R at the plasma membrane distinguishes between AP-1 and AP-2; secondly, we ask which part of the receptor's cytoplasmic tail is responsible for binding; finally, we ask whether autophosphorylation by EGF-R is essential for the interaction.

RESULTS

We demonstrate that EGF-R displays a selective association for AP-2 over AP-1 in vivo, and that this preferential interaction can also be detected using surface plasmon resonance in vitro. Using a truncated mutant and a kinase-dead mutant of EGF-R, we show that the regulatory domain of the cytoplasmic tail is essential for the recruitment of AP-2 in vivo and that this domain is required for association between purified AP-2 and EGF-R in vitro. Finally, we demonstrate, in vivo and in vitro, that tyrosine auto-phosphorylation by the receptor is not an essential pre-condition for the recruitment of AP-2.

CONCLUSIONS

EGF-R binds selectively to AP-2s, and the regulatory domain of its cytoplasmic tail is required for this interaction. The lack of correlation between receptor autophosphorylation and AP-2 recruitment suggests that activation of the EGF-R kinase stimulates endocytosis by the phosphorylation of a factor distinct from EGF-R itself, as also proposed by others based on experiments measuring receptor traffic and entrapment.

Inhibition by 5'-methylthioadenosine of cell growth and tyrosine kinase activity stimulated by fibroblast growth factor receptor in human gliomas

Stimulation of three human glioma cell lines with basic fibroblast growth factor (bFGF) led to the enhancement of cell growth and the rapid tyrosine phosphorylation of cellular proteins, including major substrates of 90 kD. A methyltransferase inhibitor, 5'-methylthioadenosine (MTA), inhibited dose dependently the bFGF-stimulated cell growth and protein tyrosine phosphorylation in glioma cells by blocking both receptor autophosphorylation and substrate phosphorylation, as shown by immunoblotting with antiphosphotyrosine antibodies and cross-linking bFGF to receptors. The antiproliferative activity of MTA correlated quantitatively with its potency as an inhibitor of bFGF-stimulated protein tyrosine kinase activity. The methyltransferase inhibitor MTA had no effect on either epidermal growth factor- or platelet-derived growth factor-stimulated protein tyrosine phosphorylation in glioma cells, but inhibited specifically bFGF-stimulated protein tyrosine kinase activity. The concentration of MTA required for inhibition of protein methylation correlated well with the concentration required for inhibition of bFGF-stimulated cell growth and protein tyrosine phosphorylation. Because MTA had no effect on numbers and dissociation constants of high- and low-affinity bFGF receptors, the inhibition of bFGF-stimulated bFGF receptor tyrosine kinase activity is not likely to be the result of a reduction in bFGF receptor and bFGF binding capacity. In fact, MTA delayed and reduced the internalization and nuclear translocation of bFGF, and the internalized bFGF was submitted to a limited proteolysis that converted it to lower molecular peptides whose presence remained for at least 22 hours. The effect of MTA on bFGF-stimulated tyrosine phosphorylation was immediate and readily reversible.

Molecular design and biological activity of potent and selective protein kinase inhibitors related to balanol

BACKGROUND

The protein kinase C (PKC) family of serine/threonine-specific protein kinases is involved in many cellular processes, and the unregulated activation of PKC has been implicated in carcinogenesis. PKC inhibitors thus have significant potential as chemotherapeutic agents. Recently, the fungal metabolite balanol was shown to be an exceptionally potent inhibitor of PKC. We previously developed a practical and efficient total synthesis of balanol. We set out to use this synthetic molecule, and several synthetic analogs, to probe the mechanism of PKC inhibition and to determine the effect of balanol on the activity of other protein kinases.

RESULTS

As well as inhibiting PKC, balanol is a potent inhibitor of cyclic AMP-dependent protein kinase (PKA), another protein serine/threonine kinase. Balanol does not, however, inhibit the Src or epidermal growth factor receptor protein tyrosine kinases. The inhibition of both PKC and PKA by balanol can be overcome by high concentrations of ATP, and molecular modeling studies suggest that balanol may function as an ATP structural analog. Although balanol discriminates rather poorly between PKC and PKA, only minor modifications to its molecular structure are required to furnish compounds that are highly specific inhibitors of PKA.

CONCLUSIONS

A number of balanol analogs have been designed and synthesized that, unlike balanol itself, exhibit dramatic selectivity between PKA and PKC. Thus, despite the substantial homology between the catalytic domains of PKA and PKC, there is enough difference to allow for the development of potent and selective inhibitors acting in this region. These inhibitors should be useful tools for analyzing signal transduction pathways and may also aid in the development of drugs with significant therapeutic potential.

Aggregation-induced activation of the epidermal growth factor receptor protein tyrosine kinase

Various agents are able to stimulate the EGF receptor protein tyrosine kinase in the absence of ligand binding. To characterize their mechanism of action, we investigated their effects on the kinase activity of the intracellular domain of the EGF receptor (EGFR-IC). EGFR-IC (67 kDa) lacking the extracellular domain and transmembrane segment of the EGF receptor, but retaining kinase and autophosphorylation domains, was produced and purified as a soluble, cytoplasmic protein from Sf9 insect cells infected with a recombinant baculovirus. EGFR-IC was able to undergo autophosphorylation in a manner similar to full-length EGFR. Synthetic substrate peptides showed similar affinity to EGFR-IC as to the full-length receptor. The activity of the EGFR-IC was found to be dependent on divalent cations, Mn2+ being a more potent activator than Mg2+. Agents capable of aggregating the kinase by direct interaction (cross-linking antibodies, polycations) or through altering the surrounding solvent structure and thereby decreasing protein solubility [ammonium sulfate, poly(ethylene glycol), 2-methyl-2,4-pentanediol] activated the kinase in a manner which correlated with their ability to precipitate the EGFR intracellular domain. The widely different chemical nature of these agents suggests that they do not act by direct interaction with specific allosteric regulatory sites, but rather by facilitating the interactions between kinase molecules. These results support the hypothesis that full-length receptor aggregation itself, induced by ligand binding to the extracellular domain, results in intracellular domain interactions and the activation of kinase activity.

Substrate-specific modulation of Src-mediated phosphorylation of Ras and caseins by sphingosines and other substrate modulators

It is important for the understanding of protein kinase action to differentiate between regulation at the enzyme and at the substrate levels. For example, the inhibitors dinitrophenol-tyrosine and tyrphostins act at the enzyme level to inhibit phosphorylation of all substrates by c-Src and v-Src kinases. In contrast, polylysine acts at the substrate level to stimulate Src-mediated phosphorylation of beta-casein but to inhibit phosphorylation of alpha-casein. Here we demonstrate novel enzyme-specific and substrate-specific modulations of Src kinase activity of potential physiological significance. At the enzyme level, we observed that c-Src kinase preferentially phosphorylates alpha-casein, while the v-Src kinase prefers beta-casein. At the substrate level we observed substrate-specific modulation by physiological factors including sphingosine, sphingosine derivatives and the ganglioside GM3. Galactosyl-sphingosine (psychosine) was more effective in stimulating phosphorylation of beta-casein and poly(E1A1Y1) than sphingosine. Glucosyl- and lactosyl-sphingosine were ineffective. Rat was extensively phosphorylated by c-Src in the presence of polylysine, and to a lesser extent in the sphingosine and galactosyl-sphingosine. These unexpected differences point out another potential mechanism for regulation of c-Src and v-Src kinase activities and may help to explain some of the pleotyptic manifestations of protein tyrosine kinase actions.

Characterization of baculovirus-expressed human alpha and beta platelet-derived growth factor receptors

In an effort to biochemically characterize PDGF receptors and their mechanism of activation, recombinant baculovirus vectors containing the cDNAs of the human alpha PDGF receptor or beta PDGF receptor were engineered. Characterization of recombinant PDGF receptor expression in infected Sf9 insect cells by immunoblot analysis with specific PDGF receptor peptide antisera revealed that the alpha and beta PDGF receptor gene products were translated as 160- and 165-kDa transmembrane proteins, respectively. Ligand binding analysis demonstrated saturable, high-affinity binding of either 125I-labeled PDGF AA or 125I-labeled PDGF BB to Sf9 cells expressing the recombinant alpha PDGF receptor. In contrast, recombinant beta PDGF receptor expressing Sf9 cells showed high-affinity binding only for PDGF BB. Analysis of the kinetics of PDGF receptor expression demonstrated that receptor number increased dramatically from 24- to 48-h postinfection. Early in infection, the PDGF receptors were present in low numbers, lacked tyrosine phosphorylation, and exhibited ligand-dependent tyrosine phosphorylation. However, with increasing time postinfection and increasing receptor number, the PDGF receptors became constitutively tyrosine-phosphorylated in serum-free culture medium. Cross-linking studies revealed that receptor activation involved ligand-independent receptor dimer formation at high receptor number. Thus, these results strongly suggest that PDGF stabilizes and increases the frequency of PDGF receptor interaction, which ultimately results in PDGF receptor activation and intracellular signaling.

Biphasic effects of exogenous ganglioside GM3 on follicle-stimulating hormone-dependent expression of luteinizing hormone receptor in cultured granulosa cells

The major ganglioside NeuAc alpha 2-->3Gal beta 1-->4Glc beta 1-->1Cer (GM3) present in cultured rat granulosa cells was examined for potential function in the expression of luteinizing hormone (LH) receptor on the cell surface in response to follicle-stimulating hormone (FSH). Synthesis of GM3 was stimulated concentration-dependently by FSH, and the stimulation was enhanced synergistically by insulin, as revealed by metabolic labeling of glycosphingolipids with [3H]galactose. When granulosa cells were cultured in the media containing GM3 (0.2-20 microM), biphasic changes in FSH-dependent expression of LH receptor were observed, as measured by the binding of 125I-deglycosylated human choriogonadotropin to the intact cells. Exogenous GM3 suppressed expression of LH receptor in the cells treated with a low dose of FSH (20 ng/ml), which was characterized by a low GM3 level, to 30% of control at 10 microM, with a half-maximal inhibitory concentration of 8 microM. In contrast, in the cells treated with a high dose of FSH (100 ng/ml) and insulin, which was characterized by a high GM3 level, expression of LH receptor was enhanced by exogenous GM3, to 148% of control at 10 microM, with a half-maximal effective concentration of 2 microM. Exogenous GM3 produced concomitant changes in the levels of extracellular cAMP. These effects of exogenous GM3 were not accompanied by changes in granulosa cell proliferation. Exogenous GM3 also modulated the LH receptor expression by the synergistic action of 12-O-tetradecanoylphorbol 13-acetate with insulin, with no significant changes in cellular DNA contents, suggesting that exogenous GM3 does not modulate directly the action of FSH at its receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

A strong protein-tyrosine kinase activity is associated with a baculovirus-expressed chicken tkl gene

We have previously described a gene named tkl (tyrosine kinase related to lck). It belongs to the src family of protein-tyrosine kinases and among these it has significant homology to the lck gene (lymphoide cell kinase). The tkl gene product may represent the avian homolog of Lck, which is believed to participate in a lymphocyte-specific signal transduction pathway by association with a membrane receptor. To study the biochemical properties of the protein, a nearly complete tkl gene (isolated from a cDNA library from chicken spleen cells) was expressed in a baculovirus system. Approximately 10% of the extracted protein consisted of the soluble 51-kDa Tkl protein (p51tkl) at 40 h post-infection. This protein was found to be phosphorylated on tyrosine and serine residues at a ratio of 5:1. As expected, glycosylation or myristoylation could not be detected. Immunocomplex kinase assays indicated strong autophosphorylation of p51tkl at tyrosine residues and phosphorylation of exogenous substrates such as D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), histones H2b and H4, and casein. This protein-tyrosine kinase activity also exhibited a marked preference for Mn2+ compared to Mg2+. The high level expression of enzymatically active Tkl should provide an excellent tool to further study the biological functions of this class of enzymes.

Large-scale purification and characterisation of a recombinant epidermal growth-factor receptor protein-tyrosine kinase. Modulation of activity by multiple factors

The human epidermal-growth-factor receptor (EGF-R) is a 170-kDa transmembrane glycoprotein that mediates the mitogenic response of cells to EGF and transforming growth factor alpha. Culture conditions have been developed for the large-scale expression of the cytoplasmic domain of the EGF-R in insect cells using a recombinant baculovirus. From 61 Sf9 cells, grown to high density using a bioreactor, 20 mg of the EGF-R kinase was purified to greater than 95% purity. Purification, which was carried out in the absence of detergents using classical purification methods, yielded an EGF-R protein that was not phosphorylated on tyrosine. This procedure has enabled us to produce high quality enzyme for both structural and biochemical studies on the EGF-R kinase. The in vitro activity of the cytoplasmic domain of the EGF-R kinase was modulated by multiple assay factors which include substrates, divalent cations and conformational modulators. Kinetic analysis in the presence of Mn2+ gave an apparent Vmax value of 20 nmol min-1 mg-1 and Km values of 4.5 microM for ATP and 1.43 mM for angiotensin II. This corresponds to a turnover number of 1.4 mol min-1 mol-1. Ammonium sulfate (1 M) resulted in an eightfold stimulation of kinase activity when assayed using angiotensin II as substrate. The specific activity of the intracellular domain of the EGF-R, when assayed at 20 degrees C in the presence of 1M ammonium sulfate, was 160 nmol min-1 mg-1. Activation of the EGF-R kinase by ammonium sulfate was found to be substrate-specific. No activation was found when assayed using polymeric substrates. Addition of Me(2+)-ATP to the purified enzyme resulted in autophosphorylation and was accompanied by retardation of SDS/PAGE migration. Kinetic constants and metal ion preferences of a number of co-polymers and peptide substrates have been compared. Dramatic differences in kinetic constants were found which were dependent on both the substrate and metal ion used. Activation of EGF-R autophosphorylation was found to be influenced by the use of charged polymers. The random polymer of Glu, Lys, Ala, Tyr (2:5:6:1), which was not phosphorylated by the EGF-R kinase, dramatically activates autophosphorylation of the EGF-R. Thus the intracellular domain of the EGF-R appears to be in a low-activity conformation which, under appropriate assay conditions, can be activated to a similar specific activity to that reported for the purified EGF-R holoenzyme.

Development of solid-phase enzyme-linked immunosorbent assays for the determination of epidermal growth factor receptor and pp60c-src tyrosine protein kinase activity

Solid-phase ELISAs for the determination of EGF receptor (EGF-R) and pp60c-src tyrosine protein kinase activity are described. The methods were developed and optimized using purified recombinant EGF-R intracellular domain (ICD) and pp60c-src tyrosine protein kinases. A standardized assay that utilizes poly (GluNa-Tyr)4:1 as substrate and a monoclonal antiphosphotyrosine antibody for detection is described. Assay conditions for both enzymes were optimized with respect to substrate and ELISA plate-coating condition, divalent metal ion preferences, enzyme concentration, apparent kinetic constants for ATP, and reaction linearity. Following standardization, a number of reference tyrosine protein kinase inhibitors were tested in the ELISAs and compared to results obtained using solution-phase radioactive tyrosine protein kinase assays, which are based on the transfer of 32P from [gamma-32P]ATP to synthetic substrate. To enable a comprehensive comparison, IC50 values obtained in the ELISA were compared with values obtained in radioactive assays using both the holo-EGF-R and EGF-R ICD kinases. No substantial qualitative differences between these assays were seen. For many routine tyrosine protein kinase assays, semiquantitative or qualitative measurement of TPK activity is adequate. For such purposes, the ELISAs would be an attractive alternative to radioactive assays.

The ligand binding domain of the epidermal growth factor receptor is not required for receptor dimerization

To examine the role of the ligand binding domain of epidermal growth factor receptor in its dimerization, we studied the dimerization of a truncated form of the receptor that resembles v-erbB in that it lacks a ligand binding domain. Receptor dimerization was determined by sedimentation analysis on sucrose density gradients at different concentrations of Triton X-100. At high concentrations of Triton X-100 (0.2%), the truncated receptor occurred as a monomer and displayed low basal autophosphorylation. By contrast, at low concentrations of Triton X-100 (0.01%), it existed as a dimer and exhibited high basal autophosphorylation. The ability of the truncated receptor to dimerize indicates that the ligand binding domain of the epidermal growth factor receptor is not required for receptor dimerization.

Potentiation of epidermal growth factor receptor protein-tyrosine kinase activity by sulfate

The protein-tyrosine kinase activity of the epidermal growth factor (EGF) receptor is critical for EGF-stimulated cell growth, although little is known about the molecular details of its enzymatic activity. Previous studies have found that EGF receptor kinase activity can be stimulated by factors such as ammonium sulfate ((NH4)2SO4), but the manner in which (NH4)2SO4 induces this effect is unclear. Therefore, we have explored the processes by which (NH4)2SO4 potentiated tyrosine kinase activity to better understand not only the molecular events involved in (NH4)2SO4 activation, but also the kinetic properties and mechanism of the EGF receptor. In this study, the addition of an optimum concentration of (NH4)2SO4 (250 mM) resulted in a 5-fold stimulation of kinase activity toward the peptide substrate, angiotensin II. The sulfate group is primarily involved in this action, since other salts containing SO4(2-) increased kinase activity similarly, whereas salts containing Cl- and F- had less of an effect, and divalent salts such as HPO4(2-) and NaVO4(2-) were inhibitory at doses of 1 mM or more. In addition, EGF receptor kinase activation by (NH4)2SO4 did not strictly correlate with changes in the ionic strength or conductivity of the solution. However, several lines of evidence suggest that SO4(2-) directly alters the kinetic properties of the EGF receptor kinase: (1) the maximum velocity (Vmax) and Km (ATP) for EGF receptor phosphorylation of angiotensin II were substantially higher in the presence of (NH4)2SO4. (2) EGF receptor kinase activity in the absence of (NH4)2SO4 required either Mn2+ or Mg2+, yet in the presence of (NH4)2SO4, only Mn2+ supported the increase in kinase activity. (3) Ammonium sulfate addition altered the product inhibition pattern of ADP versus angiotensin II, suggesting that an enzyme-angiotensin II-ADP complex can form in the presence of (NH4)2SO4 but not in its absence. (4) The near-maximal rate of self-phosphorylation was not affected by (NH4)2SO4 but the apparent Km (ATP) was greatly increased. From these results, we propose a model for (NH4)2SO4 stimulation of EGF receptor kinase activity in which SO4(2-) interacts directly with the receptor or receptor-Mn(2+)-ATP complex and alters reactant binding and the catalytic efficiency of the tyrosine kinase.