Expression, characterization, and crystallization of the catalytic core of the human insulin receptor protein-tyrosine kinase domain

A surface phospholipase is involved in the migration of plasmodium sporozoites through cells

Plasmodium sporozoites, injected by mosquitoes into the skin of the host, traverse cells during their migration to hepatocytes where they continue their life cycle. The mechanisms used by the parasite to rupture the plasma membrane of the host cells are not known. Here we report the presence of a phospholipase on the surface of Plasmodium berghei sporozoites (P. berghei phospholipase; Pb PL) and demonstrate that it is involved in the establishment of a malaria infection in vivo. Pb PL is highly conserved among the Plasmodium species. The protein is about 750 amino acids, with a predicted signal sequence and a carboxyl terminus that is 32% identical to the vertebrate lecithin:cholesterol acyltransferase, a secreted phospholipase. Pb PL contains a motif characteristic of lipases and a catalytic triad of a serine, aspartate, and histidine that is found in several phospholipases. We have verified its lipase and membrane lytic activity in vitro, using recombinant baculovirus-expressed protein. To study its role in vivo, we have disrupted the P. berghei PL open reading frame and generated mutants in its active site. During an infection through mosquito bite, the infectivity of the knock-out parasites in the liver is decreased by approximately 90%. The prepatent period of the resulting blood infection is 1 day longer as compared with wild type. Further, the mutant sporozoites are impaired in their ability to cross epithelial cell layers. Thus, the Pb PL functions as a lipase to damage cell membranes and facilitates sporozoite passage through cells during their migration from the skin to the bloodstream.

Inhibition of insulin-like growth factor I receptor tyrosine kinase by ethanol

Ethanol inhibits insulin and insulin-like growth factor-I (IGF-I) signaling in a variety of cell types leading to reduced mitogenesis and impaired survival. This effect is associated with inhibition of insulin receptor (IR) and insulin-like growth factor-I receptor (IGF-IR) autophosphorylation, which implicates these receptors as direct targets for ethanol. It was demonstrated previously that ethanol inhibits the autophosphorylation and kinase activity of the purified cytoplasmic tyrosine kinase domain of the IR. We performed computer modeling of the ethanol interaction with the IR and IGF-IR kinases (IRK and IGF-IRK). The analysis predicted binding of alcohols within the hydrophobic pocket of the kinase activation cleft, with stabilization at specific polar residues. Using IGF-IRK purified from baculovirus-infected insect cells, ethanol inhibited peptide substrate phosphorylation by non-phosphorylated IGF-IRK, but had no effect on the autophosphorylated enzyme. In common with the IRK, ethanol inhibited IGF-IRK autophosphorylation. In cerebellar granule neurons, ethanol inhibited autophosphorylation of the apo-IGF-IR, but did not reverse IGF-IR phosphorylation after IGF-I stimulation. In summary, the findings demonstrate direct inhibition of IGF-IR tyrosine kinase by ethanol. The data are consistent with a model wherein ethanol prevents the initial phase of IRK and IGF-IRK activation, by inhibiting the engagement of the kinase activation loop.

Evaluation of active recombinant catalytic domain of human ErbB-2 tyrosine kinase, and suppression of activity by a naturally derived inhibitor, ZH-4B

Human cancers frequently express high levels of ErbB-2 tyrosine kinase, which is associated with aggressive tumor behavior and poor prognosis. ErbB-2 is thus a promising target for cancer therapy. Here we express the catalytic domain of ErbB-2 as a soluble active kinase, and investigate the correlations between its activity and kinase concentration, ATP concentration, substrate concentration and divalent cation type. A simple and effective screening model is established to identify and evaluate potential inhibitors of ErbB-2 kinase. ZH-4B, a naturally derived small molecule compound that potently inhibits ErbB-2 kinase activity with an IC50 value of 2.45+/-0.56 microM, is identified. In SK-OV-3 human ovarian cancer cells and SK-BR-3 human breast carcinoma cells, ZH-4B blocks epidermal growth factor (EGF)-induced phosphorylation of ErbB-2 in a dose-dependent manner. Our data collectively indicate that ZH-4B is a potential novel anti-cancer agent that deserves further investigation.

Inhibition of insulin-like growth factor I receptor autophosphorylation by novel 6-5 ring-fused compounds

The insulin-like growth factor 1 receptor (IGF1R) plays an important role in cell transformation, and it has emerged as a target for anti-cancer drug design. IGF1R is activated by autophosphorylation at three sites in the enzyme activation loop. We describe here a group of 6-5 ring-fused compounds that are the first reported inhibitors selective for the unphosphorylated (0P) form of IGF1R. These compounds do not significantly inhibit the fully activated, triply phosphorylated (3P) form. IGF1R was produced from baculovirus-infected Spodoptera frugiperda (Sf9) cells, and the 0P and 3P forms were purified to homogeneity. We used a continuous spectrophotometric assay to measure inhibition of the 0P and 3P forms. Analysis by native gel electrophoresis confirmed that the step inhibited in the autoactivation process was the transition between the 0P and IP forms of IGF1R. The compounds were also active against IGF1R autophosphorylation in intact Chinese hamster ovary (CHO) cells. Most of the compounds also inhibited the closely related insulin receptor to varying degrees, although some compounds showed selectivity for IGF1R or insulin receptor. This class of compounds could form the basis of design efforts to selectively block the autoinhibited conformation of IGF1R.

Effect of thiol antioxidant on body fat and insulin reactivity

Insulin signaling is enhanced by moderate concentrations of reactive oxygen species (ROS) and suppressed by persistent exposure to ROS. Diabetic patients show abnormally high ROS levels and a decrease in insulin reactivity which is ameliorated by antioxidants, such as N-acetylcysteine (NAC). A similar effect of NAC has not been reported for non-diabetic subjects. We now show that the insulin receptor (IR) kinase is inhibited in cell culture by physiologic concentrations of cysteine. In two double-blind trials involving a total of 140 non-diabetic subjects we found furthermore that NAC increased the HOMA-R index (derived from the fasting insulin and glucose concentrations) in smokers and obese patients, but not in nonobese non-smokers. In obese patients NAC also caused a decrease in glucose tolerance and body fat mass. Simultaneous treatment with creatine, a metabolite utilized by skeletal muscle and brain for the interconversion of ADP and ATP, reversed the NAC-mediated increase in HOMA-R index and the decrease in glucose tolerance without preventing the decrease in body fat. As the obese and hyperlipidemic patients had lower plasma thiol concentrations than the normolipidemic subjects, our results suggest that low thiol levels facilitate the development of obesity. Supplementation of thiols plus creatine may reduce body fat without compromising glucose tolerance.

Insulin receptor knockout mice

To examine the role of the insulin receptor in fuel homeostasis, we and others have carried out genetic ablation studies in mice. Mice lacking insulin receptors are born with normal features, but develop early postnatal diabetes and die of ketoacidosis. In contrast, mice lacking insulin receptors in specific cell types as a result of conditional mutagenesis develop mild metabolic and reproductive abnormalities. These experiments have uncovered novel functions of insulin receptors in tissues such as brain and pancreatic beta-cells. Combined knockout studies of insulin and Igf1 receptors indicate that the insulin receptor also promotes embryonic growth. Experimental crosses of mice with insulin receptor haploinsufficiency have been instrumental to the genetic analysis of insulin action by enabling us to assign specific roles to different insulin receptor substrates and identify novel elements in insulin signaling.

Grb10 inhibits insulin-stimulated insulin receptor substrate (IRS)-phosphatidylinositol 3-kinase/Akt signaling pathway by disrupting the association of IRS-1/IRS-2 with the insulin receptor

Grb10 has been proposed to inhibit or activate insulin signaling, depending on cellular context. We have investigated the mechanism by which full-length hGrb10gamma inhibits signaling through the insulin receptor substrate (IRS) proteins. Overexpression of hGrb10gamma in CHO/IR cells and in differentiated adipocytes significantly reduced insulin-stimulated tyrosine phosphorylation of IRS-1 and IRS-2. Inhibition occurred rapidly and was sustained for 60 min during insulin stimulation. In agreement with inhibited signaling through the IRS/PI 3-kinase pathway, we found hGrb10gamma to both delay and reduce phosphorylation of Akt at Thr(308) and Ser(473) in response to insulin stimulation. Decreased phosphorylation of IRS-1/2 may arise from impaired catalytic activity of the receptor, since hGrb10gamma directly associates with the IR kinase regulatory loop. However, yeast tri-hybrid studies indicated that full-length Grb10 blocks association between IRS proteins and IR, and that this requires the SH2 domain of Grb10. In cells, hGrb10gamma inhibited insulin-stimulated IRS-1 tyrosine phosphorylation in a dose-dependent manner, but did not affect IR catalytic activity toward Tyr(972) in the juxtamembrane region and Tyr(1158/1162/1163) in the regulatory domain. We conclude that binding of hGrb10gamma to IR decreases signaling through the IRS/PI 3-kinase/AKT pathway by physically blocking IRS access to IR.

Practical aspects of overexpressing bacterial secondary membrane transporters for structural studies

Membrane transporter proteins play critical physiological roles in the cell and constitute 5-10% of prokaryotic and eukaryotic genomes. High-resolution structural information is essential for understanding the functional mechanism of these proteins. A prerequisite for structural study is to overexpress such proteins in large quantities. In the last few years, over 20 bacterial membrane transporters were overexpressed at a level of 1 mg/l of culture or higher, most often in Escherichia coli. In this review, we analyzed those factors that affect the quantity and quality of the protein produced, and summarized recent progress in overexpression of membrane transporters from bacterial inner membrane. Rapid progress in genome sequencing provides opportunities for expressing several homologues and orthologues of the target protein simultaneously, while the availability of various expression vectors allows flexible experimental design. Careful optimization of cell culture conditions can drastically improve the expression level and homogeneity of the target protein. New sample preparation techniques for mass spectrometry of membrane proteins have enabled one to identity the rigid protein core, which can be subsequently overexpressed. Size-exclusion chromatography on HPLC has proven to be an efficient method in screening detergent, pH an other conditions required for maintaining the stability and monodispersity of the protein. Such high-quality preparations of membrane transporter proteins will probably lead to successful crystallization and structure determination of these proteins in the next few years.

Crystal structure of the MuSK tyrosine kinase: insights into receptor autoregulation

Muscle-specific kinase (MuSK) is a receptor tyrosine kinase expressed selectively in skeletal muscle. During neuromuscular synapse formation, agrin released from motor neurons stimulates MuSK autophosphorylation in the kinase activation loop and in the juxtamembrane region, leading to clustering of acetylcholine receptors. We have determined the crystal structure of the cytoplasmic domain of unphosphorylated MuSK at 2.05 A resolution. The structure reveals an autoinhibited kinase domain in which the activation loop obstructs ATP and substrate binding. Steady-state kinetic analysis demonstrates that autophosphorylation results in a 200-fold increase in k(cat) and a 10-fold decrease in the K(m) for ATP. These studies provide a molecular basis for understanding the regulation of MuSK catalytic activity and suggest that an additional in vivo component may contribute to regulation via the juxtamembrane region.

Insulin stimulates phosphorylation of the beta 2-adrenergic receptor by the insulin receptor, creating a potent feedback inhibitor of its tyrosine kinase

Insulin counterregulates catecholamine action at several levels, primarily in liver, fat, and adipose tissue. Herein we observe that expression of increased levels of beta(2)-adrenergic receptor increasingly inhibits insulin-stimulated phosphorylation of its primary downstream substrates (IRS-1,2). In Chinese hamster ovary cells, the insulin receptor phosphorylates dominantly Tyr(364) in the C-terminal cytoplasmic domain of the beta-receptor. A Y364A mutant form of the beta(2)-adrenergic, in contrast, loses it ability to inhibit insulin-stimulated phosphorylation of IRS-1,2. Upon phosphorylation, the C-terminal cytoplasmic domain of the beta(2)-adrenergic receptor demonstrates a potent inhibitory feedback action that can block both insulin-stimulated autophosphorylation of the insulin receptor and phosphorylation of IRS-1,2 in NIH mouse 3T3-L1 adipocyte membranes. Studies in vitro with purified insulin receptor and the C-terminal cytoplasmic domain of the beta(2)-adrenergic receptor demonstrate that the tyrosine-phosphorylated beta-receptor domain is a potent counterregulatory inhibitor of the insulin receptor tyrosine kinase.

Free radicals in the physiological control of cell function

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

Multiple activation loop conformations and their regulatory properties in the insulin receptor's kinase domain

Low catalytic efficiency of protein kinases often results from intrasteric inhibition caused by the activation loop blocking the active site. In the insulin receptor's kinase domain, Asp-1161 and Tyr-1162 in the peptide substrate-like sequence of the unphosphorylated activation loop can interact with four invariant residues in the active site: Lys-1085, Asp-1132, Arg-1136, and Gln-1208. Contributions of these six residues to intrasteric inhibition were tested by mutagenesis, and the unphosphorylated kinase domains were characterized. The mutations Q1208S, K1085N, and Y1162F each relieved intrasteric inhibition, increasing catalytic efficiency but without changing the rate-limiting step of the reaction. The mutants R1136Q and D1132N were virtually inactive. Steric accessibility of the active site was ranked by relative changes in iodide quenching of intrinsic fluorescence, and A-loop conformation was ranked by limited tryptic cleavage. Together these ranked the openness of the active site cleft as R1136Q approximately D1132N > or = D1161A > Y1162F approximately K1085N > Q1208S > or = wild-type. These findings demonstrate the importance of specific invariant residues for intrasteric inhibition and show that diverse activation loop conformations can produce similar steady-state kinetic properties. This suggests a broader range of regulatory properties for the activation loop than expected from a simple off-versus-on switch for kinase activation.

Distinct and overlapping functions of insulin and IGF-I receptors

Targeted gene mutations have established distinct, yet overlapping, developmental roles for receptors of the insulin/IGF family. IGF-I receptor mediates IGF-I and IGF-II action on prenatal growth and IGF-I action on postnatal growth. Insulin receptor mediates prenatal growth in response to IGF-II and postnatal metabolism in response to insulin. In rodents, unlike humans, insulin does not participate in embryonic growth until late gestation. The ability of the insulin receptor to act as a bona fide IGF-II-dependent growth promoter is underscored by its rescue of double knockout Igf1r/Igf2r mice. Thus, IGF-II is a true bifunctional ligand that is able to stimulate both insulin and IGF-I receptor signaling, although with different potencies. In contrast, the IGF-II/cation-independent mannose-6-phosphate receptor regulates IGF-II clearance. The growth retardation of mice lacking IGF-I and/or insulin receptors is due to reduced cell number, resulting from decreased proliferation. Evidence from genetically engineered mice does not support the view that insulin and IGF receptors promote cellular differentiation in vivo or that they are required for early embryonic development. The phenotypes of insulin receptor gene mutations in humans and in mice indicate important differences between the developmental roles of insulin and its receptor in the two species.

Role of the PHTH module in protein substrate recognition by Bruton's agammaglobulinemia tyrosine kinase

Defects in Bruton's tyrosine kinase (Btk) are responsible for X chromosome-linked agammaglobulinemia in patients. Mutations in each of the structural domains of Btk have been detected in patients, yet a mechanistic explanation for most of these mutant phenotypes is lacking. To understand the possible role of the unique pleckstrin homology and Tec homology (PHTH) module of Btk, we have compared the enzymatic properties of full-length Btk and a Btk mutant lacking the PHTH module (BtkDeltaPHTH). Here we show that Btk and BtkDeltaPHTH have similar basal catalytic activity but very different abilities to recognize protein substrates. Furthermore, the catalytic domain of Btk is inactive, in contrast to the catalytic domain of the prototypical Src tyrosine kinase that retains full catalytic ability. These data suggest that the PHTH module plays an important role in protein substrate recognition, that Btk and Src likely have different interdomain organizations and regulations, and that alterations in substrate recognition might play a role in X chromosome-linked agammaglobulinemia.

Crystal structure of bisphosphorylated IGF-1 receptor kinase: insight into domain movements upon kinase activation

BACKGROUND

The insulin-like growth-factor-1 (IGF-1) receptor, which is widely expressed in cells that have undergone oncogenic transformation, is emerging as a novel target in cancer therapy. IGF-1-induced receptor activation results in autophosphorylation of cytoplasmic kinase domains and enhances their capability to phosphorylate downstream substrates. Structures of the homologous insulin receptor kinase (IRK) exist in an open, unphosphorylated form and a closed, trisphosphorylated form.

RESULTS

We have determined the 2.1 A crystal structure of the IGF-1 receptor protein tyrosine kinase domain phosphorylated at two tyrosine residues within the activation loop (IGF-1RK2P) and bound to an ATP analog. The ligand is not in a conformation compatible with phosphoryl transfer, and the activation loop is partially disordered. Compared to the homologous insulin receptor kinase, IGF-1RK2P is trapped in a half-closed, previously unobserved conformation. Observed domain movements can be dissected into two orthogonal rotational components.

CONCLUSIONS

Conformational changes upon kinase activation are triggered by the degree of phosphorylation and are crucially dependent on the conformation of the proximal end of the kinase activation loop. This IGF-1RK structure will provide a molecular basis for the design of selective antioncogenic therapeutic agents.

Intrasteric inhibition of ATP binding is not required to prevent unregulated autophosphorylation or signaling by the insulin receptor

Receptor tyrosine kinases may use intrasteric inhibition to suppress autophosphorylation prior to growth factor stimulation. To test this hypothesis we made an Asp1161Ala mutant in the activation loop that relieved intrasteric inhibition of the unphosphorylated insulin receptor (IR) and its recombinant cytoplasmic kinase domain (IRKD) without affecting the activated state. Solution studies with the unphosphorylated mutant IRKD demonstrated conformational changes and greater catalytic efficiency from a 10-fold increase in k(cat) and a 15-fold-lower K(m ATP) although K(m peptide) was unchanged. Kinetic parameters of the autophosphorylated mutant and wild-type kinase domains were virtually identical. The Asp1161Ala mutation increased the rate of in vitro autophosphorylation of the IRKD or IR at low ATP concentrations and in the absence of insulin. However, saturation with ATP (for the IRKD) or the presence of insulin (for the IR) yielded equivalent rates of autophosphorylation for mutant versus wild-type kinases. Despite a biochemically more active kinase domain, the mutant IR expressed in C2C12 myoblasts was not constitutively autophosphorylated. However, it displayed a 2.5-fold-lower 50% effective concentration for insulin stimulation of autophosphorylation and was dephosphorylated more slowly following withdrawal of insulin than wild-type IR. In tests of the regulation of the unphosphorylated basal state, these results demonstrate that neither intrasteric inhibition against ATP binding nor suppression of kinase activity is required to prevent premature autophosphorylation of the IR. Finally, the lower rate of dephosphorylation suggests invariant residues of the activation loop such as Asp1161 may function at multiple junctures in cellular regulation of receptor tyrosine kinases.

The BPS domain of Grb10 inhibits the catalytic activity of the insulin and IGF1 receptors

Grb7, Grb10 and Grb14 comprise a family of adaptor proteins that interact with numerous receptor tyrosine kinases upon receptor activation. Between the pleckstrin homology (PH) domain and the Src homology 2 (SH2) domain of these proteins is a region of approximately 50 residues known as the BPS (between PH and SH2) domain. Here we show, using purified recombinant proteins, that the BPS domain of Grb10 directly inhibits substrate phosphorylation by the activated tyrosine kinase domains of the insulin receptor and the insulin-like growth factor 1 (IGF1) receptor. Although inhibition by the BPS domain is dependent on tyrosine phosphorylation of the kinase activation loop, peptide competition experiments indicate that the BPS domain does not bind directly to phosphotyrosine. These studies provide a molecular mechanism by which Grb10 functions as a negative regulator of insulin- and/or IGF1-mediated signaling.

Crystallographic and solution studies of an activation loop mutant of the insulin receptor tyrosine kinase: insights into kinase mechanism

The tyrosine kinase domain of the insulin receptor is subject to autoinhibition in the unphosphorylated basal state via steric interactions involving the activation loop. A mutation in the activation loop designed to relieve autoinhibition, Asp-1161 --> Ala, substantially increases the ability of the unphosphorylated kinase to bind ATP. The crystal structure of this mutant in complex with an ATP analog has been determined at 2.4-A resolution. The structure shows that the active site is unobstructed, but the end of the activation loop is disordered and therefore the binding site for peptide substrates is not fully formed. In addition, Phe-1151 of the protein kinase-conserved DFG motif, at the beginning of the activation loop, hinders closure of the catalytic cleft and proper positioning of alpha-helix C for catalysis. These results, together with viscometric kinetic measurements, suggest that peptide substrate binding induces a reconfiguration of the unphosphorylated activation loop prior to the catalytic step. The crystallographic and solution studies provide new insights into the mechanism by which the activation loop controls phosphoryl transfer as catalyzed by the insulin receptor.

Protein tyrosine kinase structure and function

Tyrosine phosphorylation is one of the key covalent modifications that occurs in multicellular organisms as a result of intercellular communication during embryogenesis and maintenance of adult tissues. The enzymes that carry out this modification are the protein tyrosine kinases (PTKs), which catalyze the transfer of the phosphate of ATP to tyrosine residues on protein substrates. Phosphorylation of tyrosine residues modulates enzymatic activity and creates binding sites for the recruitment of downstream signaling proteins. Two classes of PTKs are present in cells: the transmembrane receptor PTKs and the nonreceptor PTKs. Because PTKs are critical components of cellular signaling pathways, their catalytic activity is strictly regulated. Over the past several years, high-resolution structural studies of PTKs have provided a molecular basis for understanding the mechanisms by which receptor and nonreceptor PTKs are regulated. This review will highlight the important results that have emerged from these structural studies.

Autophosphorylation of the insulin-like growth factor I receptor cytoplasmic domain

The cytoplasmic domain of the beta subunit of the insulin-like growth factor I receptor (amino acids 936-1337) was overexpressed in Sf9 insect cells using a baculovirus expression system, and the 6-His tagged receptor was purified by metal-affinity chromatography. Autophosphorylation of the receptor was concentration dependent, consistent with a trans phosphorylation mechanism. Phosphoamino acid analysis of the autophosphorylated receptor showed predominantly phosphotyrosine, but phosphoserine and phosphothreonine were also present. However, when the receptor was further purified by gel filtration on Sephadex G-100 and then autophosphorylated, phosphoamino acid analysis showed only phosphotyrosine. We conclude that the IGF-I receptor tyrosine kinase is not a dual-specificity kinase and that autophosphorylation of the beta subunit is by a trans mechanism.

Probing the catalytic mechanism of the insulin receptor kinase with a tetrafluorotyrosine-containing peptide substrate

The interaction of a synthetic tetrafluorotyrosyl peptide substrate with the activated tyrosine kinase domain of the insulin receptor was studied by steady-state kinetics and x-ray crystallography. The pH-rate profiles indicate that the neutral phenol, rather than the chemically more reactive phenoxide ion, is required for enzyme-catalyzed phosphorylation. The pK(a) of the tetrafluorotyrosyl hydroxyl is elevated 2 pH units on the enzyme compared with solution, whereas the phenoxide anion species behaves as a weak competitive inhibitor of the tyrosine kinase. A structure of the binary enzyme-substrate complex shows the tetrafluorotyrosyl OH group at hydrogen bonding distances from the side chains of Asp(1132) and Arg(1136), consistent with elevation of the pK(a). These findings strongly support a reaction mechanism favoring a dissociative transition state.

Autophosphorylation of the two C-terminal tyrosine residues Tyr1316 and Tyr1322 modulates the activity of the insulin receptor kinase in vitro

Previously, several studies have demonstrated that autophosphorylation of the C-terminal tyrosine residues (Tyr1316 and Tyr1322) affects the signaling properties of the insulin receptor in vivo. To assess the biochemical consequences of the C-terminal phosphorylation in vitro, we have constructed, purified and characterized 45 kDa soluble insulin receptor kinase domains (IRKD), either with (IRKD) or without (IRKD-Y2F) the two C-terminal tyrosine phosphorylation sites, respectively. According to HPLC phosphopeptide mapping, autophosphorylation of the three tyrosines in the activation loop of the IRKD-Y2F kinase (Tyr1146, Tyr1150, and Tyr1151) was not affected by the mutation. In addition, the Y2F mutation did not significantly change the Km values for exogenous substrates. However, the mutation in IRKD-Y2F resulted in a decrease in the maximum velocities of the phosphotransferase reaction in substrate phosphorylation reactions. Moreover, the exchange of the tyrosines in IRKD-Y2F led to an increase in the apparent Km values for ATP, suggesting a cross-talk of the C-terminus and the catalytic domain of the enzyme. In addition, as judged by size exclusion chromatography, conformational changes of the enzyme following autophosphorylation were abolished by the removal of the two C-terminal tyrosines. These data suggest a regulatory role of the two C-terminal phosphorylation sites in the phosphotransferase activity of the insulin receptor.

Conformational changes in the activation loop of the insulin receptor's kinase domain

Low catalytic efficiency of basal-state protein kinases often depends on activation loop residues blocking substrate access to the catalytic cleft. Using the recombinant soluble form of the insulin receptor's kinase domain (IRKD) in its unphosphorylated state, activation loop conformation was analyzed by limited proteolysis. The rate of activation loop cleavage by trypsin is slow in the apo-IRKD. Bound Mg-adenine nucleoside di- and triphosphates increased the cleavage rate with half-maximal effects observed at 0.4-0.9 mM nucleotide. Adenosine monophosphate at concentrations up to 10 mM was not bound appreciably by the IRKD and had virtually no impact on activation loop cleavage. Amino-terminal and carboxy-terminal core-flanking regions of the IRKD had no statistically significant impact on the ligand-dependent or -independent activation loop cleavages. Furthermore, the core-flanking regions did not change the inherent conformational stability of the active site or the global stability of the IRKD, as determined by guanidinium chloride-induced denaturation. These measurements indicate that the intrasterically inhibitory conformation encompasses > or =90% of the ligand-free basal state kinase. However, normal intracellular concentrations of Mg-adenine nucleotides, which are in the millimolar range, would favor a basal-state conformation of the activation loop that is more accessible.

Phosphorylation of the insulin receptor kinase by phosphocreatine in combination with hydrogen peroxide: the structural basis of redox priming

Signaling by insulin requires autophosphorylation of the insulin receptor kinase (IRK) at Tyr1158, Tyr1162, and Tyr1163. Earlier experiments with (32)P-gamma-ATP indicated that the nonphosphorylated IRK (IRK-0P) is relatively inactive, and crystallographic data indicated that the ATP binding site of IRK-0P is blocked by its activation loop. We now show that phosphocreatine (PCr) in combination with hydrogen peroxide serves as an alternative phosphate donor and that ATP and PCr use distinct binding sites. Whereas phosphorylation of the IRK by ATP is inhibited by the nonhydrolyzable competitor adenylyl-imidodiphosphate, phosphorylation by PCr is enhanced. The IRK mutant Tyr1158Phe showed no phosphorylation with PCr but almost normal phosphorylation with ATP, whereas Tyr1162Phe was phosphorylated well with PCr but less then normal with ATP. 3-Dimensional models of IRK-0P revealed that the conversion of any of the four cysteine residues 1056, 1138, 1234, and 1245 into sulfenic acid produces structural changes that bring Tyr1158 into close contact with Asp1083 and render the well-known catalytic site at Asp1132 and Tyr1162 accessible from a direction that differs from the known ATP binding site. The mutant Cys1138Ala, in contrast, showed relatively inaccessible catalytic sites and weak catalytic activity in functional experiments. Taken together, these findings indicate that 'redox priming' of the IRK facilitates its autophosphorylation by PCr in the activation loop.

Expression, purification, and characterization of the cytoplasmic domain of the human IGF-1 receptor using a baculovirus expression system

The cytoplasmatic domain of the beta-subunit of the human IGF-1 receptor (residues 929-1337) has been overexpressed in insect cells using the baculovirus expression system. Synthesis of the soluble protein (IGFK, M(r) 46 kDa) in Spodoptera frugiperda (Sf9) cells was detected 24 h after infection and maximal accumulation was achieved 40-48 h postinfection. Rapid purification to near homogeneity (>/=95% pure protein) was accomplished by sequential chromatography on Resource-Q and phenyl-Sepharose with a specific activity of 142 nmol/min/mg using poly[Glu:Tyr] as substrate. The purified IGFK showed a preference for Mn(2+) ions and a linear incorporation of (32)P from [gamma-(32)P]ATP over a 20-fold dilution of the protein and was stimulated 20-fold by the polycation poly-L-lysine. Interestingly, the kinase autophosphorylated on tyrosine and serine residues. In contrast, a kinase-negative mutant, IGFK-K1003A, did not undergo phosphorylation on tyrosine or serine residues, respectively, suggesting that IGF-1 receptor kinase is a dual specific kinase.

Structural analysis of receptor tyrosine kinases

Receptor tyrosine kinases (RTKs) are single-pass transmembrane receptors that possess intrinsic cytoplasmic enzymatic activity, catalyzing the transfer of the gamma-phosphate of ATP to tyrosine residues in protein substrates. RTKs are essential components of signal transduction pathways that affect cell proliferation, differentiation, migration and metabolism. Included in this large protein family are the insulin receptor and the receptors for growth factors such as epidermal growth factor, fibroblast growth factor and vascular endothelial growth factor. Receptor activation occurs through ligand binding, which facilitates receptor dimerization and autophosphorylation of specific tyrosine residues in the cytoplasmic portion. The phosphotyrosine residues either enhance receptor catalytic activity or provide docking sites for downstream signaling proteins. Over the past several years, structural studies employing X-ray crystallography have advanced our understanding of the molecular mechanisms by which RTKs recognize their ligands and are activated by dimerization and tyrosine autophosphorylation. This review will highlight the key results that have emerged from these structural studies.

Crystal structure of the kinase domain of human vascular endothelial growth factor receptor 2: a key enzyme in angiogenesis

BACKGROUND

Angiogenesis is involved in tumor growth, macular degeneration, retinopathy and other diseases. Vascular endothelial growth factor (VEGF) stimulates angiogenesis by binding to specific receptors (VEGFRs) on the surface of vascular endothelial cells. VEGFRs are receptor tyrosine kinases that, like the platelet-derived growth factor receptors (PDGFRs), contain a large insert within the kinase domain.

RESULTS

We report here the generation, kinetic characterization, and 2.4 A crystal structure of the catalytic kinase domain of VEGF receptor 2 (VEGFR2). This protein construct, which lacks 50 central residues of the 68-residue kinase insert domain (KID), has comparable kinase activity to constructs containing the entire KID. The crystal structure, determined in an unliganded phosphorylated state, reveals an overall fold and catalytic residue positions similar to those observed in other tyrosine-kinase structures. The kinase activation loop, autophosphorylated on Y1059 prior to crystallization, is mostly disordered; however, a portion of it occupies a position inhibitory to substrate binding. The ends of the KID form a beta-like structure, not observed in other known tyrosine kinase structures, that packs near to the kinase C terminus.

CONCLUSIONS

The majority of the VEGFR2 KID residues are not necessary for kinase activity. The unique structure observed for the ends of the KID may also occur in other PDGFR family members and may serve to properly orient the KID for signal transduction. This VEGFR2 kinase structure provides a target for design of selective anti-angiogenic therapeutic agents.

Vascular endothelial growth factor receptor KDR tyrosine kinase activity is increased by autophosphorylation of two activation loop tyrosine residues

Vascular endothelial growth factor is an important physiological regulator of angiogenesis. The function of this endothelial cell selective growth factor is mediated by two homologous tyrosine kinase receptors, fms-like tyrosine kinase 1 (Flt-1) and kinase domain receptor (KDR). Although the functional consequence of vascular endothelial growth factor binding to the Flt-1 receptor is not fully understood, it is well established that mitogenic signaling is mediated by KDR. Upon sequencing several independent cDNA clones spanning the cytoplasmic region of human KDR, we identified and confirmed the identity of a functionally required valine at position 848 in the ATP binding site, rather than the previously reported glutamic acid residue, which corresponds to an inactive tyrosine kinase. The cytoplasmic domain of recombinant native KDR, expressed as a glutathione S-transferase fusion protein, can undergo autophosphorylation in the presence of ATP. In addition, the kinase activity can be substantially increased by autophosphorylation at physiologic ATP concentrations. Mutation analysis indicates that both tyrosine residues 1054 and 1059 are required for activation, which is a consequence of an increased affinity for both ATP and the peptide substrate and has no effect on kcat, the intrinsic catalytic activity of the enzyme. KDR kinase catalyzes phosphotransfer by formation of a ternary complex with ATP and the peptide substrate. We demonstrate that tyrosine kinase antagonists can preferentially inhibit either the unactivated or activated form of the enzyme.

ATP-dependent desensitization of insulin binding and tyrosine kinase activity of the insulin receptor kinase. The role of endosomal acidification

Incubating endosomes with ATP decreased binding of 125I-insulin but not 125I-labeled human growth hormone. Increasing ATP concentrations from 0.1 to 1 mM increased beta-subunit tyrosine phosphorylation and insulin receptor kinase (IRK) activity assayed after partial purification. At higher (5 mM) ATP concentrations beta-subunit tyrosine phosphorylation and IRK activity were markedly decreased. This was not observed with nonhydrolyzable analogs of ATP, nor with plasma membrane IRK, nor with endosomal epidermal growth factor receptor kinase autophosphorylation. The inhibition of endosomal IRK tyrosine phosphorylation and activity was completely reversed by bafilomycin A1, indicating a role for endosomal proton pump(s). The inhibition of IRK was not due to serine/threonine phosphorylation nor was it influenced by the inhibition of phosphotyrosyl phosphatase using bisperoxo(1,10-phenanthroline)oxovanadate anion. Prior phosphorylation of the beta-subunit with 1 mM ATP did not prevent the inhibition of IRK activity on incubating with 5 mM ATP. To evaluate conformational change we incubated endosomes with dithiothreitol (DTT) followed by SDS-polyacrylamide gel electrophoresis under nonreducing conditions. Without DTT the predominant species of IRK observed was alpha2 beta2. With DTT the alpha beta dimer predominated but on co-incubation with 5 mM ATP the alpha2 beta2 form predominated. Thus, ATP-dependent endosomal acidification contributes to the termination of transmembrane signaling by, among other processes, effecting a deactivating conformational change of the IRK.

Detection of protein tyrosine kinase activity using a high-capacity streptavidin-coated membrane and optimized biotinylated peptide substrates

A protein tyrosine kinase (PTK) assay system is described that uses a series of optimized biotinylated peptide substrates in conjunction with a streptavidin-coated matrix (SAM(2)) biotin capture membrane. The SAM(2) biotin capture membrane provides low backgrounds and high linear binding capacity (up to approximately 3.6 nmol of biotinylated PTK peptide/cm(2)), resulting in high signal-to-noise ratios and greater reproducibility. Capture of the phosphorylated peptide substrates onto the SAM(2) membrane is rapid and occurs independent of the amino acid sequence of the peptide, thereby overcoming difficulties commonly encountered with other methodologies. Two broad-specificity biotinylated PTK peptide substrates were identified with optimum kinetic properties, allowing members from eight distinct classes of enzymes, including transmembrane (epidermal growth factor receptor (EGFR), fibroblast growth factor receptor, insulin receptor, and platelet-derived growth factor receptor) and cytoplasmic (p43(abl), p56(lck), p60(src), and p93(fes)) PTKs, to be analyzed. A third biotinylated peptide substrate, shown to be highly selective for the EGFR, was used to illustrate the versatility of this system for both broad specificity and highly selective detection of PTK activity. The ability to accurately detect activity under optimum conditions and with crude cell extract samples, including kinetic analysis and with enzyme detection limits in the low femtomole range, supports the utility of this assay system for studying PTK enzymes.

Crystal structure of the activated insulin receptor tyrosine kinase in complex with peptide substrate and ATP analog

The crystal structure of the phosphorylated, activated form of the insulin receptor tyrosine kinase in complex with a peptide substrate and an ATP analog has been determined at 1.9 A resolution. The activation loop (A-loop) of the kinase undergoes a major conformational change upon autophosphorylation of Tyr1158, Tyr1162 and Tyr1163 within the loop, resulting in unrestricted access of ATP and protein substrates to the kinase active site. Phosphorylated Tyr1163 (pTyr1163) is the key phosphotyrosine in stabilizing the conformation of the tris-phosphorylated A-loop, whereas pTyr1158 is completely solvent-exposed, suggesting an availability for interaction with downstream signaling proteins. The YMXM-containing peptide substrate binds as a short anti-parallel beta-strand to the C-terminal end of the A-loop, with the methionine side chains occupying two hydrophobic pockets on the C-terminal lobe of the kinase. The structure thus reveals the molecular basis for insulin receptor activation via autophosphorylation, and provides insights into tyrosine kinase substrate specificity and the mechanism of phosphotransfer.

Autophosphorylation of activation loop tyrosines regulates signaling by the TRK nerve growth factor receptor

Many receptor tyrosine kinases possess an "activation loop" containing three similarly placed tyrosine autophosphorylation sites. To examine their roles in the TRK NGF receptor, these residues (Tyr-670, Tyr-674, and Tyr-675) were mutated singly and in all combinations to phenylalanine and stably expressed in Trk-deficient PC12nnr5 cells. All mutant receptors showed significantly diminished nerve growth factor (NGF)-stimulated autophosphorylation, indicating impaired catalytic activity. NGF-induced neurite outgrowth exhibited dose-responsive behavior when transfectants were compared by relative receptor expression and exhibited a functional hierarchy: wild type > Y670F >/= Y674F >> Y675F >/= YY670/674FF = YY670/675FF >> YY674/675FF > YYY670/674/675FFF. NGF-induced tyrosine phosphorylation of Shc, ERKs, and SNT and immediate early gene inductions generally paralleled neurogenic potential. However, activation of phosphatidylinositol 3'-kinase and tyrosine phosphorylation of phospholipase Cgamma-1 was essentially abolished. The latter effect appears due to selective inability of the mutated TRKs to autophosphorylate the tyrosine residue (Tyr-785) required for binding phospholipase Cgamma-1 and indicates that the "activation loop" tyrosines participate in NGF-dependent changes in receptor conformation. Our findings stress the importance that expression levels play in assessing the consequences of receptor mutations and that all three activation loop tyrosines have roles regulating both overall and specific NGF-mediated signaling through TRK.

Biochemical characterization of the protein tyrosine kinase homology domain of the ErbB3 (HER3) receptor protein

The putative protein tyrosine kinase domain (TKD) of the ErbB3 (HER3) receptor protein was generated as a histidine-tagged recombinant protein (hisTKD-B3) and characterized enzymologically. CD spectroscopy indicated that the hisTKD-B3 protein assumed a native conformation with a secondary structure similar to that of the epidermal growth factor (EGF) receptor TKD. However, when compared with the EGF receptor-derived protein, hisTKD-B3 exhibited negligible intrinsic protein tyrosine kinase activity. Immune complex kinase assays of full-length ErbB3 proteins also yielded no evidence of catalytic activity. A fluorescence assay previously used to characterize the nucleotide-binding properties of the EGF receptor indicated that the ErbB3 protein was unable to bind nucleotide. The hisTKD-B3 protein was subsequently found to be an excellent substrate for the EGF receptor protein tyrosine kinase, which suggested that in vivo phosphorylation of ErbB3 in response to EGF could be attributed to a direct cross-phosphorylation by the EGF receptor protein tyrosine kinase.

Mechanism of activation for Zap-70 catalytic activity

There is a growing body of evidence, including data from human genetic and T-cell receptor function studies, which implicate a zeta-associated protein of M(r) 70,000 (Zap-70) as a critical protein tyrosine kinase in T-cell activation and development. During T-cell activation, Zap-70 becomes associated via its src homology type 2 (SH2) domains with tyrosine-phosphorylated immune-receptor tyrosine activating motif (ITAM) sequences in the cytoplasmic zeta chain of the T-cell receptor. An intriguing conundrum is how Zap-70 is catalytically activated for downstream phosphorylation events. To address this question, we have used purified Zap-70, tyrosine phosphorylated glutathione S-transferase (GST)-Zeta, and GST-Zeta-1 cytoplasmic domains, and various forms of ITAM-containing peptides to see what effect binding of zeta had upon Zap-70 tyrosine kinase activity. The catalytic activity of Zap-70 with respect to autophosphorylation increased approximately 5-fold in the presence of 125 nM phosphorylated GST-Zeta or GST-Zeta-1 cytoplasmic domain. A 20-fold activity increase was observed for phosphorylation of an exogenous substrate. Both activity increases showed a GST-Zeta concentration dependence. The increase in activity was not produced with nonphosphorylated GST-Zeta, phosphorylated zeta, or phosphorylated ITAM-containing peptides. The increase in Zap-70 activity was SH2 mediated and was inhibited by phenylphosphate, Zap-70 SH2, and an antibody specific for Zap-70 SH2 domains. Since GST-Zeta and GST-Zeta-1 exist as dimers, the data suggest Zap-70 is activated upon binding a dimeric form of phosphorylated zeta and not by peptide fragments containing a single phosphorylated ITAM. Taken together, these data indicate that the catalytic activity of Zap-70 is most likely activated by a trans-phosphorylation mechanism.

Phosphotyrosine specifies the phosphorylation by protein kinase CK2 of a peptide reproducing the activation loop of the insulin receptor protein tyrosine kinase

Protein kinase CK2 is a ubiquitous Ser/Thr-specific protein kinase responsible for the phosphorylation of many proteins implicated in signal transduction. It phosphorylates both threonyl and seryl residue(s) of the insulin receptor beta-subunit. In this study, a series of peptides, reproducing all the threonyl sites of the intracellular domain of the insulin receptor that display the consensus sequence for CK2, has been synthesized and used as substrate for purified rat liver CK2. The only peptide readily phosphorylated is the one reproducing the activation loop of the insulin receptor (EIYET1160DYYA), including three tyrosines (Y1158, Y1162 and Y1163) whose phosphorylation through an intermolecular autocatalytic process promotes the activation of the receptor kinase. The phosphorylation efficiency of T1160 is increased almost 20-fold if these three tyrosines are previously phosphorylated. By using variably phosphorylated peptides, the tyrosine mainly responsible for such a hierarchical phosphorylation process has been identified as Y1163. It can be concluded, from these data, that T1160 situated in the activation loop of the insulin receptor, represents an excellent target for CK2, its phosphorylation being triggered by the previous autophosphorylation of the three tyrosyl residues surrounding it, with special reference to Y1163. These data are consistent with the implication of CK2 in the regulation of the activation process of the insulin receptor protein tyrosine kinase.

Effect of tyrosine mutations on the kinase activity and transforming potential of an oncogenic human insulin-like growth factor I receptor

The tyrosines in the cytoplasmic domain of an oncogenic human insulin-like growth factor I receptor (gag-IGFR) were systematically mutated to phenylalanines to investigate the role of those tyrosines in the enzymatic and biological function of the gag-IGFR. Our results indicate that tyrosines 1131, 1135, 1136, and 1221 are important for the receptor protein-tyrosine kinase (PTK) activity. However, mutation of Tyr-1136 only slightly affects the kinase activity but dramatically reduces the transforming ability and overall substrate phosphorylation, in particular, annexin II, which is strongly phosphorylated by the gag-IGFR but not by the Phe-1136 mutant. Single mutation of either Tyr-943 or Tyr-950 resulted in significantly reduced phosphorylation of the receptor but not on its PTK activity or transforming ability. Tyr-950 together with its surrounding sequence is involved in mediating the interaction between the gag-IGFR and insulin receptor substrate 1. Our data also suggest that Tyr-1316 is involved in phosphorylation of phospholipase C-gamma, which is, however, not important for cell transforming activity. Overall, our study has identified several tyrosine residues of IGFR important for its PTK activity and substrate interaction. The transforming potential of the gag-IGFR correlates well with its ability to phosphorylate overall cellular substrates and to activate phosphatidylinositol 3-kinase via insulin receptor substrate 1.

Inhibitors of protein tyrosine kinases

The description in the past year of several novel protein tyrosine kinase inhibitors, which exhibit dramatic improvements in potency and specificity over earlier agents, will be considered a major turning point in the field. These compounds appear to have the necessary pharmacological properties to finally allow clarification of whether suppression of specific tyrosine kinases is of therapeutic benefit in certain disease states.

Activating and inhibitory mutations in adjacent tyrosines in the kinase domain of ZAP-70

ZAP-70 is an 70-kDa protein tyrosine kinase, expressed exclusively in T cells and NK cells, and plays a critical role in mediating T cell activation in response to T cell receptor engagement. The strong correlation between tyrosine phosphorylation of ZAP-70 and its acquisition of increased kinase activity suggests that is is positively regulated by tyrosine phosphorylation. Previously, we identified tyrosines 492 and 493 of ZAP-70 as being sites of in vivo phosphorylation in response to T cell receptor engagement. To determine the role of phosphorylation in regulating ZAP-70 activity, we mutated each of these tyrosines individually to phenylalanine. When expressed in COS cells, Y493F-mutated ZAP-70 demonstrated normal basal kinase activity, but, unlike wild type ZAP-70, could not be activated by tyrosine phosphorylation induced by incubation with pervanadate or by co-expression of constitutively activated Lck. This suggests that Tyr-493 phosphorylation is required for the tyrosine phosphorylation-induced activation of ZAP-70. The Y492F mutation resulted in 4-fold higher basal kinase activity, which could be stimulated further by tyrosine phosphorylation. These results reveal that critical tyrosine residues in the kinase domain of ZAP-70 are important in regulation of its catalytic activity.