The first structure of a receptor tyrosine kinase domain: a further step in understanding the molecular basis of insulin action

Crystal structure of the kinase domain of a receptor tyrosine kinase from a choanoflagellate, Monosiga brevicollis

Genomic analysis of the unicellular choanoflagellate, Monosiga brevicollis (MB), revealed the remarkable presence of cell signaling and adhesion protein domains that are characteristically associated with metazoans. Strikingly, receptor tyrosine kinases, one of the most critical elements of signal transduction and communication in metazoans, are present in choanoflagellates. We determined the crystal structure at 1.95 Å resolution of the kinase domain of the M. brevicollis receptor tyrosine kinase C8 (RTKC8, a member of the choanoflagellate receptor tyrosine kinase C family) bound to the kinase inhibitor staurospaurine. The chonanoflagellate kinase domain is closely related in sequence to mammalian tyrosine kinases (~ 40% sequence identity to the human Ephrin kinase domain EphA3) and, as expected, has the canonical protein kinase fold. The kinase is structurally most similar to human Ephrin (EphA5), even though the extracellular sensor domain is completely different from that of Ephrin. The RTKC8 kinase domain is in an active conformation, with two staurosporine molecules bound to the kinase, one at the active site and another at the peptide-substrate binding site. To our knowledge this is the first example of staurospaurine binding in the Aurora A activation segment (AAS). We also show that the RTKC8 kinase domain can phosphorylate tyrosine residues in peptides from its C-terminal tail segment which is presumably the mechanism by which it transmits the extracellular stimuli to alter cellular function.

A novel insulin receptor-signaling platform and its link to insulin resistance and type 2 diabetes

Insulin-induced insulin receptor (IR) tyrosine kinase activation and insulin cell survival responses have been reported to be under the regulation of a membrane associated mammalian neuraminidase-1 (Neu1). The molecular mechanism(s) behind this process is unknown. Here, we uncover a novel Neu1 and matrix metalloproteinase-9 (MMP-9) cross-talk in alliance with neuromedin B G-protein coupled receptor (GPCR), which is essential for insulin-induced IR activation and cellular signaling. Neu1, MMP-9 and neuromedin B GPCR form a complex with IRβ subunit on the cell surface. Oseltamivir phosphate (Tamiflu®), anti-Neu1 antibodies, broad range MMP inhibitors piperazine and galardin (GM6001), MMP-9 specific inhibitor (MMP-9i), and GPCR neuromedin B specific antagonist BIM-23127 dose-dependently inhibited Neu1 activity associated with insulin stimulated rat hepatoma cells (HTCs) that overly express human IRs (HTC-IR). Tamiflu, anti-Neu1 antibodies and MMP-9i attenuated phosphorylation of IRβ and insulin receptor substrate-1 (IRS1) associated with insulin-stimulated cells. Olanzapine, an antipsychotic agent associated with insulin resistance, induced Neu3 sialidase activity in WG544 or 1140F01 human sialidosis fibroblast cells genetically defective in Neu1. Neu3 antagonist 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA) and anti-Neu3 antibodies inhibited sialidase activity associated with olanzapine treated murine Neu4 knockout macrophage cells. Olanzapine attenuated phosphorylation of IGF-R and IRS1 associated with insulin-stimulated human wild-type fibroblast cells. Our findings identify a novel insulin receptor-signaling platform that is critically essential for insulin-induced IRβ tyrosine kinase activation and cellular signaling. Olanzapine-induced Neu3 sialidase activity attenuated insulin-induced IGF-R and IRS1 phosphorylation contributing to insulin resistance.

Properties of small molecules affecting insulin receptor function

Small molecules with insulin mimetic effects and oral availability are of interest for potential substitution of insulin injections in the treatment of diabetes. We have searched databases for compounds capable of mimicking one epitope of the insulin molecule known to be involved in binding to the insulin receptor (IR). This approach identifies thymolphthalein, which is an apparent weak agonist that displaces insulin from its receptor, stimulates auto- and substrate phosphorylation of IR, and potentiates lipogenesis in adipocytes in the presence of submaximal concentrations of insulin. The various effects are observed in the 10(-5)-10(-3) M range of ligand concentration and result in partial insulin activity. Furthermore, analogues of the related phenol red and fluorescein molecules fully displace insulin from the IR ectodomain, however, without insulin agonistic effects. The interactions are further characterized by NMR, UV-vis, and fluorescence spectroscopies. It is shown that both fluorescence and UV-vis changes in the ligand spectra induced by IR fragments occur with Kd values similar to those obtained in the displacement assay. Nevertheless, insulin itself cannot completely abolish binding of the small molecules. Determination of the binding stoichiometry reveals multiple binding sites for ligands of which one overlaps with the insulin binding site on the receptor.

The three dimensional structure of the type I insulin-like growth factor receptor

Ever since the discovery of insulin and its role in the regulation of glucose metabolism, there has been great interest in the molecule itself, the insulin-like growth factors (IGFs), and their receptors (IR and IGF-R). These receptors form a subfamily of tyrosine kinase receptors which are large, transmembrane proteins consisting of several structural domains. Their ectodomains have a similar arrangement of two homologous domains (L1 and L2) separated by a Cys rich region. The C-terminal half of their ectodomains consists of three fibronectin type 3 repeats, and an insert domain that contains the alpha-beta cleavage site. This review summarises the key developments in the understanding of the structure of this family of receptors and their relation to other multidomain proteins. Data presented will include multiple sequence analyses, single molecule electron microscope images of the IGF-1R, insulin receptor (IR), and IR-Fab complexes, and the three dimensional structure of the first three domains of the IGF-1R determined to 2.6 A resolution by x ray crystallography. The L domains each adopt a compact shape consisting of a single stranded, right handed beta-helix. The Cys rich region is composed of eight disulphide bonded modules, seven of which form a rod shaped domain with modules associated in an unusual manner.

Ligand-induced conformational change in the minimized insulin receptor

Within the class of insulin and insulin-like growth factor receptors, detailed information about the molecular recognition event at the hormone-receptor interface is limited by the absence of suitable co-crystals. We describe the use of a biologically active insulin derivative labeled with the NBD fluorophore (B29NBD-insulin) to characterize the mechanism of reversible 1:1 complex formation with a fragment of the insulin receptor ectodomain. The accompanying 40 % increase in the fluorescence quantum yield of the label provides the basis for a dynamic study of the hormone-receptor binding event. Stopped-flow fluorescence experiments show that the kinetics of complex formation are biphasic comprising a bimolecular binding event followed by a conformational change. Displacement with excess unlabeled insulin gave monophasic kinetics of dissociation. The rate data are rationalized in terms of available experiments on mutant receptors and the X-ray structure of a non-binding fragment of the receptor of the homologous insulin-like growth factor (IGF-1).

Members of the insulin receptor family contain three fibronectin type III domains

The insulin receptor is a large transmembrane dimer, comprised of several domains. Detailed 3D structural information is available for the L1-cys-rich-L2 domains in the extracellular region (ectodomain) and for the tyrosine kinase catalytic domain in the cytoplasmic portion of the receptor. In addition, previous sequence analyses have identified two fibronectin type III domains in the C-terminal half of each ectodomain monomer. In this report, evidence is provided to show that a third fibronectin type III module exists between the L2 domain and the two previously described fibronectin type III domains.

A function-structure model for NGF-activated TRK

Mechanisms regulating transit of receptor tyrosine kinases (RTKs) from inactive to active states are incompletely described, but require autophosphorylation of tyrosine(s) within a kinase domain 'activation loop'. Here, we employ functional biological assays with mutated TRK receptors to assess a 'switch' model for RTK activation. In this model: (i) ligand binding stimulates activation loop tyrosine phosphorylation; (ii) these phosphotyrosines form specific charge pairs with nearby basic residues; and (iii) the charge pairs stabilize a functionally active conformation in which the activation loop is restrained from blocking access to the kinase catalytic core. Our findings both support this model and identify residues that form specific charge pairs with each of the three TRK activation loop phosphotyrosines.

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.

Characterization of ligand binding of a soluble human insulin-like growth factor I receptor variant suggests a ligand-induced conformational change

Details of the signal transduction mechanisms of the tyrosine kinase family of growth factor receptors remain elusive. In this work, we describe an extensive study of kinetic and thermodynamic aspects of growth factor binding to a soluble extracellular human insulin-like growth factor-I receptor (sIGF-IR) variant. The extracellular receptor domains were produced fused to an IgG-binding protein domain (Z) in transfected human 293 cells as a correctly processed secreted alpha-beta'-Z dimer. The receptor was purified using IgG affinity chromatography, rendering a pure and homogenous protein in yields from 1 to 5 mg/liter of conditioned cell media. Biosensor technology (BIAcore) was applied to measure the insulin-like growth factor-I (IGF-I), des(1-3)IGF-I, insulin-like growth factor-II, and insulin ligand binding rate constants to the immobilized IGF-IR-Z. The association equilibrium constant, Ka, for the IGF-I interaction is determined to 2.8 x 10(8) M-1 (25 degrees C). Microcalorimetric titrations on IGF-I/IGF-IR-Z were performed at three different temperatures (15, 25, and 37 degrees C) and in two different buffer systems at 25 degrees C. From these measurements, equilibrium constants for the 1:1 (IGF-I:(alpha-beta'-Z)2) receptor complex in solution are deduced to 0.96 x 10(8) M-1 (25 degrees C). The determined heat capacity change for the process is large and negative, -0.51 kcal (K mol)-1. Further, the entropy change (DeltaS) at 25 degrees C is large and negative. Far- and near-UV circular dichroism measurements display significant changes over the entire wavelength range upon binding of IGF-I to IGF-IR-Z. These data are all consistent with a significant change in structure of the system upon IGF-I binding.

Bound to activate: conformational consequences of cyclin binding to CDK2

The cyclin-dependent kinases (CDKs) are among the most highly regulated enzymes in the protein-kinase family. The crystal structures of cyclin A and the CDK2-cyclin A complex spectacularly reveal the atomic basis for regulation of these enzymes and provide a template for understanding the function and regulation of other members of the CDK family.

Protein kinases--structure and function

The solution of crystal structures from half a dozen protein kinases during the last four years in different laboratories has deepened our understanding of the catalysis and regulation of this enzyme class, and given a vigorous impetus to the whole field. Due to the great degree of sequence conservation among protein kinases the informational yield with every new structure is high, as each is a representative of the enzyme family in general and most often of a subclass in particular. This review will focus on the active site structure of cAMP-dependent protein kinase (cAPK) with special regard to two new crystal structures; one of an active protein kinase CK1*, which may represent an as yet unsolved step in the kinetic pathway, and the other of the insulin receptor kinase domain, the first structure of a tyrosine kinase.

Decreased phosphorylation of mutant insulin receptor by protein kinase C and protein kinase A

We have recently reported that the Arg1152-->Gln insulin receptor mutation (QK single mutant) alters a conserved motif (RK motif) immediately next to the key tyrosine phosphorylation sites (Tyr1146, Tyr1150, Tyr1151) of the receptor and constitutively activates its kinase and metabolic signaling. To investigate further the function of the RK motif, we have expressed two additional mutant insulin receptors: a single mutant, in which the second basic residue in the RK motif (Lys1153) was substituted (RA mutant); and a double mutant, in which both the Arg and the Lys residues were replaced with noncharged amino acids (QA mutant). As compared with the transfected wild-type receptors (WT), both the single and the double mutant receptors were normally synthetized and transported to the plasma membrane and bound insulin normally. Whereas the double mutant receptor exhibited preserved insulin-dependent autophosphorylation, kinase activity, and 2-deoxyglucose uptake, all of these functions were grossly impaired in the two single mutant receptors. Two-dimensional analysis of tryptic phosphopeptides from receptor beta-subunits revealed that decreased autophosphorylation of the single mutant receptors mainly involved regulatory Tyr1150,1151 and carboxyl-terminal Tyr1316,1322. At variance with the insulin-stimulated, insulin-independent tyrosine kinase activity toward poly(Glu-Tyr) 4:1 was increased 3-fold in both the double and the single mutants. All mutant receptors induced a 2-fold increase in basal 2-deoxyglucose uptake in NIH-3T3 cells. Treatment of WT transfected cells with 12-O-tetradecanoyl-phorbol-13-acetate or 8-bromo-cAMP increased insulin receptor phosphorylation by 3-fold. No phosphorylation was observed in cells expressing the two single or the double mutant receptor. Consistently, purified preparations of PKC and PKA phosphorylated the WT but not the mutant receptors in vitro. A 17-amino acid synthetic peptide encoding the receptor sequence surrounding the RK motif inhibited phosphorylation of WT insulin receptors by both protein kinases A and C. A mutant peptide in which the RK sequence was replaced by QK (to mimic the mutation in the QK receptor) exhibited no inhibitory effect. Thus, the RK insulin receptor motif is required for insulin receptor phosphorylation by protein kinases C and A and may modulate insulin-independent receptor activity. The RK motif may also have an important structural role in allowing normal insulin regulation of the kinase.