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

Mechanoenzymatics of titin kinase

Biological responses to mechanical stress require strain-sensing molecules, whose mechanically induced conformational changes are relayed to signaling cascades mediating changes in cell and tissue properties. In vertebrate muscle, the giant elastic protein titin is involved in strain sensing via its C-terminal kinase domain (TK) at the sarcomeric M-band and contributes to the adaptation of muscle in response to changes in mechanical strain. TK is regulated in a unique dual autoinhibition mechanism by a C-terminal regulatory tail, blocking the ATP binding site, and tyrosine autoinhibition of the catalytic base. For access to the ATP binding site and phosphorylation of the autoinhibitory tyrosine, the C-terminal autoinhibitory tail needs to be removed. Here, we use AFM-based single-molecule force spectroscopy, molecular dynamics simulations, and enzymatics to study the conformational changes during strain-induced activation of human TK. We show that mechanical strain activates ATP binding before unfolding of the structural titin domains, and that TK can thus act as a biological force sensor. Furthermore, we identify the steps in which the autoinhibition of TK is mechanically relieved at low forces, leading to binding of the cosubstrate ATP and priming the enzyme for subsequent autophosphorylation and substrate turnover.

Protein microarray analysis identifies human cellular prion protein interactors

AIMS

To obtain an insight into the function of cellular prion protein (PrPC), we studied PrPC-interacting proteins (PrPIPs) by analysing a protein microarray.

METHODS

We identified 47 novel PrPIPs by probing an array of 5000 human proteins with recombinant human PrPC spanning amino acid residues 23-231 named PR209.

RESULTS

The great majority of 47 PrPIPs were annotated as proteins involved in the recognition of nucleic acids. Coimmunoprecipitation and cell imaging in a transient expression system validated the interaction of PR209 with neuronal PrPIPs, such as FAM64A, HOXA1, PLK3 and MPG. However, the interaction did not generate proteinase K-resistant proteins. KeyMolnet, a bioinformatics tool for analysing molecular interaction on the curated knowledge database, revealed that the complex molecular network of PrPC and PrPIPs has a significant relationship with AKT, JNK and MAPK signalling pathways.

CONCLUSIONS

Protein microarray is a useful tool for systematic screening and comprehensive profiling of the human PrPC interactome. Because the network of PrPC and interactors involves signalling pathways essential for regulation of cell survival, differentiation, proliferation and apoptosis, these observations suggest a logical hypothesis that dysregulation of the PrPC interactome might induce extensive neurodegeneration in prion diseases.

The insulin-like growth factor I receptor is required for Akt activation and suppression of anoikis in cells transformed by the ETV6-NTRK3 chimeric tyrosine kinase

Signaling through the insulin-like growth factor I receptor (IGF-IR) axis is essential for transformation by many dominantly acting oncoproteins. However, the mechanism by which IGF-IR contributes to oncogenesis remains unknown. To examine this, we compared transformation properties of the oncogenic ETV6-NTRK3 (EN) chimeric tyrosine kinase in IGF-IR-null R- mouse embryo fibroblasts with R- cells engineered to reexpress IGF-IR (R+ cells). We previously showed that R- cells expressing EN (R- EN cells) are resistant to transformation but that transformation is restored in R+ cells. We now show that while R- EN cells have intact Ras-extracellular signal-regulated kinase signaling and cell cycle progression, they are defective in phosphatidylinositol-3-kinase (PI3K)-Akt activation and undergo detachment-induced apoptosis (anoikis) under anchorage-independent conditions. In contrast, R+ cells expressing EN (R+ EN cells) suppress anoikis and are fully transformed. The requirement for IGF-IR in R- EN cells is overcome by ectopic expression of either activated Akt or a membrane-targeted form of EN. Moreover, compared to R- EN cells, R+ EN cells show a dramatic increase in membrane localization of insulin receptor substrate 1 (IRS-1) in association with EN. Since EN is known to bind IRS-1 as an adaptor protein, our findings suggest that IGF-IR may function to localize EN/IRS-1 complexes to cell membranes, in turn facilitating PI3K-Akt activation and suppression of anoikis.

Rapid identification of 14-3-3-binding proteins by protein microarray analysis

The 14-3-3 protein family consists of acidic 30-kDa proteins composed of seven isoforms in mammalian cells, expressed abundantly in neurons and glial cells of the central nervous system (CNS). The 14-3-3 isoforms form a dimer that acts as a molecular adaptor interacting with key signaling components involved in cell proliferation, transformation, and apoptosis. Until present, more than 300 proteins have been identified as 14-3-3-binding partners, although most of previous studies focused on a limited range of 14-3-3-interacting proteins. Here, we studied a comprehensive profile of 14-3-3-binding proteins by analyzing a high-density protein microarray using recombinant human 14-3-3 epsilon protein as a probe. Among 1752 proteins immobilized on the microarray, 20 were identified as 14-3-3 interactors, most of which were previously unreported 14-3-3-binding partners. However, 11 known 14-3-3-binding proteins, including keratin 18 (KRT18) and mitogen-activated protein kinase-activated protein kinase 2 (MAPKAPK2), were not identified as a 14-3-3-binding protein. The specific binding to 14-3-3 of EAP30 subunit of ELL complex (EAP30), dead box polypeptide 54 (DDX54), and src homology three (SH3) and cysteine rich domain (STAC) was verified by immunoprecipitation analysis of the recombinant proteins expressed in HEK293 cells. These results suggest that protein microarray is a powerful tool for rapid and comprehensive profiling of 14-3-3-binding proteins.

Nitric oxide decreases IGF-1 receptor function in vitro; glutathione depletion enhances this effect in vivo

Insulin-like growth factor-1 (IGF) helps maintain healthy articular cartilage; however, arthritic cartilage becomes less responsive to the anabolic actions of IGF. We previously showed that high concentrations of nitric oxide (NO) decrease IGF receptor tyrosine phosphorylation and response to IGF in intact chondrocytes. The current studies evaluate direct effects of NO on IGF receptor kinase (IGF-RK) in vitro. NO from S-nitroso-N-acetyl-d,l-penicillamine (SNAP) or 1-hydroxy-2-oxo-3-(N-3-methyl-aminopropyl)-3-methyl-1-triazene (NOC-7) inhibits IGF-RK auto- and substrate phosphorylation in a dose and time dependent manner. There is a linear correlation between inhibition of auto- and substrate phosphorylation (r(2)=0.98). Increasing either dithiothreitol or reduced glutathione (GSH) content of the phosphorylation buffer to protect thiol groups blocks NO inhibition of IGF-RK substrate phosphorylation. Increased S-nitrosylation of cysteines in IGF-RK after exposure to SNAP suggests that NO may react with sulfhydryl groups, form S-nitrosothiols, which may result in functional modifications. NO blockade of IGF-1 stimulated proteoglycan synthesis in intact cells is enhanced when chondrocyte glutathione is depleted. The in vitro system shows that there can be direct effects of NO on IGF-RK that modify receptor function; the intact cell studies suggest that the mechanisms identified in vitro may be important in intact chondrocyte insensitivity to IGF-1 in cells exposed to NO.

14-3-3 binding to the IGF-1 receptor is mediated by serine autophosphorylation

The phosphoserine-binding 14-3-3 proteins have been implicated in playing a role in mitogenic and apoptotic signaling pathways. Binding of 14-3-3 proteins to phosphoserine residues in the C-terminus of the insulin-like growth factor-1 receptor (IGF-1R) has been described to occur in a variety of cell systems, but the kinase responsible for this serine phosphorylation has not been identified yet. Here we present evidence that the isolated dimeric insulin-like growth factor-1 receptor kinase domain (IGFKD) contains a dual specific (i.e. tyrosine/serine) kinase activity that mediates autophosphorylation of C-terminal serine residues in the enzyme. From the total phosphate incorporation of approximately 4 mol per mol kinase subunit, 1 mol accounts for serine phosphate. However, tyrosine autophosphorylation proceeds more rapidly than autophosphorylation of serine residues (t(1/2) approximately 1 min vs. t(1/2) approximately 5 min). Moreover, dot-blot and far-Western analyses reveal that serine autophosphorylation of IGFKD is sufficient to promote binding of 14-3-3 proteins in vitro. The proof that dual kinase activity of IGFKD is necessary and sufficient for 14-3-3 binding was obtained with an inactive kinase mutant that was phosphorylated on serine residues in a stoichiometric reaction with the catalytically active enzyme. Thus, the IGF-1R itself might be responsible for the serine autophosphorylation which leads to recognition of 14-3-3 proteins in vivo.

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.

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.