The tyrosine phosphorylated carboxyterminus of the EGF receptor is a binding site for GAP and PLC-gamma

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.

Epidermal growth factor receptor: elements of intracellular communication

While EGF has an important function in cell growth regulation, the molecular mechanisms by which intracellular signal connect the EGF: receptor complex on the plasma membrane with the initiation of DNA synthesis and mitogenesis is not well understood. The discovery that rasGAP, PI-3 kinase and PLC-gamma 1 are substrates for the EGF receptor tyrosine kinase has provided a beginning in understanding the biochemistry underlying growth factor receptor transduction.

Antigen Responsive Antibody–Receptor Kinase Chimera

We have constructed chimeric receptors, combining murine IgM and the cytoplasmic portion of human epidermal growth factor receptor (EGFR), with the aim of developing a novel immunosensor with antigen-dependent phosphorylation activity. When intact IgM was used, the chimeric receptor showed both antigen binding and protein tyrosine kinase activity, but the kinase activity was constitutive and independent of antigen binding. However, with IgM lacking the CH2 domain, the autophosphorylation activity increased with increasing concentrations of anti-IgM or hapten-BSA conjugate. Monovalent hapten could not induce phosphorylation but inhibited stimulation by hapten-conjugated BSA.

Tyr721 regulates specific binding of the CSF-1 receptor kinase insert to PI 3'-kinase SH2 domains: a model for SH2-mediated receptor-target interactions

Efficient binding of active phosphatidylinositol (PI) 3'-kinase to the autophosphorylated macrophage colony stimulating factor receptor (CSF-1R) requires the noncatalytic kinase insert (KI) region of the receptor. To test whether this region could function independently to bind PI 3'-kinase, the isolated CSF-1R KI was expressed in Escherichia coli, and was inducibly phosphorylated on tyrosine. The tyrosine phosphorylated form of the CSF-1R KI bound PI 3'-kinase in vitro, whereas the unphosphorylated form had no binding activity. The p85 alpha subunit of PI 3'-kinase contains two Src homology (SH)2 domains, which are implicated in the interactions of signalling proteins with activated receptors. Bacterially expressed p85 alpha SH2 domains complexed in vitro with the tyrosine phosphorylated CSF-1R KI. Binding of the CSF-1R KI to PI 3'-kinase activity, and to the p85 alpha SH2 domains, required phosphorylation of Tyr721 within the KI domain, but was independent of phosphorylation at Tyr697 and Tyr706. Tyr721 was also critical for the association of activated CSF-1R with PI 3'-kinase in mammalian cells. Complex formation between the CSF-1R and PI 3'-kinase can therefore be reconstructed in vitro in a specific interaction involving the phosphorylated receptor KI and the SH2 domains of p85 alpha.

Tyrosine phosphorylation of vav proto-oncogene product containing SH2 domain and transcription factor motifs

Activation of receptor-linked and cytoplasmic protein tyrosine kinases is crucial in the control of normal and abnormal cell growth and differentiation. Some substrates of protein tyrosine kinases such as phospholipase C gamma and ras GTPase-activating protein (GAP) contain sequences homologous to the src protein domains SH2 and SH3 (refs 3-9). The proto-oncogene vav is expressed in haematopoietic cells and its product Vav contains sequence motifs commonly found in transcription factors, such as helix-loop-helix, leucine-zipper and zinc-finger motifs and nuclear localization signals, as well as a single SH2 and two SH3 domains. Here we show that stimulation of T-cell antigen receptor on normal human peripheral blood lymphocytes or on human leukaemic T cells, and the crosslinking of IgE receptors on rat basophilic leukaemia cells, both promote the phosphorylation of tyrosine residues in Vav. Moreover, activation of the receptor for epidermal growth factor leads to marked tyrosine phosphorylation of Vav in cells transiently expressing vav, and Vav associates with the receptor through its SH2 domain. We propose that vav encodes a new class of substrates whose tyrosine phosphorylation may provide a mechanism for direct signal transduction linking receptors at the cell surface to transcriptional control.

Tyrosine kinases and their interactions with signalling proteins

Recent progress has been made in identifying signal transduction pathways controlled by receptor protein-tyrosine kinases. The receptors for nerve growth factor and hepatocyte growth factor have been identified as the Trk and Met tyrosine kinases. The stimulation of intracellular signal transduction pathways by activated receptors appears to involve the association of SH2-containing cytoplasmic signalling proteins with autophosphorylated receptors.

Inducible expression of human phospholipase C-gamma 2 and its activation by platelet-derived growth factor B-chain homodimer and platelet-derived growth factor A-chain homodimer in transfected NIH 3T3 fibroblasts

Growth factors such as platelet-derived growth factor (PDGF) have been shown to activate phospholipase C-gamma 1 (PLC-gamma 1). We have overexpressed the human PLC-gamma 2 (hPLC-gamma 2) cDNA in murine NIH 3T3 fibroblasts using the interferon-type-I inducible murine Mx promoter. Northern blot analysis revealed an induction of hPLC-gamma 2 mRNA by interferon (IFN) alpha of about 25-fold as compared to the uninduced transcript level. Western blot analysis with anti(bovine PLC-gamma 2) antiserum showed increased hPLC-gamma 2 protein levels in hPLC-gamma 2 transfected cells. Induction with IFN alpha resulted only in a slight further increase. After labelling the cells with [35S]methionine an increase of radioactive label in a protein migrating at 148 kDa could be detected in IFN-alpha-stimulated, hPLC-gamma 2 overexpressing cells. PLC activity in homogenates from hPLC-gamma 2 overexpressing cells was increased as compared to control cells transfected with the vector lacking the hPLC-gamma 2 cDNA insert. There was no difference between in vitro PLC activity in homogenates from PDGF B-chain homodimer (BB) treated and untreated cells. PLC activity was mainly present in the soluble fraction. After incubation of hPLC-gamma 2 overexpressing cells with IFN alpha, the in vitro activity of PLC increased significantly in the soluble fraction. Stimulation with PDGF BB increased inositol phosphate production about 3.5-fold in control cells and about 10-fold in hPLC-gamma 2 overexpressing cells. PDGF A-chain homodimer (AA) showed slightly smaller effects. These results demonstrate that human PLC-gamma 2 can be expressed functionally in murine NIH 3T3 fibroblasts and can be activated by both murine PDGF receptors, alpha and beta type.

GAP domains responsible for ras p21-dependent inhibition of muscarinic atrial K+ channel currents

The interaction between the low molecular weight G protein ras p21 and a guanosine triphosphatase activating protein (GAP) uncouples a heterotrimeric G protein (Gk) from muscarinic receptors. Through the use of isolated atrial cell membranes and genetically engineered GAP deletion mutants, the src homology regions (SH2-SH3) at the amino terminus of GAP have been identified as the domains responsible for this effect. Deletion of the domain required to stimulate the guanosine triphosphatase activity of ras p21 relieves the requirement for ras p21 in this system. A model is presented that suggests that ras p21 induces a conformational change in GAP, which allows the SH2-SH3 regions of GAP to function.

Insulin Signal Transduction: The Role of Protein Phosphorylation

Recent evidence suggests that the mechanism of insulin action depends in part on protein phosphorylation on tyrosine residues. A cascade of phosphorylation/dephosphorylation reactions is proposed to modulate multiple enzymes involved in metabolism, protein synthesis, and cell growth. Direct evidence is presented for the phosphorylation of myelin basic protein and microtubule-associated protein 2 on tyrosine residues by the insulin receptor.

Regulation of phospholipase C isozymes

Phosphatidylinositol bisphosphate hydrolysis is an immediate response to many hormones, including growth factors. The hydrolysis of phosphatidylinositol bisphosphate is catalyzed by phosphatidylinositol-specific phospholipase C. A number of phospholipase C isozymes have been identified. Different isozymes are activated by different receptor classes. This review will summarize the different isozymes of phospholipase C, and the current knowledge of the mechanisms by which phospholipase C activity is modulated by growth factors.

Highly conserved eight amino acid sequence in SH2 is important for recognition of phosphotyrosine site

Src homology region 2(SH2) has been demonstrated to recognize phosphotyrosine site. To clarify the precise mechanism of the recognition, we developed in vitro binding assay system using EGF receptor and SH2/SH3 region of phospholipase C(PLC) gamma 1. Phosphorylated EGF receptor bound to immobilized SH2/SH3 of PLC gamma 1 in Sepharose beads, while nonphosphorylated EGF receptor did not bind. In SH2 domain of PLC gamma 1, there are several highly conserved amino acid sequences that are common in a variety of SH2-containing proteins. Especially the eight amino acid sequence, G(S/T)FLVR(E/D)S is highly conserved in these proteins. We synthesized several peptides related to these sequences and examined the effect of peptides on the binding of EGF receptor to SH2 of PLC gamma 1. P1, GSFLVRES was the most effective inhibitor to suppress the binding. P2, GSFLVAES in which one amino acid, arginine of P1 is substituted by alanine is still effective. But a peptide, P3, SFLVRE in which two amino acids are deleted from P1 did not inhibit markedly. Moreover, P1 peptide immobilized in Sepharose beads also bound phosphorylated EGF receptor. These data suggest that highly conserved amino acid sequence GSFLVRES is the minimum essential unit to recognize tyrosine phosphorylated site.

Dissecting the activating mutations in v-erbB of avian erythroblastosis virus strain R

The v-erbB oncogene isolated from the R (or ES4) strain of avian erythroblastosis virus is capable of inducing erythroleukemia and fibrosarcomas. This oncogene differs from the proto-oncogene c-erbB, the avian homolog of the epidermal growth factor receptor, by its lack of an intact ligand-binding domain as well as additional alterations in its cytoplasmic coding sequences. By contrast, the insertionally activated c-erbB, a variant oncogene, which encodes a product that also lacks the ligand-binding domain but is otherwise unaltered in its cytoplasmic coding sequences, is capable of inducing leukemia but cannot induce sarcomas. In this report, we show that the critical changes for activating the sarcomagenic potential displayed by v-erbB R are two point mutations within the tyrosine kinase domain and an internal deletion of 21 amino acids in the carboxyl-terminal regulatory domain. The removal of the carboxyl-terminal autophosphorylation sites is not obligatory. These activating mutations (Arg-263 to His, Ile-384 to Ser, and the deletion of residues 494 to 514), when introduced singly into the insertionally activated c-erbB, all dramatically increase fibroblast-transforming potential. Arg-263 resides near the highly conserved HRD motif of the kinase domain, and its mutation to His increases the autophosphorylation activity. The other two mutations do not alter the intrinsic kinase activity and presumably affect other aspects of the receptor involved in growth signaling. Therefore, the high transforming potential of v-erbB R is a consequence of synergism among multiple activating mutations.

Interallelic complementation among DER/flb alleles: implications for the mechanism of signal transduction by receptor-tyrosine kinases

The large number of available embryonic lethal alleles in the Drosophila EGF receptor homolog (DER)/faint little ball locus allowed us to test the possibility of positive or negative interactions among different DER alleles. These interactions were monitored by examining the embryonic cuticular phenotypes of different heteroallelic combinations. Several positive interactions were identified, while negative interactions were restricted to a single allele. This is the first example of positive interactions within the same cell type among alleles of a receptor tyrosine kinase gene. The basis for these interactions is likely to arise from the mechanism of signal transduction by receptor tyrosine kinases, which involves receptor aggregation. A combination of two different DER mutant proteins defective in temporally distinct stages of the signal transduction process, may thus form a functional heterodimer. The mutation sites in four alleles showing positive interactions were localized. They identify regions within the protein which are likely to be important for these temporally distinct signal transduction processes.

Nerve growth factor binds to the 140 kd trk proto-oncogene product and stimulates its association with the src homology domain of phospholipase C gamma 1

The cellular actions of nerve growth factor (NGF) involve regulation of protein phosphorylation. In PC-12 pheochromocytoma cells, exposure of [125I]NGF followed by crosslinking indicates that the ligand binds to two discreet receptors, the previously described 75 kd protein, as well as the trk proto-oncogene product pp140c-trk. Competition experiments reveal that of the two, pp 140c-trk binds to NGF with higher affinity. Following exposure to NGF, pp140c-trk undergoes a rapid autophosphorylation on tyrosine residues, and concomitantly phosphorylates and associates with phospholipase C gamma 1 (PLC gamma 1), through interaction with its src homology domains. The binding of NGF to pp140c-trk with high affinity, the NGF-dependent homology domains. The binding of NGF to pp140c-trk with high affinity, the NGF-dependent activation of its tyrosine kinase activity and the specific association with the effector molecule, PLC gamma 1, suggests that this is the biologically relevant signaling receptor for NGF.

Inositol phospholipids-specific phospholipase C: interaction of the gamma 1 isoform with tyrosine kinase

The generation of second messengers from inositol phospholipids is catalysed by enzymes from the phospholipase C family. Activation of phospholipase C-gamma 1 through tyrosine phosphorylation provides a link between mitogenic and inositol phospholipid signaling.

BCR sequences essential for transformation by the BCR-ABL oncogene bind to the ABL SH2 regulatory domain in a non-phosphotyrosine-dependent manner

BCR-ABL is a chimeric oncogene implicated in the pathogenesis of Philadelphia chromosome-positive human leukemias. BCR first exon sequences specifically activate the tyrosine kinase and transforming potential of BCR-ABL. We have tested the hypothesis that activation of BCR-ABL may involve direct interaction between BCR sequences and the tyrosine kinase regulatory domains of ABL. Full-length c-BCR as well as BCR sequences retained in BCR-ABL bind specifically to the SH2 domain of ABL. The binding domain has been localized within the first exon of BCR and consists of at least two SH2-binding sites. This domain is essential for BCR-ABL-mediated transformation. Phosphoserine/phosphothreonine but not phosphotyrosine residues on BCR are required for interaction with the ABL SH2 domain. These findings extend the range of potential SH2-protein interactions in growth control pathways and suggest a function for SH2 domains in the activation of the BCR-ABL oncogene as well as a role for BCR in cellular signaling pathways.

Interaction of epidermal growth factor with vasoactive hormones in the regulation of phospholipase A2

The interaction of growth factors with their receptors initiates a series of intracellular events that are of critical importance in the control of normal cell proliferation. In this regard considerable attention has focused on the coupling of phospholipase C-gamma to growth factor receptor tyrosine kinases. In contrast, the interaction of growth factors with phospholipase A2 has received less attention, most likely because the arachidonic acid release response has been considered to be an accompaniment of phospholipase C activation. Work from our laboratory using a well defined model system demonstrated a distinct coupling relationship of epidermal growth factor to phospholipase A2. This review focuses on the interaction of the epidermal growth factor receptor with phospholipases involved in both mitogenic and non-mitogenic responses and discusses their possible relation with vasoactive hormones.

Cloning of PI3 kinase-associated p85 utilizing a novel method for expression/cloning of target proteins for receptor tyrosine kinases

A novel method has been developed to allow cloning of protein targets for receptors with tyrosine kinase activity. By utilizing the carboxy-terminal tail of EGF receptor (EGFR) as a probe to screen lambda gt11 expression libraries, several EGFR-binding proteins have been cloned; two have been analyzed and contain unique SH2 and SH3 domains. One gene (GRB-1) has been fully sequenced, is expressed in various tissues and cell lines, and has a molecular mass of 85 kd. Interestingly, GRB-1 encodes the human counterpart of the PI3 kinase-associated protein p85. Advantages of this technique include the ease of cloning tyrosine kinase receptor targets present at low levels and the ability to identify proteins that are related in their capacity to bind activated receptors but contain no significant DNA sequence homology. This method, termed CORT (for cloning of receptor targets), offers a general approach for the identification and cloning of various receptor targets.