Synthesis, purification, and characterization of the cytoplasmic domain of the human insulin receptor using a baculovirus expression system

The cytoplasmic domain of the beta subunit of the human insulin receptor has been overexpressed in insect cells using the baculovirus expression system. A recombinant baculovirus (BIR-2) was constructed by inserting the human insulin proreceptor cDNA fragment that encodes the cytoplasmic domain of the receptor into the genome of Autographa californica nuclear polyhedrosis virus adjacent to the strong polyhedrin promoter. Synthesis of the protein (baculovirus insulin receptor kinase (BIRK), Mr 48,000) in BIR-2-infected Spodoptera frugiperda (Sf9) cells was detected 24 h after infection and maximal accumulation (2-3% of the cytosolic protein) was achieved 48-72 h post-infection. The expressed protein is active as a soluble protein tyrosine kinase, both in Sf9 cells and in vitro. Rapid purification to near homogeneity was accomplished by sequential chromatography on Fast-Q-Sepharose and phenyl-Superose with an overall yield of 35% and a specific activity with histone as substrate of 20 nmol/min/mg protein. Autophosphorylation activated the intrinsic kinase activity of BIRK and decreased its mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using a combination of tryptic digestion and immunoprecipitation with specific antipeptide antisera, it was ascertained that 30-40% of the 32P incorporated into BIRK by autophosphorylation is in the carboxyl-terminal domain (that includes tyrosyl residues 1316 and 1322 of the human proreceptor). Of the remaining radioactivity, 75% is in the amino-terminal domain (that includes tyrosyl residues 953, 960, 972, 999, and 1075) and 25% is in the conserved autophosphorylation domain (including tyrosyl residues 1146, 1150, and 1151). Limited digestion of BIRK with trypsin yielded a fragment, Mr 38,000, that lacks the carboxyl-terminal domain. This fragment exhibits protein tyrosine kinase activity that is stimulated by autophosphorylation. The properties of the soluble, monomeric BIRK are similar to those of the intact, activated, oligomeric insulin receptor kinase with respect to specificity, immunoreactivity, divalent cation requirements, and specific activity. These observations coupled with the ease of producing 0.4 mg of purified enzyme from 100 ml of suspension culture suggest that BIRK will be useful for biochemical and biophysical analysis of the insulin receptor protein tyrosine kinase.     

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.     

Structural analysis of the transmembrane domain of the epidermal growth factor receptor

The ligand-binding domain of the epidermal growth factor (EGF) receptor is separated from the cytoplasmic protein tyrosine kinase domain by a predicted single transmembrane segment. Antipeptide antibodies prepared against the outer portion of the predicted transmembrane segment confirmed this area was exposed only when cells were treated with permeabilizing agents. To investigate structural requirements for signal transduction by the transmembrane domain, three types of mutant EGF receptor were prepared. The first type was designed to shorten the transmembrane domain, the second to place proline substitutions within this domain, and the third to make amino acid substitutions analogous to those present in the transforming c-erbB2/neu oncoprotein. Mutant human receptors were expressed in null recipient mouse B82L and Chinese hamster ovary cells. All receptors bound EGF and exhibited EGF-stimulated protein tyrosine kinase activity in vivo as assayed using a 125I-labeled monoclonal anti-phosphotyrosine antibody. EGF stimulated growth of cells expressing each mutant receptor with similar dose-response characteristics. In contrast to other growth factor receptors, the transmembrane domain of the EGF receptor is tolerant to a variety of changes which neither mimic EGF action by constitutive activation nor interfere with ligand-induced signal transduction.     

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.     

Efficient expression in insect cells of a soluble, active human insulin receptor protein-tyrosine kinase domain by use of a baculovirus vector.

The human insulin receptor (IR) is a transmembrane glycoprotein, whose cytoplasmic domain contains an insulin-activated protein-tyrosine kinase (EC 2.7.1.112). By the use of an appropriately engineered baculovirus expression vector, a soluble cytoplasmic derivative of this domain was expressed in the insect cell line Spodoptera frugiperda (Sf9). At 24 to 48 h after Sf9 cells were infected with recombinant virus, a protein of the size expected for this domain (approximately 48 kilodaltons) constituted a major band when total cell lysates of metabolically labeled cells were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. This protein (designated AchIRPTK) was immunoprecipitated by three monoclonal antibodies, each of which recognizes a distinct antigenic site of the IR cytoplasmic domain and requires the native structure of the protein for recognition and one of which binds at or near the physiologically relevant site(s) of IR autophosphorylation. In vivo, AchIRPTK was phosphorylated on both tyrosine and serine residues. When affinity purified, the kinase was active in vitro; it autophosphorylated exclusively on tyrosine residues, and phosphorylated the exogenous substrates histone H2b and poly(Glu-Tyr). The expression of an active IR protein-tyrosine kinase molecule in this heterologous cell system provides an efficient experimental method for producing this domain in quantity for enzymatic and 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.     

The C terminus of RON tyrosine kinase plays an autoinhibitory role

RON is a receptor tyrosine kinase in the MET family. We have expressed and purified active RON using the Sf9/baculovirus system. The constructs used in this study comprise the kinase domain alone and the kinase domain plus the C-terminal region. The construct containing the kinase domain alone has a higher specific activity than the construct containing the kinase and C-terminal domains. Purified RON undergoes autophosphorylation, and the exogenous RON C terminus serves as a substrate. Peptides containing a dityrosine motif derived from the C-terminal tail inhibit RON in vitro or when delivered into intact cells, consistent with an autoinhibitory mechanism. Phenylalanine substitutions within these peptides increase the inhibitory potency. Moreover, introduction of these Phe residues into the dityrosine motif of the RON kinase leads to a decrease in kinase activity. Taken together, our data suggest a model in which the C-terminal tail of RON regulates kinase activity via an interaction with the kinase catalytic domain.     

Effect of carboxyl terminal truncation on the tyrosine kinase activity of the epidermal growth factor receptor.

The carboxyl terminal domain of the epidermal growth factor receptor (EGFR) is an important regulatory region in mediating the tyrosine kinase-dependent biological effects of EGF. The effect of a 164-amino-acid carboxyl deletion of the EGFR or the EGFR cytoplasmic kinase domain on in vitro tyrosine kinase activity was assessed. C'-terminal truncation of the EGFR resulted in dependence on Mn2+ for full activity. The EGFR kinase domain (kd EGFR) and the C'-terminally truncated kinase domain (kd c'1022 EGFR) also exhibited a strong preference for Mn2+ compared to Mg2+, with kd c'1022 EGFR being completely inactive in the presence of Mg2+ alone. Sphingosine or ammonium sulfate specifically activated both kd EGFR and kd c'1022 EGFR. EGFR and c'1022 EGFR displayed similar EGF-stimulated in vitro tyrosine kinase activities; however, kd EGFR was 5- to 10-fold more active in vitro than kd c'1022 EGFR. Thus, the regulatory contribution of the C'-terminus is most evident when the EGFR ligand binding domain is removed. These results indicate that an intact EGFR C'-terminus is necessary for the protein to assume a fully active conformation.     

Biochemical comparisons of the normal and oncogenic forms of insect cell-expressed neu tyrosine kinases.

The rat neu oncogene product is a member of the epidermal growth factor (EGF) receptor subgroup of the superfamily of growth factor receptor tyrosine kinases. The oncogenic activation of the neu protein occurs by a point mutation within its transmembrane region which results in an increase in its tyrosine kinase activity. Using three different forms of neu expressed in insect cells via baculovirus infection, we have examined the biochemical differences between the normal and transforming forms of neu and investigated the role of the transmembrane domain in its tyrosine kinase activity. One form of neu which was expressed in insect cells consisted of the complete tyrosine kinase domain but lacked the extracellular and transmembrane regions (designated NTK). The other two forms consisted of the tyrosine kinase domain, the transmembrane domain, and 40 amino acids of the extracellular domain. One of these transmembrane forms of neu contained the normal valine residue at position 664 within the transmembrane region (MS-N), while the other contained the oncogenic glutamic acid residue at this position (MS-T). Direct comparisons of NTK, MS-N, and MS-T have shown that the NTK protein is capable of the highest extents of both autophosphorylation activity and the tyrosine phosphorylation of exogenous substrate, suggesting that the presence of the transmembrane region of neu suppresses the tyrosine kinase activity of this receptor. In addition, we have found that the oncogenic point mutation within the transmembrane region stimulates the tyrosine kinase activity of the neu protein by allowing it to more effectively utilize Mg2+. Overall, the results of these studies suggest that the valine to glutamic acid substitution at position 664 may at least partially relieve a negative constraint imparted by the membrane-spanning domain on the tyrosine kinase activity of neu and enables a more effective use of Mg2+ in the catalysis of tyrosine phosphorylation of exogenous substrates.     

Antibodies against a synthetic peptide as a probe for the kinase activity of the avian EGF receptor and v-erbB protein

The transforming protein v-erbB of avian erythroblastosis virus (AEV) displays extensive sequence homology with the presumptive protein-tyrosine kinase domain of the human EGF receptor and with the src protein-tyrosine kinase family of oncogenes. However, no kinase activity has previously been demonstrated for the v-erbB protein. Here antibodies generated against a synthetic peptide from the C terminus of human EGF receptor are shown to immunoprecipitate the EGF receptor from human and avian cells, as well as the v-erbB proteins from AEV-transformed cells that become phosphorylated on tyrosine residues upon the addition of gamma-32P-ATP. The immunoprecipitates are also able to phosphorylate exogenous tyrosine-containing substrates. Hence, it is likely that both avian EGF receptor and v-erbB proteins are protein tyrosine-specific protein kinases. Since the kinase activity of v-erbB protein cannot be regulated by EGF, it is proposed that the tyrosine protein kinase function of v-erbB may be constitutively activated.     

Bruton酪氨酸激酶在昆虫细胞中表达的初步研究

用杆状病毒表达载体系统表达小鼠Ｂｒｕｔｏｎ酪氨酸激酶（Ｂｒｕｔｏｎｔｙｒｏｓｉｎｅｋｉｎａｓｅ,Ｂｔｋ）．构建重组转染载体时,于Ｂｔｋ起始码的上游插入了一段Ｈ９０２序列．用重组转染载体、苜蓿夜蛾核型多角体病毒线性ＤＮＡ和质脂体共转染Ｓｆ９昆虫细胞,经过对重组杆状病毒三轮扩增后,用Ｈ９０２抗体检测表明,昆虫细胞中Ｂｔｋ的表达已达最高水平．对表达后Ｂｔｋ自身磷酸化检测表明,该激酶具有自身磷酸化活性．从而证实Ｂｔｋ在昆虫细胞中的表达获得了成功     

Functional characterization and purification of the secretin receptor expressed in baculovirus-infected insect cells

Structural insights into Class II G protein-coupled receptors have been limited by the absence of a plentiful and highly enrichable source such as rhodopsin in the Class I family. With structural differences predicted to exist between these families, and with the key importance of an intact, disulfide-bonded amino-terminal domain for the Class II receptors, an overproduction and purification scheme is critically important. In this work, we have established and characterized a baculoviral expression and purification system for the secretin receptor. Hemagglutinin epitope-tagged wild-type rat secretin receptor construct was expressed using the recombinant baculovirus/Sf9 insect cell-based system, achieving a level of expression substantially higher than that previously achieved in Chinese hamster ovary (CHO-SecR) cells. Receptor expressed in Sf9 cells had similar affinity for secretin (Ki=1.4+/-0.2 nM) and similar potency to stimulate intracellular cAMP in response to this hormone (EC50=194+/-45 pM) as did wild-type receptor expressed in CHO cells. Receptors from Sf9 cells were also affinity labeled saturably and specifically by a photolabile secretin analogue. The receptors were purified to homogeneity by solubilization with sodium deoxycholate, selective ammonium sulfate precipitation, gel filtration and immunoaffinity purification. This expression system should facilitate the structural characterization of this receptor and its important amino-terminal domain.     

Inhibition of stimulus-dependent epidermal growth factor receptor and transforming growth factor-alpha mRNA accumulation by the protein kinase C inhibitor staurosporine

The ability of staurosporine, a potent inhibitor of protein kinase C, to block certain cellular events initiated by 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor (EGF) was examined. Treatment of MDA468 breast cancer cells with TPA decreases EGF binding to the cell surface and this effect is blocked by pretreatment with staurosporine with an IC50 of 30 nM. Either 10(-9) M EGF or 100 ng/ml TPA stimulated the accumulation of both EGF receptor and TGF-alpha mRNA and staurosporine (50 nM) completely abolished these mRNA accumulations. Staurosporine did not block EGF-stimulated tyrosine phosphorylation of its receptor as measured by immunoblotting with anti-phosphotyrosine antibodies. The ability of staurosporine to block the mRNA responses of either EGF or TPA suggests that these two agents have common signaling pathways and it implies a role for protein kinase C in the control of EGF receptor and TGF-alpha expression.     

A mutant epidermal growth factor receptor with defective protein tyrosine kinase is unable to stimulate proto-oncogene expression and DNA synthesis.

Cultured NIH-3T3 cells devoid of endogenous epidermal growth factor (EGF) receptors were transfected with cDNA expression constructs encoding either normal human EGF receptor or a receptor mutated in vitro at Lys-721, a residue that is thought to function as part of the ATP-binding site of the kinase domain. Unlike the wild-type EGF-receptor expressed in these cells, which exhibited EGF-dependent protein tyrosine kinase activity, the mutant receptor lacked protein tyrosine kinase activity and was unable to undergo autophosphorylation and to phosphorylate exogenous substrates. Despite this deficiency, the mutant receptor was normally expressed on the cell surface, and it exhibited both high- and low-affinity binding sites. The addition of EGF to cells expressing wild-type receptors caused the stimulation of various responses, including enhanced expression of proto-oncogenes c-fos and c-myc, morphological changes, and stimulation of DNA synthesis. However, in cells expressing mutant receptors, EGF was unable to stimulate these responses, suggesting that the tyrosine kinase activity is essential for EGF receptor signal transduction.     

Phosphatidylinositol kinase or an associated protein is a substrate for the insulin receptor tyrosine kinase.

The tyrosine kinase activity intrinsic to the insulin receptor is thought to be important in eliciting the intracellular responses to insulin; however, it has been difficult to determine the biochemical functions of the proteins which are substrates for this receptor. Treatment of Chinese hamster ovary (CHO) cells overexpressing the human insulin receptor (CHO.T) with insulin results in a 38 +/- 11 (mean +/- S.E., n = 9)-fold increase in a phosphatidylinositol (PtdIns) kinase activity in anti-phosphotyrosine immunoprecipitates of whole cell lysates. One minute of treatment of cells with insulin causes a dramatic increase in the PtdIns kinase activity in the anti-phosphotyrosine immunoprecipitates; the activity peaks within 5 min and remains elevated for at least 60 min after addition of insulin to the cells. This response is only slightly delayed compared with the time course we observe for activation of the insulin receptor tyrosine kinase. The insulin dose-response curves are also very similar for the activation of the insulin receptor tyrosine kinase activity and for the appearance of PtdIns kinase in the anti-phosphotyrosine immunoprecipitates. Stimulation of the endogenous insulin receptor of CHO cells also results in the association of PtdIns kinase activity with phosphotyrosine-containing proteins. However, CHO cells are less sensitive to insulin than CHO.T cells, and the maximal PtdIns kinase activity in antiphosphotyrosine immunoprecipitates from CHO cells is one-sixth that of CHO.T cells. In contrast, immunoprecipitates from CHO.T cells made with anti-insulin receptor antibodies do not contain significant levels of PtdIns kinase activity. This demonstrates that the PtdIns kinase is either a substrate for the insulin receptor tyrosine kinase or is tightly associated with another tyrosine phosphoprotein, which is not the insulin receptor.     

Biochemical properties of the Cdc42-associated tyrosine kinase ACK1. Substrate specificity, authphosphorylation, and interaction with Hck

ACK1 (activated Cdc42-associated kinase 1) is a nonreceptor tyrosine kinase and the only tyrosine kinase known to interact with Cdc42. To characterize the enzymatic properties of ACK, we have expressed and purified active ACK using the baculovirus/Sf9 cell system. This ACK1 construct contains (from N to C terminus) the kinase catalytic domain, SH3 domain, and Cdc42-binding Cdc42/Rac interactive binding (CRIB) domain. We characterized the substrate specificity of ACK1 using synthetic peptides, and we show that the specificity of the ACK1 catalytic domain most closely resembles that of Abl. Purified ACK1 undergoes autophosphorylation, and autophosphorylation enhances kinase activity. We identified Tyr284 in the activation loop of ACK1 as the primary autophosphorylation site using mass spectrometry. When expressed in COS-7 cells, the Y284F mutant ACK1 showed dramatically reduced levels of tyrosine phosphorylation. Although the SH3 and CRIB domains of purified ACK1 are able to bind ligands (a polyproline peptide and Cdc42, respectively), the addition of ligands did not stimulate tyrosine kinase activity. To characterize potential interacting partners for ACK1, we screened several SH2 and SH3 domains for their ability to bind to full-length ACK1 or to the catalytic-SH3-CRIB construct. ACK1 interacts most strongly with the SH3 domains of Src family kinases (Src or Hck) via its C-terminal proline-rich domain. Co-expression of Hck with kinase-inactive ACK1(K158R) in mammalian cells resulted in tyrosine phosphorylation of ACK1, suggesting that ACK1 is a substrate for Hck. Our data suggest that Hck is a novel binding partner for ACK1 that can regulate ACK1 activity by phosphorylation.     

Epidermal growth factor (EGF) stimulates association and kinase activity of Raf-1 with the EGF receptor.

Raf-1 serine- and threonine-specific protein kinase is transiently activated in cells expressing the epidermal growth factor (EGF) receptor upon treatment with EGF. The stimulated EGF receptor coimmunoprecipitates with Raf-1 kinase and mediates protein kinase C-independent phosphorylation of Raf-1 on serine residues. Hyperphosphorylated Raf-1 has lower mobility on sodium dodecyl sulfate gels and has sixfold-increased activity in immunocomplex kinase assay with histone H1 or Raf-1 sequence-derived peptide as a substrate. Raf-1 activation requires kinase-active EGF receptor; a point mutant lacking tyrosine kinase activity in inactive in Raf-1 coupling and association. It is noteworthy that tyrosine phosphorylation of c-Raf-1 induced by EGF was not detected in these cells. These observations suggest that Raf-1 kinase may act as an important downstream effector of EGF signal transduction.     

Regulation of the epidermal growth factor receptor by phosphorylation

The receptor for epidermal growth factor (EGF) is a glycosylated transmembrane phosphoprotein that exhibits EGF-stimulable protein tyrosine kinase activity. On EGF stimulation, the receptor undergoes a self-phosphorylation reaction at tyrosine residues located primarily in the extreme carboxyl-terminal region of the protein. Using enzymatically active EGF receptor purified by immunoaffinity chromatography from A431 human epidermoid carcinoma cells, the self-phosphorylation reaction has been characterized as a rapid, intramolecular process which is maximal at 30-37 degrees C and exhibits a very low Km for ATP (0.2 microM). When phosphorylation of exogenous peptide substrates was measured as a function of receptor self-phosphorylation, tyrosine kinase activity was found to be enhanced two to threefold at 1-2 mol of phosphate per mol of receptor. Analysis of the dependence of the tyrosine kinase activity on ATP concentration yielded hyperbolic kinetics when plotted in double-reciprocal fashion, indicating that ATP can serve as an activator of the enzyme. Higher concentrations of peptide substrates were found to inhibit both the self- and peptide phosphorylation, but this inhibition could be overcome by first self-phosphorylating the enzyme. These results suggest that self-phosphorylation can remove a competitive/inhibitory constraint so that certain exogenous substrates can have greater access to the enzyme active site. In addition to self-phosphorylation, the EGF receptor can be phosphorylated on threonine residues by the calcium- and phospholipid-dependent protein kinase C. The sites on the EGF receptor phosphorylated in vitro by protein kinase C are identical to the sites phosphorylated on the receptor isolated from A431 cells exposed to the tumor promoters 12-O-tetradecanoylphorbol 13-acetate or teleocidin. This phosphorylation of the EGF receptor results in a suppression of its tyrosine kinase and EGF binding activities both in vivo and in vitro. The EGF receptor can thus be variably regulated by phosphorylation: self-phosphorylation can enhance tyrosine kinase activity whereas protein kinase C-catalyzed phosphorylation can depress enzyme activity. Because these two phosphorylations account for only a fraction of the phosphate present in the EGF receptor in vivo, other protein kinases can apparently phosphorylate the receptor and these may exert additional controls on EGF receptor/kinase function.     

Ligand-induced protein tyrosine kinase activity in living cells coexpressing intact EGF receptors and receptors with an extensive cytosolic deletion.

A population of stable NIH 3T3 transfectants with two molecular weight classes of membrane-bound EGF receptors encoded by a human EGF receptor cDNA has been identified and characterized. In addition to intact EGF receptors, these cells also express a molecule with an extensive cytosolic deletion. This deletion includes the ligand-activated intrinsic protein tyrosine kinase catalytic domain. Treatment with EGF caused dimerization of intact and truncated receptors, allowing us to assess protein tyrosine kinase activity in the heterodimer isolated from living cells. In contrast to homodimeric complexes with intact EGF receptor only, heterodimers were deficient in protein tyrosine kinase activity. Moreover, physical association between intact and truncated molecules suppressed receptor auto-phosphorylation by EGF receptor protein tyrosine kinase activated by antibody binding in vitro. Evidence presented here supports the idea that protein tyrosine kinase activation is facilitated by interaction between adjacent receptor molecules with intact catalytic domains. Furthermore, molecules with cytoplasmic deletions that are physically associated with kinase-active EGF receptors appear to behave as dominant negative mutations. The HerC cl cells used in this study were selected with methotrexate to amplify the EGF receptor cDNA, and in that sense may resemble certain tumor-derived cells characterized by overexpressed and rearranged EGF receptor genes.     

Intrapeptide autophosphorylation of the epidermal growth factor receptor: regulation of kinase catalytic function by receptor dimerization

The epidermal growth factor (EGF) receptor is a transmembrane polypeptide of 170 000 daltons (Da) with a cytoplasmically facing protein kinase domain. The regulation of the tyrosine kinase activity of the EGF receptor by added EGF and by receptor association state was studied in an in vitro system. The rate of autophosphorylation of the solubilized and purified EGF receptor was found to be independent of receptor concentration. To determine whether the zero-order kinetics observed point to intrapeptide phosphorylation, we measured the sedimentation characteristics of the undenatured solubilized receptor. The receptor was found to exist in two association-dissociation states-a monomeric 7.7S form and a dimeric 12S form. The 7.7S form is an active tyrosine kinase; it has high basal activity, and the activity is not further stimulated by EGF; it appears to be an EGF-independent form of the receptor kinase. The 12S form is devoid of catalytic activity, but in the presence of EGF it dissociates into the active monomeric form. Freshly purified receptor preparations contain mainly the monomeric receptor, have high basal kinase activity, and show low EGF stimulatability (less than 1.3-fold). Aging of the receptor results in progressive dimerization and decay of EGF-independent kinase activity (and increase in EGF stimulatability). All of these processes are reversed in the presence of EGF or dithiothreitol.(ABSTRACT TRUNCATED AT 250 WORDS)