Epidermal-growth-factor-dependent transformation by a human EGF receptor proto-oncogene.

The epidermal growth factor (EGF) receptor gene EGFR has been placed in a retrovirus vector to examine the growth properties of cells that experimentally overproduce a full-length EGF receptor. NIH 3T3 cells transfected with the viral DNA or infected with the corresponding rescued retrovirus developed a fully transformed phenotype in vitro that required both functional EGFR expression and the presence of EGF in the growth medium. Cells expressing 4 x 10(5) EGF receptors formed tumors in nude mice, while control cells did not. Therefore, the EGFR retrovirus, which had a titer on NIH 3T3 cells that was greater than 10(7) focus-forming units per milliliter, can efficiently transfer and express this gene, and increased numbers of EGF receptors can contribute to the transformed phenotype.

Functional heterogeneity of proto-oncogene tyrosine kinases: the C terminus of the human epidermal growth factor receptor facilitates cell proliferation.

Previous reports have indicated that the C termini of the membrane-associated tyrosine kinases encoded by c-src and c-fms proto-oncogenes have a negative effect on their biological activity and that this effect is mediated by their C-terminal tyrosine residue. To determine whether this was true for the human epidermal growth factor (EGF) receptor, which is also a membrane-associated tyrosine kinase proto-oncogene, we have constructed two premature termination mutants, dc19 and dc63, that delete the C-terminal 19 and 63 amino acids, respectively, from the human full-length receptor (hEGFR). The smaller deletion removes the C-terminal tyrosine residue, while the larger deletion removes the two most C-terminal tyrosines; similar deletions are found in v-erbB. As previously shown for the gene encoding the full-length EGF receptor, the two C-terminal mutants induced EGF-dependent focal transformation and anchorage-independent growth of NIH 3T3 cells. However, both dc19 and dc63 were quantitatively less efficient than the gene encoding the full-length receptor, with dc63 being less active than dc19. Although the C-terminal mutants displayed lower biological activity than the gene encoding the full-length receptor, the mutant receptors were found to be similar in several respects to the full-length receptor. These parameters included receptor localization, stability in the absence of EGF, receptor half-life in the presence of EGF, EGF binding, extent of EGF-dependent autophosphorylation in vitro, and EGF-dependent phosphorylation of an exogenous substrate in vitro. Therefore, the C-terminal 63 amino acids of the human receptor have no detectable influence on EGF-dependent early events. We conclude that in contrast

Phosphorylation of tyrosine 992, 1068, and 1086 is required for conformational change of the human epidermal growth factor receptor c-terminal tail.

We reported previously that a conformation-specific antibody, Ab P2, to a 16-amino acid peptide (Glu-Gly-Tyr-Lys-Lys-Lys-Tyr-Gln-Gln-Val-Asp-Glu-Glu-Phe-Leu-Arg) of the cytoplasmic domain of the beta-type platelet-derived growth factor receptor also recognizes the epidermal growth factor (EGF) receptor. Although the antibody is not directed to phosphotyrosine, it recognizes in immunoprecipitation the activated and hence phosphorylated form of both receptors. In P2 peptide, there are two tripeptide sequences, Asp-Glu-Glu and Tyr-Gln-Gln, that are also present in the EGF receptor. Our present studies using either EGF receptor C-terminal deletion mutants or point mutations (Tyr-->Phe) and our previous studies on antibody inhibition by P2-derived peptides suggest that Gln-Gln in combination with Asp-Glu-Glu forms a high-affinity complex with Ab P2 and that such complex formation is dependent on tyrosine phosphorylation. Of the five phosphate acceptor sites in the EGF receptor, clustered in the extreme C-terminal tail, phosphorylation of three tyrosine residues (992, 1068, and 1086) located between Asp-Glu-Glu and Gln-Gln is necessary for Ab P2 binding. In contrast, the acceptor sites Tyr 1173 and 1148 play no role in the conformation change. Asp-Glu-Glu and Gln-Gln are located 169 amino acids apart, and it is highly likely that the interactions among three negatively charged phosphotyrosine residues in the receptor C terminus may result in the bending of the peptide chain in such a way that these two peptides come close to each other to form an antibody-binding site. Such a possibility is also supported by our finding that receptor dephosphorylation results in complete loss of Ab P2-binding activity. In conclusion, we have identified a domain within the cytoplasmic part of the EGF receptor whose conformation is altered by receptor phosphorylation; furthermore, we have identified the tyrosine residues that positively regulate this conformation.

Potent SHC tyrosine phosphorylation by epidermal growth factor at low receptor density or in the absence of receptor autophosphorylation sites.

The importance of epidermal growth factor (EGF) receptor expression level and autophosphorylation sites in src homology and collagen protein (SHC) tyrosine phosphorylation has been studied. In contrast to EGF-induced tyrosine phosphorylation of the GTPase-activating protein for ras (rasGAP) and phospholipase C-gamma 1 (PLC-gamma 1), SHC tyrosine phosphorylation occurs at a very low receptor density in parental NIH3T3 mouse fibroblasts expressing less than 1 x 10(4) EGF receptors per cell. In transfected NIH3T3 cells expressing human EGF receptors (approximately 4 x 10(5) receptors per cell), maximal levels of SHC and PLC-gamma 1 tyrosine phosphorylation occur when approximately 4 x 10(4) receptors or more are occupied by ligand. At lower levels of receptor occupancy only SHC phosphorylation was significant. Also, EGF treatment of mouse keratinocytes, which represent a physiological target of EGF, express a low number of EGF receptors (approximately 2 x 10(4) receptors per cell), and stringently require EGF to grow, results in intense SHC tyrosine phosphorylation, compared to rasGAP or PLC-gamma 1. SHC is also efficiently tyrosine phosphorylated by an EGF receptor deletion mutant (Dc214) that is devoid of autophosphorylation sites, but which remains mitogenically responsive to EGF. The EGF receptor mutant Dc214 is able to activate the ras guanine nucleotide exchanger and phosphorylate mitogen-activated protein kinase (MAPK), presumable as a result of complex formation between tyrosine phosphorylated SHC and GRB2. These results indicate that potent EGF-induced SHC tyrosine phosphorylation can be triggered in cells having relatively few receptors. Also, our data show that EGF receptors are able to phosphorylate SHC, activate the exchange of guanine nucleotide on ras and phosphorylate MAPK by a mechanism that does not require receptor autophosphorylation sites and, therefore, the src homology 2 (SH2):phosphotyrosine-dependent interaction of SHC or GRB2 with the EGF receptor.

The biological activity of the human epidermal growth factor receptor is positively regulated by its C-terminal tyrosines.

The epidermal growth factor receptor (EGF-R) C-terminus contains three conserved tyrosines (Y-1068, Y-1148, Y-1173) which are phosphorylated upon EGF activation. To clarify the functional role of these tyrosines, each has been mutated to phenylalanine and studied as single, double and triple mutants in the full length receptor. EGF-dependent transforming ability of the single point mutants is similar to that of the wild type, while that of double mutants is decreased and an even lower activity is present in the triple mutant. In each bioassay, including EGF-dependent focal transformation, growth in agar and growth in low serum, mutant receptors display a similar hierarchy of activity. The lower activity is intrinsic in the mutants since they are expressed at similar level as the wild type and bind EGF with similar affinity. Deletion mutants lacking the last 19 or 63 amino acids (Velu et al., 1989a) show a similar decline in biological activity when compared to the corresponding point mutants, although the reduction is more pronounced than with the point mutants. Deletion of the last 123 aa, which removes all three tyrosines (Dc123), results in a receptor that is almost inactive biologically. The EGF-R kinase activity is affected by tyrosine substitution since in vitro phosphorylation of exogenous substrates is reduced in the double and triple mutants. Autophosphorylation, in vivo and in vitro, is also reduced, but not totally abolished in the triple point mutant and Dc123 indicating the existence of other autophosphorylation sites. A new site of autophosphorylation is found in the Dc123 mutant. We conclude, therefore, that the tyrosines at the extreme C-terminus positively regulate the biological and transforming activity of the EGF-R, probably via autophosphorylation.

Synthesis of epidermal growth factor (EGF) receptor in vitro using SP6 RNA polymerase-transcribed template mRNA.

The epidermal growth factor (EGF) receptor plays a key role in the control cellular proliferation, and its homology to the avian erythroblastosis virus erb B oncogene implicates its involvement in cellular transformation. The establishment of a correlation between the various structural domains of the EGF receptor and their functional counterparts would greatly advance our understanding of these processes. To this end, we have constructed an expression vector containing the SP6 viral promoter and an adjacent cDNA fragment encoding the full-length EGF receptor. Upon addition of SP6 RNA polymerase, this DNA is capable of generating large amounts of EGF receptor mRNA; this RNA can then be translated in vitro into immunoprecipitable EGF receptor protein. The translational efficiency of this EGF receptor RNA was found to be relatively low: approx. 100-fold lower than globin RNA synthesized using SP6 RNA polymerase. Use of these tools should now permit the synthesis and analysis of mutated EGF receptor protein in an effort to clarify the role of this receptor in growth control.

Transmembrane signalling at the epidermal growth factor receptor.

The EGF receptor, which is homologous to the v-erb-B oncogene product, has intrinsic tyrosine kinase activity, and mediates an increase in polyphosphoinositide turnover and [Ca2+]i. Recently, great progress has been made in understanding the mechanism of signal transduction at this receptor. Jacopo Meldolesi and colleagues discuss how this knowledge may lead to a better understanding of the control of cell proliferation.

Defective insulin action in fibroblasts from noninsulin-dependent diabetes mellitus patients with Gln1152 insulin receptor mutation.

Insulin action was investigated in cultured skin fibroblasts from two consanguineous patients with a heterozygous point mutation in the insulin receptor kinase (Arg1152-Gln). In spite of normal binding, Gln1152 insulin receptor exhibited 20% increased basal kinase activity, but significantly reduced insulin-dependent autophosphorylation and kinase activity compared to controls from either weight-matched noninsulin-dependent diabetic patients (n = 4) or normal subjects (n = 5). In fibroblasts from the mutant patients, basal alpha-aminoisobutyric acid and 2-deoxyglucose (2-DG) uptake, cytochalasin-B (CB) plasma membrane binding, and glycogen synthase activity were increased to levels similar to those in maximally insulin-stimulated control cells. No insulin stimulation of these metabolic effects was detected in the mutant cells. In spite of the high basal 2-DG uptake and CB binding and the lack of further insulin response, fibroblasts from the mutant patients responded to 12-O-tetradecanoylphorbol-13-acetate with a further 50% increase in 2-DG uptake and CB binding. The magnitude of the effects of insulin and 12-O-tetradecanoylphorbol-13-acetate in control cells were nearly identical. We conclude that the Gln1152 insulin receptor impairs insulin regulation of metabolic responses in patient cells. Its presence in fibroblasts from the mutant patients appears to be accompanied by an increased pool of glucose transporters.

Alteration of erythrocyte membrane lipid fluidity in human obesity.

The lipophilic probe 1,6-diphenyl-1,3,5-hexatriene was incorporated into erythrocyte ghosts of either normal or obese humans, and the polarization of fluorescence was measured between 0 and 40 C. The membrane lipid fluidity, evaluated by fluorescence polarization, was consistently higher in the ghosts from obese subjects. A strong correlation was found between increased 1,6-diphenyl-1,3,5-hexatriene fluorescence polarization and excess body weight. Measurements of cholesterol and phospholipids indicated increased cholesterol and decreased phospholipids in erythrocyte ghosts from obese subjects. These data suggest that alterations in lipid composition in erythrocytes of obese subjects are responsible for abnormal physical properties of plasma membranes, which, in turn, may cause altered enzymatic activities.

Conformational analysis of the phosphorylated epidermal growth factor receptor.

Phosphorylation-induced conformational changes have been well documented with different receptor tyrosine kinases. However. the susceptible epitopes and the tyrosine residue(s) involved in particular structural alteration mostly remain to be determined. Using a conformation-specific anti-peptide antibody, we have not only identified one such domain in the C-terminal tail of the EGF receptor but also identified the phosphate acceptor sites that are involved in the conformational change.

Internalization and down-regulation of the human epidermal growth factor receptor are regulated by the carboxyl-terminal tyrosines.

The C terminus of the epidermal growth factor receptor (EGF-R) contains three tyrosines (Y1068, Y1148, and Y1173) which correspond to the major autophosphorylation sites. To investigate the role of the tyrosines in internalization and down-regulation of the EGF-R, mutational analysis was performed with receptors in which 1, 2, or all 3 tyrosines were changed to phenylalanines. The triple point mutant EGF-R, expressed in NIH-3T3, exhibited low autophosphorylation in vivo, low biological and reduced kinase activities. Single and double point mutants were down-regulated, as well as wild type EGF-R in response to EGF showing a half-life of about 1 h. Degradation of the triple point mutant, however, was impaired and resulted in a half-life of 4 h in the presence of EGF. EGF-dependent down-regulation of surface receptors was decreased in the triple point mutant EGF-R as was internalization and degradation of EGF. The specific rate of internalization of the triple point mutant was reduced. By contrast, intracellular processing of ligand previously internalized at 20 degrees C was similar between wild type and mutant receptors. Taken together the data indicate that the delay in degradation observed in cells expressing the triple point mutant EGF-R can be attributed mainly to a slower removal from the cell surface. Our results show that in the full-length EGF-R all three C-terminal tyrosines are necessary for rapid internalization, suggesting that autophosphorylation is required for efficient EGF-dependent receptor endocytosis.

Individual epidermal growth factor receptor autophosphorylation sites do not stringently define association motifs for several SH2-containing proteins.

To determine whether individual autophosphorylation sites in the epidermal growth factor (EGF) receptor define specific interaction sites for the in vivo association of signal transduction proteins that contain src homology 2 (SH2) domains, the capacity of wild-type and mutant EGF receptors to associate with several SH2 domain-containing proteins has been assayed. Mutants included receptors with single autophosphorylation site mutations at each of five autophosphorylation sites and receptors in which multiple autophosphorylation sites were removed by point mutation or deletion of carboxyl-terminal residues. Receptor association, as measured by coimmunoprecipitation, has been determined for phospholipase C-gamma 1, the ras GTPase-activating protein, the p85 subunit of phosphatidylinositol 3-kinase, and the src homology and collagen protein. In contrast to data obtained with single autophosphorylation site mutants of other receptor tyrosine kinases, none of the EGF receptor single site mutants was dramatically impaired in its capacity to associate with any of these SH2-containing proteins. However, association was completely abrogated when all five autophosphorylation sites were mutated or removed by deletion. These results indicate that individual autophosphorylation sites in the EGF receptor are not stringently required for the recognition and association of different SH2-containing substrates. Thus, EGF receptor autophosphorylation sites seem to be flexible and/or compensatory in their capacity to mediate association with these four SH2-containing substrates.

Tyrosine phosphorylation of ras GTPase-activating protein does not require association with the epidermal growth factor receptor.

The importance of the carboxyl-terminal domain of the epidermal growth factor (EGF) receptor and its five autophosphorylation sites in the in vivo interaction and tyrosine phosphorylation of the ras GTPase-activating protein (rasGAP) has been investigated, using NIH 3T3 cells transfected with mutant EGF receptors. Phosphorylation of rasGAP by EGF receptor mutants, in which one to four autophosphorylation sites (Tyr-1173, -1148, -1086, and -1068) were mutated to phenylalanine, was reduced by 50-60% compared to the wild-type receptor. Elimination of these four autophosphorylation sites by truncation of 123 carboxyl-terminal residues of the EGF receptor paralleled results obtained with point mutants. Substantial inhibition (about 90%) of rasGAP tyrosine phosphorylation by the EGF receptor occurred only when the remaining autophosphorylation site (Tyr-992) was mutated, in the context of this truncated receptor or in the full-length receptor mutated at all four other autophosphorylation sites. However, a point mutation of only Tyr-992 in the full-length receptor suppressed tyrosine phosphorylation of rasGAP only by 50%. In contrast, an EGF receptor lacking the last 214 amino acid residues (Dc214), which encompasses all five autophosphorylation sites, phosphorylated rasGAP to the same extent as the wild-type receptor. However, this truncated receptor was significantly impaired in its capacity to phosphorylate phospholipase C-gamma 1. Interestingly, while EGF receptor autophosphorylation sites are required for EGF-induced rasGAP association with the receptor, maximal phosphorylation of rasGAP by the truncated receptor Dc214 occurred without detectable formation of receptor-rasGAP complexes. Furthermore, the capacity of mutated EGF receptors to bring about focal transformation was correlated with their capacity to phosphorylate rasGAP.

Transmembrane signalling at epidermal growth factor receptors overexpressed in NIH 3T3 cells. Phosphoinositide hydrolysis, cytosolic Ca2+ increase and alkalinization correlate with epidermal-growth-factor-induced cell proliferation.

NIH 3T3 cells, which express a small number of EGF (epidermal growth factor) receptors, are poorly responsive to EGF. However, when the same cells overexpress the cloned human EGF receptor (EGFR T17 cells), they display EGF-dependent transformation. In EGFR T17 cells (but not in the parental NIH 3T3 cells), EGF is shown here to trigger polyphosphoinositide hydrolysis as well as the generation of the ensuing intracellular signals, the increase in the cytosolic Ca2+ concentration ([Ca2+]i) and pH. EGF induced a large accumulation of inositol 1,4,5-trisphosphate, with a peak at 15-30 s and a slow decline thereafter. Other inositol phosphates (1,3,4-trisphosphate and 1,3,4,5-tetrakisphosphate) increased less rapidly and to a lesser degree. [Ca2+]i increased after a short lag, reached a peak at 25 s and remained elevated for several minutes. By use of incubation media with and without Ca2+, the initial phase of the EGF-induced [Ca2+]i increase was shown to be due largely to Ca2+ release from intracellular stores. In contrast with previous observations in human A431 cells, the concentration-dependence of the EGF-triggered [Ca2+]i increase in EGFR T17 cells paralleled that of [3H]thymidine incorporation. It is concluded that polyphosphoinositide hydrolysis, [Ca2+]i increase and cytoplasmic alkalinization are part of the spectrum of intracellular signals generated by the activation of one single EGF receptor type. These processes might be triggered by the receptor via activation of the intrinsic tyrosine kinase activity. Large stimulation of DNA synthesis and proliferation by EGF in EGFR T17 cells could be due to a synergistic interplay between the two signal pathways initiated by tyrosine phosphorylation and polyphosphoinositide hydrolysis.

EGF-R antisense RNA blocks expression of the epidermal growth factor receptor and suppresses the transforming phenotype of a human carcinoma cell line.

We have used an antisense approach to investigate the role of overexpression of the normal human epidermal growth factor (EGF) receptor in the transformed phenotype of KB cells, which are a tumor derived human cell line. Initial experiments performed in vitro, showed that antisense RNA complementary to the entire coding region (AS-FL) or to parts of the EGF-R mRNA (AS-3', AS-5', and AS-K) effectively blocked translation of EGF-R mRNA. In addition, upon microinjection into KB cells, the in vitro synthesized antisense RNAs were able to inhibit transiently the synthesis of EGF-R. Inhibition was concentration-dependent, both in vitro and in cells, and the most effective constructs were those complementary to the entire coding region (AS-FL) or to the 3'-coding end of the mRNA (AS-3'). Transfection of the same EGF-R antisense RNA constructs into the human epidermoid carcinoma KB cell line gave rise to several clones stably expressing elevated levels of antisense RNA and resulting in low residual levels of EGF receptor. The most reduced clones exhibited a totally restored serum-dependent growth and were severely impaired in colony formation and growth in agar. In addition the severity of the phenotype was directly proportional to the residual amount of EGF-R expressed. We conclude that over-expression of normal EGF-R plays a direct primary role in the development of the transformed phenotype of this human cancer cell line.

Lysosomal targeting of epidermal growth factor receptors via a kinase-dependent pathway is mediated by the receptor carboxyl-terminal residues 1022-1123.

Binding of epidermal growth factor (EGF) to its receptor induces rapid internalization and degradation of both ligand and receptor via the lysosomal pathway. To study the mechanism of intracellular sorting of EGF-EGF receptor complexes to lysosomes, NIH 3T3 cells transfected with wild-type and mutant EGF receptors were employed. The kinetics of 125I-EGF trafficking was analyzed using low concentrations of the ligand to avoid saturation of the specific sorting system. The relative size of the pool of internalized 125I-EGF-receptor complexes that were capable of recycling decreased as receptors traversed the endosomal system. The rate of 125I-EGF sequestration from the recycling pathway correlated with the rate of 125I-EGF transition from early to late endosomes as measured by Percoll gradient fractionation. Deletion of the last 63 amino acids of the EGF receptor cytoplasmic tail did not inhibit the process of sequestration and targeting to the late endosomes and lysosomes. Truncation of the 123 residues, however, resulted in impaired lysosomal targeting and increased recycling of EGF. Receptor mutant in which 165 residues were deleted displayed maximal ability to recycle and a minimal extent of sorting to the late endosomes. The data suggest that two regions of the EGF receptor molecule, residues 1022-1063 and to a lesser extent residues 1063-1123, contribute in the regulation of routing of EGF receptors to the degradation pathway. The kinase-negative receptor mutant recycled EGF more intensively compared with the wild-type receptor, and the transport of this mutant to late endosomes was inhibited. These results support the view that the receptor kinase activity is important for ligand-induced sorting of EGF receptors to the pathway of lysosomal degradation.

Biochemical characterization of human GCF transcription factor in tumor cells.

GCF is a transcriptional regulator that was found to repress transcription of the epidermal growth factor (EGF) receptor and several other genes and is encoded by a 3-kb mRNA (R. Kageyama and I. Pastan, Cell, 59: 815-825, 1989; A. C. Johnson et al., J. Biol. Chem., 267: 1689-1694, 1992). To identify and characterize the GCF gene product at the cellular level, we have developed antibodies against a bacterially expressed GCF fusion protein. GCF antibodies recognize GCF present in extracts from human cells and causes a "supershift" of a protein DNA complex containing a GCF oligonucleotide binding site. The major form of GCF has a molecular weight of approximately M(r) 97,000, identical to that of GCF transiently expressed in CV1 cells by the vaccinia virus system. In addition, other less abundant species with slightly higher and lower apparent molecular weight are specifically recognized, suggesting extensive posttranslational modification. GCF is highly expressed in EGF receptor-negative human cell lines (HUT102, U266, and CA46) and in lower amounts in several EGF receptor-expressing cells (KB, A431, TMK, and HeLa). Cell fractionation studies indicate that GCF is predominantly localized in the nucleus. GCF is a stable protein with a relatively long half-life. In addition, GCF is a phosphoprotein, and the phosphorylated form is found to be associated with the nuclear compartment in both HUT102 and KB cells. Phosphorylation occurs on serine and threonine residues and is stimulated by okadaic acid, phorbol myristate acetate, and cyclic AMP, but not vanadate.(ABSTRACT TRUNCATED AT 250 WORDS)

SHC and GRB-2 are constitutively by an epidermal growth factor receptor with a point mutation in the transmembrane domain.

A single point mutation, Glu627--> Val, equivalent to the activating mutation in the Neu oncogene, was inserted in the transmembrane domain of the human epidermal growth factor (EGF) receptor. Unlike the wild type, Glu627-EGF receptor, transfected in NIH3T3 cells, gave rise to focal transformation and growth in agar even in the absence EGF. Constitutive activity of mutant EGF receptor amounted to 20% of that of wild type receptor stimulated by EGF. In addition, the mutant receptor was more sensitive to EGF, reaching maximum transforming activity at 5 ng/ml EGF. NIH3T3 cells expressing Glu627-EGF receptor showed a transformed phenotype and were not arrested in G0 upon serum deprivation. The mutant receptor was constitutively autophosphorylated, and several other cellular proteins were phosphorylated on tyrosine in absence of the ligand. Among these, the SHC adaptor protein was phosphorylated in absence of EGF, the other adaptor, GRB-2 was constitutively associated with the Glu627-EGF receptor in vivo and in vitro, and mitogen-activated protein kinase was constitutively phosphorylated. In contrast, other EGF receptor substrates, like phospholipase C gamma, were not phosphorylated in absence of EGF. The mutant receptor showed a higher sensitivity to cleavage by calpain both in absence and presence of EGF, appeared as a 170- and 150-kDa doublet in cell extracts, and a specific calpain inhibitor blocked the appearance of the 150-kDa form. Since the calpain cleavage site is located in the receptor cytoplasmic tail, this finding suggests that the Glu627 mutation induces a slightly different conformation in the EGF receptor intracellular domain. In conclusion, our data show that a point mutation in the EGF receptor transmembrane domain was able to constitutively activate the receptor and to induce transformation via constitutive activation of the Ras pathway.

Transmembrane signalling at the epidermal growth factor receptor. Positive regulation by the C-terminal phosphotyrosine residues.

Mutant epidermal growth factor (EGF) receptors (obtained by substitution of one, two or three C-terminal autophosphorylable tyrosine residues with phenylalanine residues or by deletion of the C-terminal 19 amino acids, including the distal tyrosine) were expressed in mouse NIH-3T3 fibroblast clones at densities comparable (less than 25% difference) with those in control clones expressing the wild-type receptor. Total EGF-induced phosphorylation of the mutated receptors was not appreciably changed with respect to controls, whereas autophosphorylation at tyrosine residues was decreased, especially in the double and the triple mutants. In the latter mutant, expression of the EGF-receptor-activated lipolytic enzyme phospholipase C gamma was unchanged, whereas its tyrosine phosphorylation induced by the growth factor was lowered to approx. 25% of that in the controls. In all of the cell clones employed, the accumulation of inositol phosphates induced by treatment with fetal calf serum varied only slightly, whereas the same effect induced by EGF was consistently lowered in those lines expressing mutated receptors. This decrease was moderate for those receptors missing only the distal tyrosine (point and deletion mutants), intermediate in the dual mutants and almost complete in the triple mutants. Likewise, increases in intracellular Ca2+ concentrations [( Ca2+]i) induced by fibroblast growth factor were approximately the same in all of the clones, whereas those induced by EGF were decreased in the mutants, again in proportion to the loss of the phosphorylable C-terminal tyrosine residues. The same trend occurred with membrane hyperpolarization, an effect secondary to the increase in [Ca2+]i via the activation of Ca2(+)-dependent K+ channels. We conclude that C-terminal autophosphorylable tyrosine residues play a positive role in the regulation of transmembrane signalling at the EGF receptor. The stepwise decrease in signal generation observed in single, double and triple point mutants suggest that the role of phosphotyrosine residues is not in the participation in specific amino acid sequences, but rather in the introduction of strong negative charges at strategic sites of the receptor tail. As a consequence of autophosphorylation, the receptor could become competent for specific association with phospholipase C gamma, with ensuing activation by tyrosine phosphorylation followed by the chains of intracellular responses ultimately leading to DNA synthesis and cell duplication.