Isolation of antibodies for phosphotyrosine by immunization with a v-abl oncogene-encoded protein

A single amino acid substitution in the v-Eyk intracellular domain results in activation of Stat3 and enhances cellular transformation

The receptor tyrosine kinase Eyk, a member of the Axl/Tyro3 subfamily, activates the STAT pathway and transforms cells when constitutively activated. Here, we compared the potentials of the intracellular domains of Eyk molecules derived from c-Eyk and v-Eyk to transform rat 3Y1 fibroblasts. The v-Eyk molecule induced higher numbers of transformants in soft agar and stronger activation of Stat3; levels of Stat1 activation by the two Eyk molecules were similar. A mutation in the sequence Y933VPL, present in c-Eyk, to the v-Eyk sequence Y933VPQ led to increased activation of Stat3 and increased transformation efficiency. However, altering another sequence, Y862VNT, present in both Eyk molecules to F862VNT markedly decreased transformation without impairing Stat3 activation. These results indicate that activation of Stat3 enhances transformation efficiency and cooperates with another pathway to induce transformation.

The human leukemia oncogene bcr-abl abrogates the anchorage requirement but not the growth factor requirement for proliferation

Proliferation of normal cells in a multicellular organism requires not only growth factors but also the proper attachment to the extracellular matrix. A hallmark of neoplastic transformation is the loss of anchorage dependence which usually accompanies the loss of growth factor requirement. The Bcr-Abl tyrosine kinase of human leukemias is shown here to abrogate only the anchorage, not the growth factor, requirement. Bcr-Abl-transformed cells grow in soft agar but do not proliferate in serum-free media. Bcr-Abl does not activate the mitogenic pathway, as indicated by its inability to induce enhancers such as the serum response element or the tetradecanoyl phorbol acetate response element (TRE). However, Bcr-Abl can alleviate the anchorage requirement for the induction of the TRE enhancer; i.e., it allows serum to activate the TRE in detached cells. This activity is dependent on the association of an active Bcr-Abl tyrosine kinase with the actin filaments. Despite its association with the adapter protein Grb2, Bcr-Abl's effect on the TRE enhancer is not blocked by dominant negative Ras or Raf. The finding that Bcr-Abl tyrosine kinase abrogates only anchorage dependence may have important implications on the pathogenesis of chronic myelogenous leukemia.

Autophosphorylation is required for high kinase activity and efficient transformation ability of proteins encoded by host range alleles of v-src

pp60v-src is a nonreceptor protein tyrosine kinase that can transform both chicken and rodent fibroblasts. The src homology 2 (SH2) domain of this protein serves a critical role in the regulation of protein tyrosine kinase activity. The host range proteins pp60v-src-L, which contains a deletion of a highly conserved residue (Phe-172) in the SH2 domain, and pp60v-src-PPP, which contains a change from a Leu to a Phe at amino acid 186 in the SH2 domain, transform chicken but not rat cells and have slightly reduced kinase activity measured in vitro. The data presented here show that these altered proteins require autophosphorylation on Tyr-416 for high kinase activity and transforming ability. In the absence of autophosphorylation, there is a further decrease of at least threefold in in vitro kinase activity relative to the phosphorylated host range parental protein, no morphological transformation, a reduction in anchorage independent growth, and no disruption of the actin cytoskeleton. In addition, these SH2 mutations abolish the ability of the SH2 domain to bind a phosphorylated peptide that corresponds to the autophosphorylation site of pp60src. Thus, like mutant alleles of c-src encoding transformation competent proteins, and unlike v-src, transformation by pp60v-src-F172 delta and pp60v-src-L186F is dependent on phosphorylation of Y-416 for high kinase activity and transformation ability. The dependence of transformation on phosphotyrosine is not a reflection of an intramolecular interaction between the autophosphorylation site and the SH2 domains since purified SH2 domains are incapable of binding phosphorylated autophosphorylation site peptides in vitro.

A coiled-coil oligomerization domain of Bcr is essential for the transforming function of Bcr-Abl oncoproteins

In Philadelphia chromosome-positive human leukemias, the c-abl proto-oncogene on chromosome 9 becomes fused to the bcr gene on chromosome 22, and chimeric Bcr-Abl proteins are produced. The fused Bcr sequences activate the tyrosine kinase, actin-binding, and transforming functions of Abl. Activation of the Abl transforming function has been shown to require two distinct domains of Bcr: domain 1 (Bcr amino acids 1 to 63) and domain 2 (Bcr amino acids 176 to 242). The amino acid sequence of domain 1 indicates that it may be a coiled-coil oligomerization domain. We show here that domain 1 of Bcr forms a homotetramer. Tetramerization of Bcr-Abl through Bcr domain 1 correlates with activation of the tyrosine kinase and F-actin-binding functions of Abl. Disruption of the coiled coil by insertional mutagenesis inactivates the oligomerization function as well as the ability of Bcr-Abl to transform Rat-1 fibroblasts or to abrogate interleukin-3 dependence in lymphoid cells. These results strongly suggest that Bcr-Abl oligomers are the active entities in transformation.

Abundant but inactive-state gp140proto-trk is expressed in neuroblastomas of patients with good prognosis

Steady-state levels of gp140proto-trk in cell lines and tumor tissues of neuroblastoma were examined by immunoblotting with anti-gp140proto-trk. The level of gp140proto-trk varied but showed good correlations with the stage of the tumor and the age of the patients at the time of diagnosis. Moreover, patients with higher expression of gp140proto-trk clearly had a far better survival rate than those with lower expression, suggesting that suppression of gp140proto-trk strongly correlates with the malignant conversion of the tumor. However, we found that neither autophosphorylation of gp140proto-trk nor tyrosine phosphorylation of cellular proteins was elevated in tumors of the higher expression group. These results suggest that gp140proto-trk does not actively participate in the process of transformation or the suppression of malignant conversion. Rather, the higher level of gp140proto-trk may reflect the greater level of differentiation of tumor cells.

Tyrosine phosphorylation of mammalian RNA polymerase II carboxyl-terminal domain

The carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II is composed of tandem repeats of the consensus sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser. Phosphorylation of the CTD occurs during formation of the initiation complex and is correlated with the transition from complex assembly to elongation. Previously, serine and threonine residues within the CTD have been shown to be modified by the addition of phosphate and by the addition of O-linked GlcNAc. Our results establish that the CTD is also modified in vivo by phosphorylation on tyrosine. Furthermore, a nuclear tyrosine kinase encoded by the c-abl protooncogene phosphorylates the CTD to a high stoichiometry in vitro. Under conditions of maximum phosphorylation, approximately 30 mol of phosphate are incorporated per mol of CTD. The observation that the CTD is not phosphorylated by c-Src tyrosine kinase under identical conditions indicates that the CTD is not a substrate of all tyrosine kinases. Phosphorylation of tyrosine residues within the CTD may modulate the interaction of RNA polymerase II with the preinitiation complex and, hence, may be important in regulating gene expression.

A coiled-coil oligomerization domain of Bcr is essential for the transforming function of Bcr-Abl oncoproteins

In Philadelphia chromosome-positive human leukemias, the c-abl proto-oncogene on chromosome 9 becomes fused to the bcr gene on chromosome 22, and chimeric Bcr-Abl proteins are produced. The fused Bcr sequences activate the tyrosine kinase, actin-binding, and transforming functions of Abl. Activation of the Abl transforming function has been shown to require two distinct domains of Bcr: domain 1 (Bcr amino acids 1 to 63) and domain 2 (Bcr amino acids 176 to 242). The amino acid sequence of domain 1 indicates that it may be a coiled-coil oligomerization domain. We show here that domain 1 of Bcr forms a homotetramer. Tetramerization of Bcr-Abl through Bcr domain 1 correlates with activation of the tyrosine kinase and F-actin-binding functions of Abl. Disruption of the coiled coil by insertional mutagenesis inactivates the oligomerization function as well as the ability of Bcr-Abl to transform Rat-1 fibroblasts or to abrogate interleukin-3 dependence in lymphoid cells. These results strongly suggest that Bcr-Abl oligomers are the active entities in transformation.

Functional coupling of the src-family protein tyrosine kinases p59fyn and p53/56lyn with the interleukin 2 receptor: implications for redundancy and pleiotropism in cytokine signal transduction

The binding of interleukin 2 (IL-2) to the IL-2 receptor (IL-2R) induces a rapid increase in tyrosine phosphorylation of cellular proteins. In a previous study, we have shown that p56lck (lck), a src-family protein tyrosine kinase (src-PTK), physically and functionally associates with the IL-2R beta chain (IL-2R beta). To further investigate a role of src-PTKs in IL-2 signaling, we analyzed a mouse pro-B-cell line, in which lck is not expressed detectably. We observed that in this cell line, IL-2 induces activation of at least two src-PTKs, p59fyn (fyn) and p53/56lyn (lyn). Interestingly, stimulation of this cell line with IL-3 also induces activation of src-PTKs. The activation of fyn or lyn seems to be selective for stimulation with IL-2 or IL-3 since stimulation with IL-6 fails to activate them. Furthermore, we provide evidence for the physical association of fyn with IL-2R beta. Taken together with previous results, our current study suggests that different src-PTKs, each of which is expressed in a cell-type-specific manner, can participate in the IL-2 signal transduction.

Synthetic phosphopeptide immunogens yield activation-specific antibodies to the c-erbB-2 receptor

We inoculated rabbits with synthetic phosphopeptides, duplicating a major autophosphorylation site of the c-erbB-2 protooncogene product. The rabbits produced antisera that, after reverse immunoaffinity purification, selectively recognize the erbB-2 protein in its enzymatically active configuration. These anti-phosphopeptide antisera identify a subset of erbB-2-positive human cell lines wherein the protein is constitutively active as a tyrosine kinase. Synthetic phosphopeptides incorporating informative protein phosphorylation sites may prove useful for generating antibodies that indicate the activation state of additional tyrosine kinases and perhaps other proteins phosphorylated on serine and threonine residues.

Increased tyrosine kinase activity of c-Src during calcium-induced keratinocyte differentiation

In cultured human epidermal keratinocytes, induction of differentiation by Ca2+ and ionophore treatment was found to result in rapid elevation of c-Src tyrosine kinase activity and inactivation of the c-Yes tyrosine kinase. Activation of c-Src kinase was accompanied by tyrosine dephosphorylation, which might be explained by a rapid increase in intracellular protein-tyrosine phosphatase activity. Ca(2+)-induced differentiation was also associated with altered tyrosine phosphorylation of several cellular proteins and correlated with a marked redistribution of intracellular phosphotyrosine from membrane and adhesion sites to the nucleus. Some of the c-Src protein was also found in the nucleus after Ca2+ treatment, and Ca(2+)-activated c-Src bound to three cellular proteins (120 kDa, 65 kDa, and 34 kDa). In agreement with these results, immunohistochemistry on human epidermis revealed an increase in c-Src expression and tyrosine phosphorylation in cells undergoing differentiation, which strongly suggests a possible role of non-receptor tyrosine kinases in epithelial cell maturation.

Activation of Src-like protein-tyrosine kinase Lyn and its association with phosphatidylinositol 3-kinase upon B-cell antigen receptor-mediated signaling

Crosslinking of membrane-bound immunoglobulins, which are B-cell antigen receptors, causes proliferation and differentiation of B cells or the inhibition of their growth. The receptor-mediated signaling involves tyrosine phosphorylation of cellular proteins. The Src-like protein-tyrosine kinase Lyn is expressed preferentially in B cells and is an intracytoplasmic constituent of the B-cell antigen receptor complex. Crosslinking of membrane-bound immunoglobulin M with antibody induced rapid increases in the kinase activities of Lyn and Lyn-associated phosphatidylinositol 3-kinase. Crosslinking of B-cell antigen receptor also induced association of Lyn with an 85-kDa noncatalytic subunit of phosphatidylinositol 3-kinase. Thus, Lyn is functionally associated with membrane-bound immunoglobulin M and seems likely to participate in B-cell antigen receptor-mediated signaling.

Aberrant elevation of tyrosine-specific phosphorylation in human gastric cancer cells

Phosphotyrosine-containing proteins in various human cancer cell lines were studied by immunoblotting with anti-phosphotyrosine antibody. Of 29 cell lines derived from oral epidermoid cancer, esophageal cancer, gastric cancer, colon cancer, pancreatic cancer, hepatocellular carcinoma and malignant melanoma, 3 of the 6 gastric cancer cells showed aberrant elevation of tyrosine-specific phosphorylation. On the other hand, both esophageal cancer cells and colon cancer cells, which were reported to have amplified epidermal growth factor receptor and activated p60v-src kinase, respectively, showed no apparent elevation of tyrosine-specific phosphorylation, and their profiles of phosphorylation were similar to that of normal human fibroblasts. Two gastric cancer cells, NUGC-4 and MKN-45, showed similar profiles of phosphorylation but their responses to growth factors differed from each other. Tyrosine phosphorylation in NUGC-4 was strongly activated by treatment with epidermal growth factor and quickly reduced by the acid treatment which is effective in removing growth factors from cellular surface receptors. On the contrary, phosphorylation in MKN-45 did not respond to either growth factor or acid treatment. These results suggest that NUGC-4 and MKN-45 have tyrosine kinases which are activated by different mechanisms but share similar substrates.

Specific proto-oncogenic tyrosine kinases of src family are enriched in cell-to-cell adherens junctions where the level of tyrosine phosphorylation is elevated

To approach the transmembrane signaling pathway in the cell-to-cell adherens junctions (AJ), AJ-specific tyrosine phosphorylation was analyzed. When various types of rat adult tissues were pretreated with sodium orthovanadate, a potent inhibitor of tyrosine phosphatase, immunofluorescence microscopy showed that anti-phosphotyrosine polyclonal antibody specifically stained the undercoat of the cell-to-cell AJ. This indicates that the tyrosine kinase activity is elevated at the undercoat of the cell-to-cell AJ of adult tissues. To identify tyrosine kinases responsible for the high level of tyrosine phosphorylation at AJ, we have performed in vitro phosphorylation experiments with cell-to-cell AJ isolated from rat liver (Tsukita, Sh. and Sa. Tsukita. 1989. J. Cell Biol. 108:31-41) and immunoblotting analyses with specific antibodies for tyrosine kinases. As a result, three proto-oncogenic tyrosine kinases of src family, c-yes, c-src, and lyn kinases, were identified as major tyrosine kinases in the cell-to-cell AJ of hepatocytes. Furthermore, it was immunofluorescently shown that at least two of these kinases, c-yes and c-src kinases, were enriched at the cell-to-cell AJ of various types of cells including hepatocytes. Based on these findings, it is concluded that, in various types of cells, specific proto-oncogenic tyrosine kinases of src-family (c-yes and c-src) are enriched to work as signal mediators in the cell-to-cell AJ where the level of tyrosine phosphorylation is elevated.

Membrane-anchored form of v-sis/PDGF-B induces mitogenesis without detectable PDGF receptor autophosphorylation

The v-sis protein is structurally and functionally related to PDGF. Forms of the v-sis protein which are anchored to the cell membrane via the transmembrane domain of the vesicular stomatitis virus G protein have been previously described (Hannink, M., and D.J. Donoghue. 1986. J. Cell Biol. 103:2311-2322). Several of these fusion proteins were shown to interact productively with the PDGF receptor (PDGFR) based on their ability to transform NIH 3T3 cells. In this report, we further characterized one of these membrane-anchored v-sis proteins, designated v-sis239-G. The gene encoding v-sis239-G was placed under control of the Drosophila melanogaster hsp70 promotor and synthesis of this protein was shown to induce a mitogenic response in NIH 3T3 cells. Unexpectedly, v-sis239-G did not induce detectable autophosphorylation of the PDGFR, in contrast to a similarly expressed secreted form of the v-sis protein. Thus, it appears that a PDGFR-mediated mitogenic response may be dissociated from detectable receptor autophosphorylation. Furthermore, induced synthesis of v-sis239-G was shown to lead to c-fos expression even in the absence of detectable receptor autophosphorylation. Interestingly, a nonmitogenic membrane-anchored form of the v-sis protein, designated v-sis239-G338, also induced c-fos without receptor autophosphorylation. These results raise interesting questions regarding the roles of autophosphorylation and c-fos induction in PDGFR-mediated signal transduction and suggest the possibility of an autophosphorylation-independent signal transduction pathway.

Nerve growth factor stimulates protein tyrosine phosphorylation in PC-12 pheochromocytoma cells

The cellular actions of nerve growth factor (NGF) and epidermal growth factor (EGF) may be mediated by changes in protein phosphorylation. The tyrosine phosphorylation of two predominant proteins of molecular mass 40 and 42 kDa is seen in PC-12 cells treated with NGF or EGF, correlating with activation of a previously identified serine/threonine protein kinase that phosphorylates microtubule-associated protein (MAP). Stimulation of phosphoprotein (pp) 40 and 42 phosphorylation and MAP kinase activity by NGF but not EGF is selectively attenuated by staurosporine and K-252A. Moreover, the time courses of pp40/42 phosphorylation and MAP kinase activation produced by NGF or EGF are identical. Chromatography of lysates from growth factor-treated cells on ion-exchange or hydrophobic-interaction HPLC resolves MAP kinase into two peaks, neither of which precisely coelutes with pp40 or pp42. One of these peaks (II) exhibits no detectable phosphotyrosine. The other peak (I) has some overlap with pp40. However, the activity residing in both peaks is almost completely inhibited after treatment with alkaline phosphatase, suggesting that, at least, serine/threonine phosphorylation is required for the activity of these enzymes. These data indicate that while tyrosine phosphorylation appears to be a critical early event in NGF action, the role of this modification in activation of MAP kinases remains unclear.

Activation of tyrosinase kinase and microfilament-binding functions of c-abl by bcr sequences in bcr/abl fusion proteins

Chronic myelogenous leukemia and one type of acute lymphoblastic leukemia are characterized by a 9;22 chronosome translocation in which 5' sequences of the bcr gene become fused to the c-abl proto-oncogene. The resulting chimeric genes encode bcr/abl fusion proteins which have deregulated tyrosine kinase activity and appear to play an important role in induction of these leukemias. A series of bcr/abl genes were constructed in which nested deletions of the bcr gene were fused to the c-abl gene. The fusion proteins encoded by these genes were assayed for autophosphorylation in vivo and for differences in subcellular localization. Our results demonstrate that bcr sequences activate two functions of c-abl; the tyrosine kinase activity and a previously undescribed microfilament-binding function. Two regions of bcr which activate these functions to different degrees have been mapped: amino acids 1 to 63 were strongly activating and amino acids 64 to 509 were weakly activating. The tyrosine kinase and microfilament-binding functions were not interdependent, as a kinase defective bcr/abl mutant still associated with actin filaments and a bcr/abl mutant lacking actin association still had deregulated kinase activity. Modification of actin filament functions by the bcr/abl tyrosine kinase may be an important event in leukemogenesis.

Expression of v-src in a murine T-cell hybridoma results in constitutive T-cell receptor phosphorylation and interleukin 2 production

Ligand binding to the T-cell antigen receptor results in phosphatidylinositol hydrolysis and the resultant activation of protein kinase C, as well as the activation of a receptor-coupled protein-tyrosine kinase. As a model for tyrosine kinase activation in T cells, we used retroviral gene transfer to express the v-src oncogene in an antigen-specific murine T-cell hybridoma. Clones that expressed v-src mRNA demonstrated constitutive tyrosine phosphorylation of several cellular substrates, including the zeta chain of the T-cell receptor, and constitutive interleukin 2 production. Thus, expression of a constitutively active protein-tyrosine kinase such as pp60v-src appears to be sufficient to induce the expression of at least one gene critical to the process of T-cell activation.

Activation of tyrosinase kinase and microfilament-binding functions of c-abl by bcr sequences in bcr/abl fusion proteins

Chronic myelogenous leukemia and one type of acute lymphoblastic leukemia are characterized by a 9;22 chronosome translocation in which 5' sequences of the bcr gene become fused to the c-abl proto-oncogene. The resulting chimeric genes encode bcr/abl fusion proteins which have deregulated tyrosine kinase activity and appear to play an important role in induction of these leukemias. A series of bcr/abl genes were constructed in which nested deletions of the bcr gene were fused to the c-abl gene. The fusion proteins encoded by these genes were assayed for autophosphorylation in vivo and for differences in subcellular localization. Our results demonstrate that bcr sequences activate two functions of c-abl; the tyrosine kinase activity and a previously undescribed microfilament-binding function. Two regions of bcr which activate these functions to different degrees have been mapped: amino acids 1 to 63 were strongly activating and amino acids 64 to 509 were weakly activating. The tyrosine kinase and microfilament-binding functions were not interdependent, as a kinase defective bcr/abl mutant still associated with actin filaments and a bcr/abl mutant lacking actin association still had deregulated kinase activity. Modification of actin filament functions by the bcr/abl tyrosine kinase may be an important event in leukemogenesis.

Role of platelet membrane glycoprotein IIb-IIIa in agonist-induced tyrosine phosphorylation of platelet proteins

Treatment of platelets with thrombin was shown previously to induce rapid changes in tyrosine phosphorylation of several platelet proteins. In this report, we demonstrate that a variety of agonists which induce platelet aggregation also stimulate tyrosine phosphorylation of three proteins with apparent molecular masses of 84, 95, and 97 kD. Since platelet aggregation requires the agonist-induced activation of an integrin receptor (GP IIb-IIIa) as well as the binding of fibrinogen to this receptor, we examined the relationship between tyrosine phosphorylation and the function of GP IIb-IIIa. When platelets were examined under conditions that either precluded the activation of GP IIb-IIIa (prior disruption of the complex by EGTA at 37 degrees C) or the binding of fibrinogen (addition of RGDS or an inhibitory mAb), tyrosine phosphorylation of the 84-, 95-, and 97-kD proteins was not observed. However, although both GP IIb-IIIa activation and fibrinogen binding were necessary for tyrosine phosphorylation, they were not sufficient since phosphorylation was observed only under conditions in which the activated platelets were stirred and allowed to aggregate. In contrast, tyrosine phosphorylation was not dependent on another major platelet response, dense granule secretion. Furthermore, granule secretion did not require tyrosine phosphorylation of this set of proteins. These experiments demonstrate that agonist-induced tyrosine phosphorylation is linked to the process of GP IIb-IIIa-mediated platelet aggregation. Thus, tyrosine phosphorylation may be required for events associated with platelet aggregation or for events that follow aggregation.

Tubulin is phosphorylated at tyrosine by pp60c-src in nerve growth cone membranes

We show here that tubulin is the major in vivo substrate of the tyrosine-specific protein kinase pp60c-src in nerve growth cone membranes. Phosphotyrosine antibodies were used to demonstrate phosphotyrosyl residues in a subpopulation of alpha- and beta-tubulin that was highly enriched in a subcellular fraction of growth cone membranes from fetal rat brain. The presence of phosphotyrosine-modified isoforms of alpha- and beta-tubulin in vivo was confirmed by 32p labeling of rat cortical neurons in culture. Tubulin in growth cone membranes was phosphorylated at tyrosine in endogenous membrane phosphorylation reactions (0.068 mol phosphotyrosine/mol alpha-tubulin and 0.045 mol phosphotyrosine/mol beta-tubulin), and phosphorylation was specifically inhibited by antibodies directed against pp60c-src, which is localized in the growth cone membranes. pp60c-src was capable of directly phosphorylating tubulin as shown in immune complex kinase assays with purified brain tubulin. Phosphopeptide mapping revealed a limited number of sites of tyrosine phosphorylation in alpha- and beta-tubulin, with similar phosphopeptides observed in vivo and in vitro. These results reveal a novel posttranslational modification of tubulin that could regulate microtubule dynamics at the growth cone.

Introduction of nerve growth factor (NGF) receptors into a medulloblastoma cell line results in expression of high- and low-affinity NGF receptors but not NGF-mediated differentiation

Expression of the cloned human nerve growth factor receptor (NGFR) cDNA in cell lines can generate both high- and low-affinity binding sites. Since the inability to respond appropriately to differentiation factors such as NGF may contribute to determining the malignant phenotype of neuroblastomas, we sought to determine whether the same is true of medulloblastomas. To generate a human central nervous system neuronal cell line that would respond to NGF, we infected the medulloblastoma cell line D283 MED with a defective retrovirus carrying the cDNA coding for the human NGFR. The resultant cells (MED-NGFR) expressed abundant low- and high-affinity NGFRs, and NGF treatment induced a rapid transient increase of c-fos mRNA in the NGFR-expressing cells but not in the parent line or in cells infected with virus lacking the cDNA insert. However, the MED-NGFR cells did not internalize the NGFR at high efficiency, nor did they differentiate in response to NGF. Three important conclusions emerge from this study: (i) internalization of NGFRs is not necessary for some early rapid transcriptional effects of NGF; (ii) an unknown factor(s) that cooperates with the cloned NGFR in allowing high-affinity NGF binding is found in a primitive central nervous system cell line; and (iii) NGFRs introduced into and expressed by D283 MED (i.e., MED-NGFR) cells are partially functional but are unable to induce differentiation in these primitive neuron-like tumor cells, implying that high-efficiency receptor-mediated endocytosis of NGF and its receptor may be a necessary step in the cascade of events leading to NGF-mediated differentiation.

An alternative non-tyrosine protein kinase product of the c-src gene in chicken skeletal muscle

While the c-src locus is expressed as a 4.0-kilobase (kb) mRNA coding for pp60c-src in various chicken tissues, including embryonic muscle, it is expressed as a novel 3.0-kb mRNA in adult skeletal muscle. We have analyzed the primary structure of this alternatively transcribed and spliced c-src mRNA. The sequence revealed three open reading frames, with the previously defined c-src exons 1 through 5 or 6 comprising the third, on the 3' untranslated region of this 3-kb mRNA. The exons coding for the tyrosine kinase domain of pp60c-src were excluded. On the 5' side, 2 kb of sequence upstream from the previously defined exon 1 of the c-src gene was included in this mRNA. The start site for the 3-kb mRNA probably lies downstream of that for the 4-kb mRNA. The first reading frame of the 3.0-kb mRNA, called sur (for src upstream region), encoded a 24-kilodalton (kDa) protein product rich in cysteine and proline residues. In vitro analysis indicated that the 24-kDa sur protein was membrane associated. Antibodies to sur protein detected in vivo a 24-kDa muscle-specific protein which was developmentally regulated and corresponded to the switch from the 4-kb to the 3-kb c-src mRNA. A striking kinetic pattern of appearance of sur protein and disappearance of pp60c-src suggests that the expression of these two proteins is inversely related.

An alternative non-tyrosine protein kinase product of the c-src gene in chicken skeletal muscle

While the c-src locus is expressed as a 4.0-kilobase (kb) mRNA coding for pp60c-src in various chicken tissues, including embryonic muscle, it is expressed as a novel 3.0-kb mRNA in adult skeletal muscle. We have analyzed the primary structure of this alternatively transcribed and spliced c-src mRNA. The sequence revealed three open reading frames, with the previously defined c-src exons 1 through 5 or 6 comprising the third, on the 3' untranslated region of this 3-kb mRNA. The exons coding for the tyrosine kinase domain of pp60c-src were excluded. On the 5' side, 2 kb of sequence upstream from the previously defined exon 1 of the c-src gene was included in this mRNA. The start site for the 3-kb mRNA probably lies downstream of that for the 4-kb mRNA. The first reading frame of the 3.0-kb mRNA, called sur (for src upstream region), encoded a 24-kilodalton (kDa) protein product rich in cysteine and proline residues. In vitro analysis indicated that the 24-kDa sur protein was membrane associated. Antibodies to sur protein detected in vivo a 24-kDa muscle-specific protein which was developmentally regulated and corresponded to the switch from the 4-kb to the 3-kb c-src mRNA. A striking kinetic pattern of appearance of sur protein and disappearance of pp60c-src suggests that the expression of these two proteins is inversely related.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Activation of the proto-oncogene p60c-src by point mutations in the SH2 domain

To investigate the importance of a conserved region spanning residues 137 to 241 in the noncatalytic domain of p60c-src (SH2 region), we used oligonucleotide-directed mutagenesis to change residues that are highly conserved in this region. Chicken embryo fibroblasts infected with a p60c-src variant containing arginine instead of tryptophan at residue 148 (W148R) appeared more rounded than cells overexpressing a normal c-src gene, and they formed colonies in soft agar. p60c-src variants containing serine instead of arginine at residue 155 (R155S) or isoleucine instead of glycine at residue 170 (G170I) also appeared transformed and were anchorage independent, but to a lesser extent than W148R. Mutation of residue 201 from histidine to leucine (H201L) had no observable effect. The in vitro kinase activity of cells infected with W148R or G170I was elevated twofold. Expression of p60W148R (or, to a lesser extent, of p60G170I) increased the number of proteins phosphorylated on tyrosine in infected cells. All of the mutants were phosphorylated in vivo on Tyr-527, instead of Tyr-416 as observed for p60v-src. Immunoprecipitated p60W148R and p60G170I were found to be associated with a phosphatidylinositol kinase activity, a factor which appears to be necessary for transformation by tyrosine-specific protein kinases. These results show that a single point mutation in the SH2 region of the cellular src gene can activate its transforming potential. This type of activation is in a new category of alterations at the amino terminus that activate but do not cause a shift in phosphorylation at the carboxy terminus.

Activation of the proto-oncogene p60c-src by point mutations in the SH2 domain

To investigate the importance of a conserved region spanning residues 137 to 241 in the noncatalytic domain of p60c-src (SH2 region), we used oligonucleotide-directed mutagenesis to change residues that are highly conserved in this region. Chicken embryo fibroblasts infected with a p60c-src variant containing arginine instead of tryptophan at residue 148 (W148R) appeared more rounded than cells overexpressing a normal c-src gene, and they formed colonies in soft agar. p60c-src variants containing serine instead of arginine at residue 155 (R155S) or isoleucine instead of glycine at residue 170 (G170I) also appeared transformed and were anchorage independent, but to a lesser extent than W148R. Mutation of residue 201 from histidine to leucine (H201L) had no observable effect. The in vitro kinase activity of cells infected with W148R or G170I was elevated twofold. Expression of p60W148R (or, to a lesser extent, of p60G170I) increased the number of proteins phosphorylated on tyrosine in infected cells. All of the mutants were phosphorylated in vivo on Tyr-527, instead of Tyr-416 as observed for p60v-src. Immunoprecipitated p60W148R and p60G170I were found to be associated with a phosphatidylinositol kinase activity, a factor which appears to be necessary for transformation by tyrosine-specific protein kinases. These results show that a single point mutation in the SH2 region of the cellular src gene can activate its transforming potential. This type of activation is in a new category of alterations at the amino terminus that activate but do not cause a shift in phosphorylation at the carboxy terminus.

Association of the v-crk oncogene product with phosphotyrosine-containing proteins and protein kinase activity

An antiserum specific for P47gag-crk, the oncogene product of avian sarcoma virus CT10, was used to investigate possible crk-binding proteins. In in vitro kinase assays, four proteins were phosphorylated in anti-crk immunoprecipitates. Tyrosine, serine, and threonine residues were phosphorylated. A number of tyrosine-phosphorylated proteins were identified in anti-crk immunoprecipitates of 32P-labeled cells, including the three major phosphotyrosine-containing proteins of CT10-infected cells. These three proteins also bound to bacterially synthesized crk protein. These results suggest that the crk transforming protein can stably associate with both endogenous kinases and cellular kinase substrates.

A glycoprotein in the plasma membrane matrix as a major potential substrate of p60v-src

A potential substrate of p60v-src in Rous sarcoma virus-transformed cells was found to be a 130-kilodalton (kDa) glycoprotein which binds to lectin-Sepharose and can be immunoprecipitated by an anti-phosphotyrosine antibody. This glycoprotein was shown to be distinct from the fibronectin receptor and a cellular protein phosphorylated in p60v-src immune complexes. The protein was a transmembrane protein localized in the plasma membrane and resistant to extraction with Triton X-100. The 130-kDa protein was also highly phosphorylated in cells transformed by Fujinami sarcoma virus or Y73 but not in cells infected with Rous sarcoma virus mutants that encode p60v-src lacking myristoylated N termini. Phosphorylation of this glycoprotein was temperature dependent in cells infected with temperature-sensitive mutants. The good correlation between its phosphorylation and morphological transformation, together with its relative abundance among phosphorylated proteins and its subcellular localization, suggests that phosphorylation of the 130-kDa glycoprotein is one of the primary events important for cell transformation by p60v-src and related oncogene products.

A glycoprotein in the plasma membrane matrix as a major potential substrate of p60v-src

A potential substrate of p60v-src in Rous sarcoma virus-transformed cells was found to be a 130-kilodalton (kDa) glycoprotein which binds to lectin-Sepharose and can be immunoprecipitated by an anti-phosphotyrosine antibody. This glycoprotein was shown to be distinct from the fibronectin receptor and a cellular protein phosphorylated in p60v-src immune complexes. The protein was a transmembrane protein localized in the plasma membrane and resistant to extraction with Triton X-100. The 130-kDa protein was also highly phosphorylated in cells transformed by Fujinami sarcoma virus or Y73 but not in cells infected with Rous sarcoma virus mutants that encode p60v-src lacking myristoylated N termini. Phosphorylation of this glycoprotein was temperature dependent in cells infected with temperature-sensitive mutants. The good correlation between its phosphorylation and morphological transformation, together with its relative abundance among phosphorylated proteins and its subcellular localization, suggests that phosphorylation of the 130-kDa glycoprotein is one of the primary events important for cell transformation by p60v-src and related oncogene products.

Mutations of the platelet-derived growth factor receptor that cause a loss of ligand-induced conformational change, subtle changes in kinase activity, and impaired ability to stimulate DNA synthesis

The wild-type and two mitogenic-defective mutants of the type beta receptor for platelet-derived growth factor (PDGF) were expressed in Chinese hamster ovary cells. In the first mutant, delta Ki, 82 of 104 amino acids in the kinase insert region were deleted. This mutant was recently reported to be defective in mediating DNA synthesis. In the second mutant, Y825F, tyrosine 825 was converted to phenylalanine by a point mutation. We report here that this mutant is also defective in mediating PDGF-stimulated DNA synthesis. Both mutants were capable of eliciting many of the early responses to PDGF, including receptor autophosphorylation. However, neither mutant was capable of undergoing PDGF-stimulated change in receptor conformation or of phosphorylating exogenous substrate in an in vitro assay. These data suggest that changes in receptor conformation and efficient utilization of specific tyrosine kinase substrates are important for the stimulation of cell proliferation of PDGF and that phosphorylation of tyrosine 825 may be involved in signal transduction.

Mutations of the platelet-derived growth factor receptor that cause a loss of ligand-induced conformational change, subtle changes in kinase activity, and impaired ability to stimulate DNA synthesis

The wild-type and two mitogenic-defective mutants of the type beta receptor for platelet-derived growth factor (PDGF) were expressed in Chinese hamster ovary cells. In the first mutant, delta Ki, 82 of 104 amino acids in the kinase insert region were deleted. This mutant was recently reported to be defective in mediating DNA synthesis. In the second mutant, Y825F, tyrosine 825 was converted to phenylalanine by a point mutation. We report here that this mutant is also defective in mediating PDGF-stimulated DNA synthesis. Both mutants were capable of eliciting many of the early responses to PDGF, including receptor autophosphorylation. However, neither mutant was capable of undergoing PDGF-stimulated change in receptor conformation or of phosphorylating exogenous substrate in an in vitro assay. These data suggest that changes in receptor conformation and efficient utilization of specific tyrosine kinase substrates are important for the stimulation of cell proliferation of PDGF and that phosphorylation of tyrosine 825 may be involved in signal transduction.

Identification of a developmentally regulated protein-tyrosine kinase by using anti-phosphotyrosine antibodies to screen a cDNA expression library

To identify the protein-tyrosine kinases that are expressed during chicken embryonic development, a 10-day chicken embryo cDNA expression library was screened with anti-phosphotyrosine antibodies. Of the positive clones isolated, many encoded the same protein-tyrosine kinase, which we designate Cek1 (chicken embryo kinase 1). Its amino acid sequence suggests that the Cek1 protein is a transmembrane tyrosine kinase and presumably the receptor for an unknown ligand. Antibodies prepared to the cloned Cek1 kinase recognize, in immunoblotting experiments, two protein bands with apparent molecular weights of 100,000 and 110,000. The Cek1 protein was detected in many chicken embryonic tissues, but not in the corresponding adult tissues (with the exception of brain). The Cek1 kinase appears to be very closely related to two protein-tyrosine kinases whose partial sequences have been recently determined, human Flg and mouse Bek. Cloning using anti-phosphotyrosine antibodies has allowed us to detect, in addition to Cek1, several other protein-tyrosine kinases that are expressed during chicken embryonic development, some of which have not been previously identified.

Deletions within the amino-terminal half of the c-src gene product that alter the functional activity of the protein

To examine how amino acid sequences outside of the catalytic domain of pp60c-src influence the functional activity of this protein, we have introduced deletion mutations within the amino-terminal half of pp60c-src. These mutations caused distinct changes in the biochemical properties of the c-src gene products and in the properties of cells infected with retroviruses carrying these mutant c-src genes. Cells expressing the c-srcNX protein, which contains a deletion of amino acids 15 to 89, displayed a refractile, spindle-shaped morphology, formed intermediate-sized, tightly packed colonies in soft agar, and contained elevated levels of cellular phosphotyrosine-containing proteins. Thus, deletion of amino acids 15 to 89 can activate the kinase activity and transforming potential of the c-src gene product. Deletion of amino acids 112 to 225, however, did not increase the kinase activity or transforming ability of pp60c-src; indeed, deletion of these sequences in c-srcHP suppressed phenotypic alterations induced by pp60c-src. Cells expressing the c-srcNP or c-srcBS gene products (containing deletions of amino acids 15 to 225 and 55 to 169, respectively) displayed a fusiform, refractile morphology and formed diffuse colonies in soft agar; the mutant proteins displayed an increased in vitro protein-tyrosine kinase activity. However, only a few cellular proteins contained elevated levels of phosphotyrosine in vivo. Thus, deletions downstream of amino acid 89 severely restricted the ability of c-src to phosphorylate cellular substrates in vivo without affecting the intrinsic tyrosine kinase activity of the c-src gene product. These results suggest the existence of at least two modulatory regions within the amino-terminal half of pp60c-src that are important for the regulation of tyrosine kinase activity and for the interaction of pp60c-src with cellular substrates.

Novel tyrosine kinase substrates from Rous sarcoma virus-transformed cells are present in the membrane skeleton

We have previously reported the production of monoclonal antibodies directed against phosphotyrosine, which is the modification induced by many oncogene products and growth factor receptors. One of these antiphosphotyrosine antibodies (py20) was used in affinity chromatography to isolate phosphotyrosine (PY)-containing proteins from Rous sarcoma virus-transformed chick embryo fibroblasts (RSV-CEFs). Mice were immunized with these PY-proteins for the production of monoclonal antibodies to individual substrates. Fifteen antibodies were generated in this way to antigens with molecular masses of 215, 76, 60, and 22 kD. Antibodies to individual substrates were used to analyze the subcellular location in normal and RSV-CEFs. Antibodies to the 215- and 76-kD antigen stained focal contacts when used in immunofluorescence microscopy while anti-22-kD protein antibodies resulted in punctate staining concentrated in the margins of cells and in parallel arrays. Both distributions were altered in transformed cells. When cells were extracted with nonionic detergent under conditions that stabilize the cytoskeleton, 50% of the 76-kD protein and greater than 90% of the 22-kD protein fractionated with the cytoskeleton. This study offers a new approach to both the identification of membrane skeletal proteins in fibroblasts and changes that occur upon transformation by an activated tyrosine kinase.

Heat shock induces protein tyrosine phosphorylation in cultured cells

We examined the effect of heat shock on protein tyrosine phosphorylation in cultured animal cells using antiphosphotyrosine antibodies in immunoblotting and immunofluorescence microscopy experiments. Heat shock significantly elevated the level of phosphotyrosine in proteins in most of the cultured cells examined, including fibroblasts, epithelial cells, nerve cells, and muscle cells, but not in Rous sarcoma virus-transformed fibroblasts. The increase in protein tyrosine phosphorylation induced by heat shock occurred in proteins with a wide range of molecular masses and was dependent on the temperature and duration of the heat shock.

Role of tyrosine kinase and membrane-spanning domains in signal transduction by the platelet-derived growth factor receptor

Three types of mutations were introduced into the platelet-derived growth factor (PDGF) receptor to cause a loss of PDGF-stimulated tyrosine kinase activity: (i) a point mutation of the ATP-binding site, (ii) a deletion of the carboxyl-terminal region, and (iii) replacement of the membrane-spanning sequences by analogous transmembrane sequences of other receptors. Transfectants expressing mutated receptors bind, 125I-labeled PDGF with a high affinity but had no PDGF-sensitive tyrosine kinase activity, phosphatidylinositol turnover, increase in the intracellular calcium concentration, change in cellular pH, or stimulation of DNA synthesis. However, PDGF-induced receptor down regulation was normal in the mutant cells. These results indicate that the transmembrane sequence has a specific signal-transducing function other than merely serving as a membrane anchor and that the receptor kinase activity is necessary for most responses to PDGF but is not required for receptor down regulation.

Novel tyrosine kinase identified by phosphotyrosine antibody screening of cDNA libraries

In an attempt to clone protein tyrosine kinases, antiphosphotyrosine antibodies were used to screen lambda gt11 cDNA expression libraries. By this method, a 2.5-kilobase cDNA encoding a novel tyrosine kinase was isolated from a mouse liver cDNA library. This new gene is most closely related to the receptor tyrosine kinases ret, fms, and kit.

Deletions within the amino-terminal half of the c-src gene product that alter the functional activity of the protein

To examine how amino acid sequences outside of the catalytic domain of pp60c-src influence the functional activity of this protein, we have introduced deletion mutations within the amino-terminal half of pp60c-src. These mutations caused distinct changes in the biochemical properties of the c-src gene products and in the properties of cells infected with retroviruses carrying these mutant c-src genes. Cells expressing the c-srcNX protein, which contains a deletion of amino acids 15 to 89, displayed a refractile, spindle-shaped morphology, formed intermediate-sized, tightly packed colonies in soft agar, and contained elevated levels of cellular phosphotyrosine-containing proteins. Thus, deletion of amino acids 15 to 89 can activate the kinase activity and transforming potential of the c-src gene product. Deletion of amino acids 112 to 225, however, did not increase the kinase activity or transforming ability of pp60c-src; indeed, deletion of these sequences in c-srcHP suppressed phenotypic alterations induced by pp60c-src. Cells expressing the c-srcNP or c-srcBS gene products (containing deletions of amino acids 15 to 225 and 55 to 169, respectively) displayed a fusiform, refractile morphology and formed diffuse colonies in soft agar; the mutant proteins displayed an increased in vitro protein-tyrosine kinase activity. However, only a few cellular proteins contained elevated levels of phosphotyrosine in vivo. Thus, deletions downstream of amino acid 89 severely restricted the ability of c-src to phosphorylate cellular substrates in vivo without affecting the intrinsic tyrosine kinase activity of the c-src gene product. These results suggest the existence of at least two modulatory regions within the amino-terminal half of pp60c-src that are important for the regulation of tyrosine kinase activity and for the interaction of pp60c-src with cellular substrates.

Phosphorylation of cellular proteins in Rous sarcoma virus-infected cells: analysis by use of anti-phosphotyrosine antibodies

The protein substrates for the tyrosine protein kinases in cells transformed by avian sarcoma viruses were analyzed by gel electrophoresis in combination with immunoblotting or immunoprecipitation by antibodies against phosphotyrosine. We found that greater than 90% of phosphotyrosine-containing cellular proteins can be immunoprecipitated by these antibodies. The level of phosphotyrosine-containing cellular proteins detectable by this method markedly increased upon transformation with Rous sarcoma virus, and more than 20 distinct bands of such proteins were found in lysates of Rous sarcoma virus-transformed cells. Most of these phosphotyrosine-containing proteins had not been identified by other methods, and their presence appeared to correlate with morphological transformation in cells infected with various Rous sarcoma virus mutants and Y73, PRCII, and Fujinami sarcoma viruses. However, considerably different patterns were obtained with cells infected with nontransforming Rous sarcoma virus mutants that encode nonmyristylated src kinases, indicating that most substrates that correlate with transformation can only be recognized by p60v-src associated with the plasma membrane.

Thrombin treatment induces rapid changes in tyrosine phosphorylation in platelets

We previously demonstrated that platelets express high levels of the tyrosine protein kinase pp60c-src. By a quantitative immunoblot assay, it is shown in this report that pp60c-src represents 0.2-0.4% of total platelet protein. The expression of high levels of pp60c-src in platelets correlated with high levels of total cell phosphotyrosine. Unstimulated platelets were shown to possess numerous phosphotyrosine-containing proteins by immunoblot analysis using antibodies that specifically recognize phosphotyrosine residues. To examine whether the pattern of phosphotyrosine-containing proteins changes upon platelet activation, lysates from thrombin- and phorbol ester-treated platelets were subjected to immunoblot analysis. Novel phosphotyrosine-containing proteins were detected within seconds following platelet stimulation. These results suggest that tyrosine phosphorylation, perhaps mediated by pp60c-src, may be involved in events associated with platelet activation.

A mutation in v-src that removes a single conserved residue in the SH-2 domain of pp60v-src restricts transformation in a host-dependent manner

The v-src oncogene of Rous sarcoma virus (RSV) is able to transform both avian and mammalian cells, but the mutant allele v-src-L displays a host range dependence for transformation, transforming chicken but not rat cells with wild-type efficiency. This host range restriction can be detected by measuring growth in low serum, saturation density, and anchorage independent growth. In addition, rat cells expressing v-src-L do not form tumors in syngeneic rats or nude mice, but RSV carrying the mutant allele causes tumors in chicks, although at a reduced efficiency and with increased latency. To determine the lesion responsible for this phenotype, we sequenced the entire v-src gene from the parental B77 strain of RSV, as well as the mutant allele. v-src-L is missing 3 nucleotides present in the wild-type parent, RSV B31, eliminating Phe-172, an invariant residue in a conserved region of src-related proteins known as SH-2. The kinase activity of pp60v-src-L was indistinguishable from that of the wild type in chicken cells but was significantly reduced in rat cells as assayed by an in vitro immune complex assay; in vivo phosphorylation of one specific substrate, p36 (calpactin I heavy chain); and total phosphotyrosine-containing proteins. In addition, the pattern of phosphotyrosine-containing proteins in rat cells was qualitatively different when cells containing pp60v-src-L were compared with cells with wild-type pp60v-src, even though both pp60v-src proteins were membrane associated. The data are consistent with a role for the SH-2 region in substrate specificity.

Deletions and insertions within an amino-terminal domain of pp60v-src inactivate transformation and modulate membrane stability

We previously showed (V. W. Raymond and J. T. Parsons, Virology 160:400-410, 1987) that variants of the Prague A strain of Rous sarcoma virus containing large deletions impinging on a region of the src gene encoding amino acid residues 143 to 169 were defective for transformation of chicken cells in culture. Here we report that introduction of small (tri-and tetrapeptide) deletions into a region of pp60v-src containing amino acid residues 155 to 175 was found to inactivate transformation. In addition, insertion of four, but not one, amino acid residues at position 161 also inhibited transformation. Biochemical analysis of the src proteins encoded by individual transformation-defective variants revealed that the structural alterations introduced into this domain had only marginal effects upon src tyrosine-specific protein kinase activity. However, the src proteins encoded by defective variants exhibited a significantly shorter half-life within the cell, although these proteins efficiently and rapidly associated with cellular membranes. Our results suggest that the structural domain encompassing residues 155 to 177 may influence the stability of pp60src in the cellular membrane, possibly via the interaction of src with a cellular membrane component(s) or substrate(s).

Role of tyrosine kinase and membrane-spanning domains in signal transduction by the platelet-derived growth factor receptor

Three types of mutations were introduced into the platelet-derived growth factor (PDGF) receptor to cause a loss of PDGF-stimulated tyrosine kinase activity: (i) a point mutation of the ATP-binding site, (ii) a deletion of the carboxyl-terminal region, and (iii) replacement of the membrane-spanning sequences by analogous transmembrane sequences of other receptors. Transfectants expressing mutated receptors bind, 125I-labeled PDGF with a high affinity but had no PDGF-sensitive tyrosine kinase activity, phosphatidylinositol turnover, increase in the intracellular calcium concentration, change in cellular pH, or stimulation of DNA synthesis. However, PDGF-induced receptor down regulation was normal in the mutant cells. These results indicate that the transmembrane sequence has a specific signal-transducing function other than merely serving as a membrane anchor and that the receptor kinase activity is necessary for most responses to PDGF but is not required for receptor down regulation.

Novel tyrosine kinase identified by phosphotyrosine antibody screening of cDNA libraries

In an attempt to clone protein tyrosine kinases, antiphosphotyrosine antibodies were used to screen lambda gt11 cDNA expression libraries. By this method, a 2.5-kilobase cDNA encoding a novel tyrosine kinase was isolated from a mouse liver cDNA library. This new gene is most closely related to the receptor tyrosine kinases ret, fms, and kit.

A recessive cellular mutation in v-fes-transformed mink cells restores contact inhibition and anchorage-dependent growth

A contact-inhibited revertant of mink cells transformed by the Gardner-Arnstein strain of feline sarcoma virus was isolated by fluorescence-activated sorting of cells stained with the mitochondria-specific dye rhodamine 123. The revertant cell line exhibited a decrease in its proliferative rate and saturation density and a complete loss of its capacity for anchorage-independent growth, but it remained tumorigenic when inoculated into nude mice. The revertant cells retained a rescuable Gardner-Arnstein feline sarcoma provirus, expressed high levels of the v-fes oncogene product and its associated tyrosine kinase activity, manifested elevated levels of phosphotyrosine-containing cellular proteins similar to those observed in v-fes-transformed cells, and were refractory to retransformation by retroviruses containing the v-fes, v-fms, and v-ras oncogenes. Fusion of the revertant and parental cells generated somatic cell hybrids which formed colonies in semisolid medium, indicating that the block in transformation was recessive. These data together with the observation that the revertant phenotype is unstable in continuous culture suggest that the loss of transformation is due to the presence of limiting quantities of a gene product which functions downstream of the v-fes-coded kinase in the mitogenic pathway.

Nerve growth factor induces protein-tyrosine phosphorylation

When the sympathetic nerve-like cell line PC12 is exposed to nerve growth factor (NGF), there is a rapid and transient phosphorylation of tyrosine residues in cellular proteins, as demonstrated by immunoblotting of cell extracts with high-affinity polyclonal antibodies specific for phosphotyrosine residues. Epidermal growth factor (EGF), which does not cause the morphological differentiation of PC12 cells that is produced by NGF, also induces protein-tyrosine phosphorylation. The methyltransferase inhibitor, 5'-methylthioadenosine, which is known to block the NGF-mediated morphological differentiation of PC12 cells, also inhibits the induction of protein-tyrosine phosphorylation by NGF. 5'-Methylthioadenosine has no effect, however, on EGF-stimulated phosphorylation of tyrosine residues in cellular proteins. In addition, low temperature markedly slows the rate of protein-tyrosine phosphorylation stimulated by NGF, but it has no effect on the time course of protein-tyrosine phosphorylation induced by EGF. These data suggest that NGF and EGF induce protein-tyrosine phosphorylation in PC12 cells by different mechanisms.

Phosphorylation of cellular proteins in Rous sarcoma virus-infected cells: analysis by use of anti-phosphotyrosine antibodies

The protein substrates for the tyrosine protein kinases in cells transformed by avian sarcoma viruses were analyzed by gel electrophoresis in combination with immunoblotting or immunoprecipitation by antibodies against phosphotyrosine. We found that greater than 90% of phosphotyrosine-containing cellular proteins can be immunoprecipitated by these antibodies. The level of phosphotyrosine-containing cellular proteins detectable by this method markedly increased upon transformation with Rous sarcoma virus, and more than 20 distinct bands of such proteins were found in lysates of Rous sarcoma virus-transformed cells. Most of these phosphotyrosine-containing proteins had not been identified by other methods, and their presence appeared to correlate with morphological transformation in cells infected with various Rous sarcoma virus mutants and Y73, PRCII, and Fujinami sarcoma viruses. However, considerably different patterns were obtained with cells infected with nontransforming Rous sarcoma virus mutants that encode nonmyristylated src kinases, indicating that most substrates that correlate with transformation can only be recognized by p60v-src associated with the plasma membrane.

Immunological characterization of proteins detected by phosphotyrosine antibodies in cells transformed by Rous sarcoma virus

Phosphotyrosine antibodies were used to identify tyrosine-phosphorylated proteins in Rous sarcoma virus (RSV)-transformed chicken embryo fibroblasts. A large number of tyrosine phosphoproteins were detected. A similar set of proteins was observed in RSV-transformed murine cells. An 85,000-dalton protein, however, was present in transformed avian cells but missing in transformed murine cells. Neither the 85,000-dalton protein nor any of the other tyrosine phosphoproteins appeared to be viral structural proteins. Use of RSV mutants encoding partially deleted src gene products enabled us to identify a 60,000-dalton cellular tyrosine phosphoprotein that comigrated with wild-type pp60v-src. With the exception of calpactin I, the major tyrosine phosphoproteins detected in immunoblots appeared to be different from several previously characterized substrates of pp60v-src with similar molecular masses (ezrin, vinculin, and the fibronectin receptor).

Hematopoietic growth factors activate the tyrosine phosphorylation of distinct sets of proteins in interleukin-3-dependent murine cell lines

By immunoblotting with antibodies for phosphotyrosine, we have demonstrated that the hematopoietic growth factors interleukin-2, interleukin-3, interleukin-4, and granulocyte-macrophage colony-stimulating factor stimulate the tyrosine phosphorylation of specific sets of proteins in murine hematopoietic progenitor cell lines. The stimulation of tyrosine phosphorylation is a receptor-dependent transient event. The effect of these hematopoietic growth factors on protein tyrosine phosphorylation was not mediated through protein kinase C.