Abi-2, a novel SH3-containing protein interacts with the c-Abl tyrosine kinase and modulates c-Abl transforming activity

Identification of a candidate human spectrin Src homology 3 domain-binding protein suggests a general mechanism of association of tyrosine kinases with the spectrin-based membrane skeleton

Spectrin is a widely expressed protein with specific isoforms found in erythroid and nonerythroid cells. Spectrin contains an Src homology 3 (SH3) domain of unknown function. A cDNA encoding a candidate spectrin SH3 domain-binding protein was identified by interaction screening of a human brain expression library using the human erythroid spectrin (alphaI) SH3 domain as a bait. Five isoforms of the alphaI SH3 domain-binding protein mRNA were identified in human brain. Mapping of SH3 binding regions revealed the presence of two alphaI SH3 domain binding regions and one Abl-SH3 domain binding region. The gene encoding the candidate spectrin SH3 domain-binding protein has been located to human chromosome 10p11.2 --> p12. The gene belongs to a recently identified family of tyrosine kinase-binding proteins, and one of its isoforms is identical to e3B1, an eps8-binding protein (Biesova, Z., Piccoli, C., and Wong, W. T. (1997)Oncogene 14, 233-241). Overexpression of the green fluorescent protein fusion of the SH3 domain-binding protein in NIH3T3 cells resulted in cytoplasmic punctate fluorescence characteristic of the reticulovesicular system. This fluorescence pattern was similar to that obtained with the anti-human erythroid spectrin alphaI SigmaI/betaI SigmaI antibody in untransfected NIH3T3 cells; in addition, the anti-alphaI SigmaI/betaI SigmaI antibody also stained Golgi apparatus. Immunofluorescence obtained using antibodies against alphaI SigmaI/++betaI SigmaI spectrin and Abl tyrosine kinase but not against alphaII/betaII spectrin colocalized with the overexpressed green fluorescent protein-SH3-binding protein. Based on the conservation of the spectrin SH3 binding site within members of this protein family and published interactions, a general mechanism of interactions of tyrosine kinases with the spectrin-based membrane skeleton is proposed.

Age-related changes in thyroid hormone responsive protein (THRP) expression in cerebral tissue of rats

To determine if aging in rats is associated with insensitivity of cerebral tissue to thyroid hormones (TH), the expression of a TH responsive protein or (THRP) in cerebral tissue was studied in male Fischer rats at 4, 12 and 24 months of age during euthyroid, hypothyroid and hyperthyroid states. The basal levels of THRP mRNA was significantly increased in 24-month-old and in 12-month-old rats while THRP mass measured by Western blots was decreased compared to 4-month-old rats. Compared to euthyroid rats, hyperthyroidism in 4-month-old rats was associated with 5.1-fold increase in THRP mRNA and 3.7-fold increase in protein content while in hyperthyroid aged rats, the increase of THRP mRNA was only 1.6-fold and the increase in the protein was 2.4-fold. Hypothyroidism did not significantly alter THRP or its mRNA in either young or aged rats. It is concluded that aging in rats is associated with reduced cerebral tissue responsiveness to thyroid hormones.

Oncogenic Abl and Src tyrosine kinases elicit the ubiquitin-dependent degradation of target proteins through a Ras-independent pathway

Oncogenic forms of the Abl and Src tyrosine kinases trigger the destruction of the Abi proteins, a family of Abl-interacting proteins that antagonize the oncogenic potential of Abl after overexpression in fibroblasts. The destruction of the Abi proteins requires tyrosine kinase activity and is dependent on the ubiquitin-proteasome pathway. We show that degradation of the Abi proteins occurs through a Ras-independent pathway. Significantly, expression of the Abi proteins is lost in cell lines and bone marrow cells isolated from patients with aggressive Bcr-Abl-positive leukemias. These findings suggest that loss of Abi proteins may be a component in the progression of Bcr-Abl-positive leukemias and identify a novel pathway linking activated nonreceptor protein tyrosine kinases to the destruction of specific target proteins through the ubiquitin-proteasome pathway.

Identification of sirm, a novel insulin-regulated SH3 binding protein that associates with Grb-2 and FYN

We have previously developed a mouse model of insulin-resistant diabetes by targeted inactivation of the insulin receptor gene. During studies of gene expression in livers of insulin receptor-deficient mice, we identified a novel cDNA, which we have termed sirm (Son of Insulin Receptor Mutant mice). sirm is largely, albeit not exclusively, expressed in insulin-responsive tissues. Insulin is a potent modulator of sirm expression, and sirm mRNA levels correlate with tissue sensitivity to insulin. The product of the sirm gene is a serine/threonine-rich protein with several proline-rich motifs and an NPNY motif, conforming to the consensus sequence recognized by the phosphotyrosine binding domains of insulin receptor substrate and Shc proteins. However, Sirm bears no extended homologies with other known proteins. Based on the sequences of the proline-rich domains, we sought to determine whether Sirm binds to the SH3 domains of FYN and Grb-2. We demonstrate here that Sirm binds to FYN and Grb-2 in 3T3-L1 adipocytes and that insulin treatment results in the dissociation of the Sirm.FYN and Sirm.Grb-2 complexes. We also show that Sirm is a substrate for the kinase activity of FYN in vitro. Based on the patterns of expression of sirm, its regulation by insulin, and the interactions with molecules in the insulin signaling pathway, we surmise that Sirm plays a role in modulating tissue sensitivity to insulin.

An intramolecular SH3-domain interaction regulates c-Abl activity

The ABL1 proto-oncogene encodes a cytoplasmic and nuclear protein tyrosine kinase (c-Abl) that has been implicated in processes of cell differentiation, cell division, cell adhesion and stress response. Alterations of ABL1 by chromosomal rearrangement or viral transduction can lead to malignant transformation. Activity of the c-Abl protein is negatively regulated by its SH3 domain through an unknown mechanism, and deletion of the SH3 domain turns ABL1 into an oncogene. We present evidence for an intramolecular inhibitory interaction of the SH3 domain with the catalytic domain and with the linker between the SH2 and catalytic domain (SH2-CD linker). Site-directed mutations in each of these three elements activate c-Abl. Mutations in the linker cause a conformational change of the molecule and increase binding of the SH3 domain to peptide ligands. Individual mutation of two charged residues in the SH3 and catalytic domain activates c-Abl, while inhibition is restored in the double reciprocal mutant. We propose that regulators of c-Abl will have opposite effects on its activity depending on their ability to favour or disrupt these intramolecular interactions.

The SH3 Domain Contributes to BCR/ABL-Dependent Leukemogenesis In Vivo: Role in Adhesion, Invasion, and Homing

To determine the possible role of the BCR/ABL oncoprotein SH3 domain in BCR/ABL-dependent leukemogenesis, we studied the biologic properties of a BCR/ABL SH3 deletion mutant (▵SH3 BCR/ABL) constitutively expressed in murine hematopoietic cells. ▵SH3 BCR/ABL was able to activate known BCR/ABL-dependent downstream effector molecules such as RAS, PI-3kinase, MAPK, JNK, MYC, JUN, STATs, and BCL-2. Moreover, expression of ▵SH3 BCR/ABL protected 32Dcl3 murine myeloid precursor cells from apoptosis, induced their growth factor-independent proliferation, and resulted in transformation of primary bone marrow cells in vitro. Unexpectedly, leukemic growth from cells expressing ▵SH3 BCR/ABL was significantly retarded in SCID mice compared with that of cells expressing the wild-type protein. In vitro and in vivo studies to determine the adhesive and invasive properties of ▵SH3 BCR/ABL-expressing cells showed their decreased interaction to collagen IV- and laminin-coated plates and their reduced capacity to invade the stroma and to seed the bone marrow and spleen. The decreased interaction with collagen type IV and laminin was consistent with a reduced expression of α2 integrin by ▵SH3 BCR/ABL-transfected 32Dcl3 cells. Moreover, as compared with wild-type BCR/ABL, which localizes primarily in the cytoskeletal/ membrane fraction, ▵SH3 BCR/ABL was more evenly distributed between the cytoskeleton/membrane and the cytosol compartments. Together, the data indicate that the SH3 domain of BCR/ABL is dispensable for in vitro transformation of hematopoietic cells but is essential for full leukemogenic potential in vivo.

The SH3 Domain Contributes to BCR/ABL-Dependent Leukemogenesis In Vivo: Role in Adhesion, Invasion, and Homing

To determine the possible role of the BCR/ABL oncoprotein SH3 domain in BCR/ABL-dependent leukemogenesis, we studied the biologic properties of a BCR/ABL SH3 deletion mutant (▵SH3 BCR/ABL) constitutively expressed in murine hematopoietic cells. ▵SH3 BCR/ABL was able to activate known BCR/ABL-dependent downstream effector molecules such as RAS, PI-3kinase, MAPK, JNK, MYC, JUN, STATs, and BCL-2. Moreover, expression of ▵SH3 BCR/ABL protected 32Dcl3 murine myeloid precursor cells from apoptosis, induced their growth factor-independent proliferation, and resulted in transformation of primary bone marrow cells in vitro. Unexpectedly, leukemic growth from cells expressing ▵SH3 BCR/ABL was significantly retarded in SCID mice compared with that of cells expressing the wild-type protein. In vitro and in vivo studies to determine the adhesive and invasive properties of ▵SH3 BCR/ABL-expressing cells showed their decreased interaction to collagen IV- and laminin-coated plates and their reduced capacity to invade the stroma and to seed the bone marrow and spleen. The decreased interaction with collagen type IV and laminin was consistent with a reduced expression of α2 integrin by ▵SH3 BCR/ABL-transfected 32Dcl3 cells. Moreover, as compared with wild-type BCR/ABL, which localizes primarily in the cytoskeletal/ membrane fraction, ▵SH3 BCR/ABL was more evenly distributed between the cytoskeleton/membrane and the cytosol compartments. Together, the data indicate that the SH3 domain of BCR/ABL is dispensable for in vitro transformation of hematopoietic cells but is essential for full leukemogenic potential in vivo.

Phosphorylation of Enabled by the Drosophila Abelson tyrosine kinase regulates the in vivo function and protein-protein interactions of Enabled

Drosophila Enabled (Ena) is a member of a family of cytoskeleton-associated proteins including mammalian vasodilator-stimulated phosphoprotein and murine Enabled that regulate actin cytoskeleton assembly. Mutations in Drosophila ena were discovered as dominant genetic suppressors of mutations in the Abelson tyrosine kinase (Abl), suggesting that Ena and Abl function in the same pathway or process. We have identified six tyrosine residues on Ena that are phosphorylated by Abl in vitro and in vivo. Mutation of these phosphorylation sites to phenylalanine partially impaired the ability of Ena to restore viability to ena mutant animals, indicating that phosphorylation is required for optimal Ena function. Phosphorylation of Ena by Abl inhibited the binding of Ena to SH3 domains in vitro, suggesting that one effect of Ena phosphorylation may be to modulate its association with other proteins.

The amphiphysin-like protein 1 (ALP1) interacts functionally with the cABL tyrosine kinase and may play a role in cytoskeletal regulation

cABL is a protooncogene, activated in a subset of human leukemias, whose protein product is a nonreceptor tyrosine kinase of unknown function. cABL has a complex structure that includes several domains and motifs found in proteins implicated in signal transduction pathways. An approach to elucidate cABL function is to identify proteins that interact directly with cABL and that may serve as regulators or effectors of its activity. To this end, a protein-interaction screen of a phage expression library was undertaken to identify proteins that interact with specific domains of cABL. An SH3-domain-containing protein has been identified that interacts with sequences in the cABL carboxyl terminus. The cDNA encoding ALP1 (amphiphysin-like protein 1) was isolated from a 16-day mouse embryo. ALP1 has high homology to BIN1, a recently cloned myc-interacting protein, and also shows significant homology to amphiphysin, a neuronal protein cloned from human and chicken. The amino terminus has homology to two yeast proteins, Rvs167 and Rvs161, which are involved in cell entry into stationary phase and cytoskeletal organization. ALP1 binds cABL in vitro and in vivo. Expression of ALP1 results in morphological transformation of NIH 3T3 fibroblasts in a cABL-dependent manner. The properties of ALP1 suggest that it may be involved in possible cytoskeletal functions of the cABL kinase. Additionally, these results provide further evidence for the importance of the cABL carboxyl terminus and its binding proteins in the regulation of cABL function.

Cloning and characterization of a complementary DNA for a thyroid hormone-responsive protein in mature rat cerebral tissue

A gene responsive to thyroid hormone (TH) has been identified in the adult rat brain cerebral tissue. A cDNA probe differentially expressed in euthyroid, hypothyroid and hyperthyroid rat cerebral tissue, generated by reverse transcriptase-PCR differential display of mRNA, was used to screen the rat brain cDNA library. A 3.4 kb positive clone hybridized in Northern blots with a 3.8 kb mRNA that proved to be TH responsive (THR). The remaining coding sequence and a part of the 5' untranslated region of this cDNA were obtained by 5' rapid amplification of cDNA ends. The deduced amino acid sequence revealed that THR protein (THRP), a 68 kDa moiety, has 83% sequence similarity with c-Abl interactor protein (Abi-2), which is a substrate for tyrosine kinase activity of c-Abl. The extensive similarity between the two proteins suggests a potential role for THRP as a substrate for c-Abl. Northern analysis showed that the expression of THR mRNA in hyperthyroid rats is 6-fold that in euthyroid rats. There is also a 4-6-fold increase in the concentration of THRP, as analysed by Western analysis. Owing to the extensive similarity between rat THRP and human Abi-2, a polyclonal anti- (human Abi-2) antibody was successfully used for Western analysis of proteins from the rat tissues. The observed increase in both the mRNA and the protein did not decline after beta-adrenergic system blockade with propranolol, suggesting that the action of TH on the expression of this gene is not mediated through the beta-adrenergic system. Immunohistochemical studies revealed that neuronal cells were particularly rich in THRP. Both THR mRNA and THRP are rapidly induced in vivo after intravenous administration of thyroxine. Tissue distribution studies indicated that the cerebral tissue was particularly enriched with THR mRNA and 68 kDa THRP. A cDNA clone for a THR gene could provide a useful tool to study the molecular mechanisms of TH effects on cerebral tissue in adult animals.

The PAG gene product, a stress-induced protein with antioxidant properties, is an Abl SH3-binding protein and a physiological inhibitor of c-Abl tyrosine kinase activity

Biochemical and genetic evidence suggests that the tyrosine kinase activity of c-Abl is tightly regulated in vivo by a cellular factor binding to the Src homology 3 (SH3) domain of Abl. We used the yeast two-hybrid system to identify a gene, PAG, whose protein product (Pag) interacts specifically with the Abl SH3 domain. Pag, also known as macrophage 23-kD stress protein (MSP23), is a member of a novel family of proteins with antioxidant activity implicated in the cellular response to oxidative stress and in control of cell proliferation and differentiation. In a co-expression assay, Pag associates with c-Abl in vivo and inhibits tyrosine phosphorylation induced by overexpression of c-Abl. Inhibition requires the Abl SH3 and kinase domains and is not observed with other Abl SH3-binding proteins. Expression of Pag also inhibits the in vitro kinase activity of c-Abl, but not SH3-mutated Abl or v-Abl. When transfected in NIH-3T3 cells, Pag is localized to nucleus and cytoplasm and rescues the cytostatic effect induced by c-Abl. These observations suggest Pag is a physiological inhibitor of c-Abl in vivo.

The murine AIDS virus Gag precursor protein binds to the SH3 domain of c-Abl

The Pr60gag protein of the murine AIDS (MAIDS) defective virus promotes the proliferation of the infected target B cells and is responsible for inducing a severe immunodeficiency disease. Using the yeast two-hybrid system, we identified the SH3 domain of c-Abl as interacting with the proline-rich p12 domain of Pr60gag. The two proteins were shown to associate in vitro and in vivo in MAIDS virus-infected B cells. Overexpression of Pr60(gag) in these cells led to a detectable increase of the levels of c-Abl protein and to its translocation at the membrane. These results suggest that this viral protein serves as a docking site for signaling molecules and that c-Abl may be involved in the proliferation of infected B cells.

Regulation of DNA damage-induced apoptosis by the c-Abl tyrosine kinase

Activation of the c-Abl protein tyrosine kinase by certain DNA-damaging agents contributes to downregulation of Cdk2 and G1 arrest by a p53-dependent mechanism. The present work investigates the potential role of c-Abl in apoptosis induced by DNA damage. Transient transfection studies with wild-type, but not kinase-inactive, c-Abl demonstrate induction of apoptosis. Cells that stably express inactive c-Abl exhibit resistance to ionizing radiation-induced loss of clonogenic survival and apoptosis. Cells null for c-abl are also impaired in the apoptotic response to ionizing radiation. We further show that cells deficient in p53 undergo apoptosis in response to expression of c-Abl and exhibit decreases in radiation-induced apoptosis when expressing inactive c-Abl. These findings suggest that c-Abl kinase regulates DNA damage-induced apoptosis.

Co-expression with BCR induces activation of the FES tyrosine kinase and phosphorylation of specific N-terminal BCR tyrosine residues

The human BCR gene encodes a protein with serine/threonine kinase activity and regulatory domains for the small G-proteins RAC and CDC42. Previous work in our laboratory has established that BCR is a substrate for c-FES, a non-receptor tyrosine kinase linked to myeloid growth and differentiation. Tyrosine phosphorylation led to the association of BCR with the RAS guanine nucleotide exchange complex GRB2-SOS in vivo via the GRB2 SH2 domain, linking BCR to RAS signaling (Maru, Y., Peters, K. L., Afar, D. E. H., Shibuya, M., Witte, O. N., and Smithgall, T. E. (1995) Mol. Cell. Biol. 15, 835-842). In the present study, we demonstrate that BCR Tyr-246 and at least one of the closely spaced tyrosine residues, Tyr-279, Tyr-283, and Tyr-289 (3Y cluster), are phosphorylated by FES both in vitro and in 32Pi-labeled cells. Mutagenesis of BCR Tyr-177 to Phe completely abolished FES-induced BCR binding to the GRB2 SH2 domain, identifying Tyr-177 as an additional phosphorylation site for FES. Co-expression of BCR and FES in human 293T cells stimulated the tyrosine autophosphorylation of FES. By contrast, tyrosine phosphorylation of BCR by FES suppressed BCR serine/threonine kinase activity toward the 14-3-3 protein and BCR substrate, BAP-1. These data show that tyrosine phosphorylation by FES affects the interaction of BCR with multiple signaling partners and suggest a general role for BCR in non-receptor protein-tyrosine kinase regulation and signal transduction.

Signal transduction-based strategies for the treatment of chronic myelogenous leukemia

Recent studies of the BCR-ABL fusion protein, the product of the oncogene responsible for chronic myelogenous leukemia, have identified a number of signal transduction pathways that are activated by this tyrosine kinase. In some cases, these pathways are critical mediators of the growth stimulatory effects of the oncogene on hemopoietic cells. This knowledge has been translated into therapeutic strategies that directly target BCR-ABL or the signaling pathways that BCR-ABL activates. Promising results in animal models have led to the design of Phase I clinical trials, which are in progress or will be under way shortly. These studies are among the first to target a specific genetic abnormality in human cancer.

Genotoxic drugs induce interaction of the c-Abl tyrosine kinase and the tumor suppressor protein p53

The function of the c-Abl protein tyrosine kinase is unknown. The present studies demonstrate that the antimetabolite 1-beta-D-arabinofuranosylcytosine (ara-C) induces binding of c-Abl and p53. Ara-C treatment of cells that express wild type or a dominant negative, kinase-inactive c-Abl(K-R) was associated with formation of c-Abl-p53 complexes and increased expression of the cyclin-dependent kinase (Cdk) inhibitor p21. However, down-regulation of Cdk2 by ara-C was found in cells expressing wild type c-Abl and not in cells expressing c-Abl(K-R) or those deficient in p53. Similar findings were obtained following treatment of cells with the alkylating agent methyl methanesulfonate (MMS). Cells that express the c-Abl dominant negative or are null for c-Abl exhibited partial abrogation of Cdk2 down-regulation and G1 arrest in response to MMS exposure. Cells lacking the c-abl gene also responded to ara-C and MMS with increases in p53 levels and induction of p21. These findings indicate that the cellular response to certain genotoxic drugs involves binding of c-Abl to p53 and down-regulation of Cdk2 by a c-Abl kinase/p53-dependent mechanism.

Intramolecular interactions of the regulatory domains of the Bcr-Abl kinase reveal a novel control mechanism

BACKGROUND

The Abl nonreceptor tyrosine kinase is implicated in a range of cellular processes and its transforming variants are involved in human leukemias. The N-terminal regulatory region of the Abl protein contains Src homology domains SH2 and SH3 which have been shown to be important for the regulation of its activity in vivo. These domains are often found together in the same protein and biochemical data suggest that the functions of one domain can be influenced by the other.

RESULTS

We have determined the crystal structure of the Abl regulatory region containing the SH3 and SH2 domains. In general, the individual domains are very similar to those of previously solved structures, although the Abl SH2 domain contains a loop which is extended so that one side of the resulting phosphotyrosine-binding pocket is open. In our structure the protein exists as a monomer with no intermolecular contacts to which a biological function may be attributed. However, there is a significant intramolecular contact between a loop of the SH3 domain and the extended loop of the SH2 domain. This contact surface includes the SH2 loop segment that is responsible for binding the phosphate moiety of phosphotyrosine-containing proteins and is therefore critical for orienting peptide interactions.

CONCLUSIONS

The crystal structure of the composite Abl SH3-SH2 domain provides the first indication of how SH2 and SH3 domains communicate with each other within the same molecule and why the presence of one directly influences the activity of the other. This is the first clear evidence that these two domains are in contact with each other. The results suggest that this direct interaction between the two domains may affect the ligand binding properties of the SH2 domain, thus providing an explanation for biochemical and functional data concerning the Bcr-Abl kinase.

A potential SH3 domain-binding site in the Crk SH2 domain

The Src homology 2 (SH2) domain of the mammalian adaptor protein Crk-II contains a proline-rich insert, predicted to lie within an extended DE loop, which is dispensable for phosphopeptide binding. Using the yeast two-hybrid system, this region of the Crk-II SH2 domain was found to interact with a subset of SH3 domains, notably the Abl SH3 domain. Furthermore, this proline-rich insert was found to modify the efficiency with which Crk-II was phosphorylated by the p140(c-abl) tyrosine kinase. In vitro, the interaction of full-length non-phosphorylated Crk-II with a glutathione S-transferase-Abl SH3 domain fusion protein was very weak. However, phosphorylation of Crk-II on Tyr-221 which induces an intramolecular association with the SH2 domain, or addition of a phosphopeptide corresponding to the Crk-II Tyr-221 phosphorylation site, stimulated association of Crk-II with the Abl SH3 domain. NMR spectroscopic analysis showed that binding of the Tyr-221 phosphopeptide to the Crk SH2 domain induced a chemical shift change in Val-71, located in the proline-rich insert, indicative of a change in the structure of the proline-rich loop in response of Crk SH2-pTyr-221 interaction. These results suggest that the proline-rich insert in the Crk SH2 domain constitutes an SH3 domain-binding site that can be regulated by binding of a phosphopeptide ligand to the Crk SH2 domain.

The stress response to ionizing radiation involoves c-Abl-dependent phosphorylation of SHPTP1

c-Abl is a nonreceptor tyrosine kinase that is activated by certain DNA-damaging agents. The present studies demonstrate that nuclear c-Abl binds constitutively to the protein tyrosine phosphatase SHPTP1. Treatment with ionizing radiation is associated with c-Abl-dependent tyrosine phosphorylation of SHPTP1. The results demonstrate that the SH3 domain of c-Abl interacts with a WPDHGVPSEP motif (residues 417-426) in the catalytic domain of SHPTP1 and that c-Abl phosphorylates C terminal Y536 and Y564 sites. The functional significance of the c-Abl-SHPTP1 interaction is supported by the demonstration that, like c-Abl, SHPTP1 regulates the induction of Jun kinase activity following DNA damage. These findings indicate that SHPTP1 is involved in the response to genotoxic stress through a c-Abl-dependent mechanism.

A functional screen in yeast for regulators and antagonizers of heterologous protein tyrosine kinases

Tyrosine phosphorylation exerts a pivotal role in cell regulation processes of higher eukaryotes. Tight control of the activity of protein tyrosine kinases is crucial for ordered phosphorylation to occur. We have developed a functional screen for tyrosine kinase regulators using c-Src, the first cellular protein tyrosine kinase described, as a prototype; and fission yeast, Schizosaccharomyces pombe, as a genetically amenable host system. Inducible expression of c-Src in fission yeast is lethal. We have screened human cDNA libraries for clones able to counteract the lethal effect of Src. Two different classes of cDNAs, which we called SAS for sequences antagonizing Src, were obtained. The first class encodes for the protein tyrosine kinase Csk, known to regulate Src activity through phosphorylation of the C-terminal tyrosine. The second class consists of clones encoding three different tyrosine phosphatases, counteracting Src action by dephosphorylation of Src substrates and by dephosphorylation of Src itself. The system described here can be applied to identify regulators of other heterologous tyrosine kinases, including receptor-type tyrosine kinases, which impair growth of S. pombe.

Nuclear tyrosine kinases: from Abl to WEE1

The notion of a critical role for protein tyrosine kinases in the nucleus is supported by recent findings linking these proteins with components of the cell cycle and with the transcription machinery. Several of these tyrosine kinases localize to both nuclear and cytoplasmic compartments of the cell, and may coordinate signal transduction events in the cytoplasm with specific changes in the nucleus. Among these proteins are Abl, Rak, Fes and Fer. The past year has brought significant progress both towards the elucidation of the pathways that lead to activation of the Abl tyrosine kinase and towards the identification of novel Abl targets. Recent advances have also been made in understanding the regulation of the nucleus-specific human WEE1 tyrosine kinase. Nuclear tyrosine kinases may participate in the regulation of multiple cellular processes including transcription, DNA repair and the cell cycle.

Deletion of the ABL SH3 domain reactivates de-oligomerized BCR-ABL for growth factor independence

Biological activities of BCR-ABL, an activated tyrosine kinase oncogene responsible for pathogenesis of human leukemias, can be completely inactivated by a deletion of the BCR aminoterminal sequence with tetramerizing property (BCR-ABL delta 1-40). We attempted several ways to restore the ability to induce growth factor independence to the de-oligomerized BCR-ABL delta 1-40 and found that an additional deletion of the ABL SH3 domain could. In BCR-ABL delta 1-40 reactivated by the SH3 deletion, transphosphoryation of other cellular proteins like p62 or SHC in vivo and autophosphorylation with recruitment of GRB-2 were also recovered.