Mutagenic analysis of the roles of SH2 and SH3 domains in regulation of the Abl tyrosine kinase

Expanded functions in the apical cell domain to regulate the growth rate of imaginal discs

The Drosophila expanded (ex) gene encodes a product (Ex) that shares homology with the Protein 4.1 family of proteins, many of which are enriched at specific lateral cell junctions and the apical cellular domain. Ex colocalizes with actin in the apical domain of imaginal disc epithelial cells, where it partially overlaps the distribution of phosphotyrosine (PY)-containing proteins. This suggests that Ex is present in or associated with adherens junctions. Genetic studies show that Ex is necessary for proper regulation of final cell number in adult wings and for the formation of eyes, distal leg, and distal antennal segments. We have generated mitotic clones that lack Ex using the twin spot technique, and demonstrated that the primary function of Ex is to regulate cell proliferation. Overexpressing Ex protein results in a decrease in final cell number in wings, suggesting a direct relationship between Ex function and proliferation rate.

Regulation of the hTERT telomerase catalytic subunit by the c-Abl tyrosine kinase

BACKGROUND

Telomeres consist of repetitive (TTAGGG) DNA sequences that are maintained by the multisubunit telomerase ribonucleoprotein. Telomerase consists of an RNA, which serves as template for the sequence tracts, and a catalytic subunit that functions in reverse transcription of the RNA template. Cloning and characterization of the human catalytic subunit of telomerase (hTERT) has supported a role in cell transformation. How telomerase activity is regulated, however, is largely unknown.

RESULTS

We show here that hTERT associates directly with the c-Abl protein tyrosine kinase. We also found that c-Abl phosphorylates hTERT and inhibits hTERT activity. Moreover, our findings demonstrate that exposure of cells to ionizing radiation induces tyrosine phosphorylation of hTERT by a c-Abl-dependent mechanism. The functional significance of the c-Abl-hTERT interaction is supported by the demonstration that cells deficient in c-Abl show telomere lengthening.

CONCLUSIONS

The ubiquitously expressed c-Abl tyrosine kinase is activated by DNA double-strand breaks. Our finding of telomere lengthening in c-Abl-deficient cells and the functional interactions between c-Abl and hTERT support a role for c-Abl in the regulation of telomerase function.

Interaction between protein kinase C delta and the c-Abl tyrosine kinase in the cellular response to oxidative stress

Protein kinase C (PKC) isoforms are phosphorylated on tyrosine in the response of cells to oxidative stress. The present studies demonstrate that treatment of cells with hydrogen peroxide (H(2)O(2)) induces binding of the PKCdelta isoform and the c-Abl protein-tyrosine kinase. The results show that c-Abl phosphorylates PKCdelta in the H(2)O(2) response. We also show that PKCdelta phosphorylates and activates c-Abl in vitro. In cells, induction of c-Abl activity by H(2)O(2) is attenuated by the PKCdelta inhibitor, rottlerin, and by overexpression of the regulatory domain of PKCdelta. These findings support a functional interaction between PKCdelta and c-Abl in the cellular response to oxidative stress.

The SH3 and SH2 domains are capable of directing specificity in protein interactions between the non-receptor tyrosine kinases cSrc and cYes

The c-src and c-yes proto-oncogenes encode 60 000 and 62 000 Dalton non-receptor tyrosine kinases of the Src family, pp60c-src and pp62c-yes, respectively. These kinases are over 80% homologous outside of their unique amino termini, yet several studies suggest that differences exist in the regulation, activation, and function of cSrc and cYes. The determinants of specificity in signaling between these proteins, however, remain unclear. In order to investigate the roles of the Src Homology (SH) 3 and 2 domains in mediating signaling specificity between cSrc and cYes, chimeras were created in which the SH3 and/or SH2 domains of cSrc or the fully activated variant Src527F were replaced by the corresponding domains of cYes. These constructs were used to assess the effects of the Yes SH3 and SH2 domains on the ability of Src to form stable complexes with and induce tyrosine phosphorylation of Src SH3 and SH2 domain binding partners in vivo. Both the Yes SH3 and SH2 domains were found to alter the capacity of Src to form stable associations with heterologous proteins. The Yes SH3 domain was unable to affinity absorb the Src SH3/SH2 binding partner AFAP-110 from COS-1 cell lysates, and chimeric constructs of Src527F containing the cYes SH3 domain were unable to efficiently co-immunoprecipitate with AFAP-110 from chicken embryo fibroblasts. Interactions with the Src SH2 domain binding partner pp130cas were unaffected. Additionally, only chimeras containing the cYes SH2 domain were able to co-immunoprecipitate with an unidentified 87 kDa tyrosine-phosphorylated protein. These results indicate that the SH3 and SH2 domains are capable of directing specificity in substrate binding between Src and Yes, suggesting potential mechanisms for generating specificity in signaling between these two highly related non-receptor tyrosine kinases.

c-Abl: activation and nuclear targets

The c-Abl tyrosine kinase and its transforming variants have been implicated in tumorigenesis and in many important cellular processes. c-Abl is localized in the nucleus and the cytoplasm, where it plays distinct roles. The effects of c-Abl are mediated by multiple protein-protein and protein-DNA interactions and its tyrosine kinase domain. At the biochemical level, the mechanism of c-Abl kinase activation and the identification of its target proteins and cellular machineries have in part been solved. However, the phenotypic outcomes of these molecular events remained in large elusive. c-Abl has been shown to regulate the cell cycle and to induce under certain conditions cell growth arrest and apoptosis. In this respect the interaction of c-Abl with p53 and p73 has attracted particular attention. Recent findings have implicated c-Abl in an ionizing irradiation signaling pathway that elicits apoptosis. In this pathway p73 is an important immediate downstream effector. Here I review the current knowledge about these nuclear processes in which c-Abl is engaged and discuss some of their possible implications on cell physiology. Cell Death and Differentiation (2000) 7, 10 - 16.

Activation of the Abl tyrosine kinase in vivo by Src homology 3 domains from the Src homology 2/Src homology 3 adaptor Nck

The nonreceptor tyrosine kinase c-Abl is tightly regulated in vivo, but the mechanisms that normally repress its activity are not well understood. We find that a construct encoding the first two Src homology 3 (SH3) domains of the Src homology 2/SH3 adaptor protein Nck can activate c-Abl in human 293T cells. A myristoylated Nck SH3 domain construct, which is expected to localize to membranes, potently activated Abl when expressed at low levels. An unmyristoylated Nck SH3 domain construct, which localizes to the cytosol and nucleus, also activated Abl but only at high levels of expression. Activation by both myristoylated and unmyristoylated Nck constructs required the C terminus of Abl; a C-terminally truncated form of Abl was not activated, although this construct could still be activated by deletion of its SH3 domain. Activation did not require the major binding sites in the Abl C terminus for Nck SH3 domains, however, suggesting that the mechanism of activation does not require direct binding to the C terminus. Activation of c-Abl by Nck SH3 domains provides a robust experimental system for analyzing the mechanisms that normally repress Abl activity and how that normal regulation can be perturbed.

Drosophila abelson interacting protein (dAbi) is a positive regulator of abelson tyrosine kinase activity

Human and mouse Abelson interacting proteins (Abi) are SH3-domain containing proteins that bind to the proline-rich motifs of the Abelson protein tyrosine kinase. We report a new member of this gene family, a Drosophila Abi (dAbi) that is a substrate for Abl kinase and that co-immunoprecipitates with Abl if the Abi SH3 domain is intact. We have identified a new function for both dAbi and human Abi-2 (hAbi-2). Both proteins activate the kinase activity of Abl as assayed by phosphorylation of the Drosophila Enabled (Ena) protein. Removal of the dAbi SH3 domain eliminates dAbi's activation of Abl kinase activity. dAbi is an unstable protein in cells and is present at low steady state levels but its protein level is increased coincident with phosphorylation by Abl kinase. Expression of the antisense strand of dAbi reduces dAbi protein levels and abolishes activation of Abl kinase activity. Modulation of Abi protein levels may be an important mechanism for regulating the level of Abl kinase activity in the cell.

Tec family of protein-tyrosine kinases: an overview of their structure and function

The Tec family is a recently emerging subfamily of non-receptor protein-tyrosine kinases (PTKs) represented by its first member, Tec. This family is composed of five members, namely Tec, Btk. Itk/Emt/Tsk, Bmx and Txk/Rlk. The most characteristic feature of this family is the presence of a pleckstrin homology (PH) domain in their protein structure. The PH domain is known to bind phosphoinositides; on this basis, Tec family PTKs may act as merge points of phosphotyrosine-mediated and phospholipid-mediated signaling systems. Many Tec family proteins are abundantly expressed in hematopoietic tissues, and are presumed to play important roles in the growth and differentiation processes of blood cells. Supporting this, mutations in the Btk gene cause X chromosome-linked agammaglobulinemia (XLA) in humans and X chromosome-linked immunodeficiency (Xid) in mice, indicating that Btk activity is indispensable for B-cell ontogeny. In addition, Tec family kinases have been shown to be involved in the intracellular signaling mechanisms of cytokine receptors, lymphocyte surface antigens, heterotrimeric G-protein-coupled receptors and integrin molecules. Efforts are being made to identify molecules which interact with Tec kinases to transfer Tec-mediated signals in vivo. Candidates for such second messengers include PLC-gamma2, guanine nucleotide exchange factors for RhoA and TFII-I/BAP-135. This review summarizes current knowledge concerning the input and output factors affecting the Tec kinases.

Identification of residues involved in v-Src substrate recognition by site-directed mutagenesis

To study the role of the catalytic domain in v-Src substrate specificity, we engineered three site-directed mutants (Leu-472 to Tyr or Trp and Thr-429 to Met). The mutant forms of Src were expressed in Sf9 cells and purified. We analyzed the substrate specificities of wild-type v-Src and the mutants using two series of peptides that varied at residues C-terminal to tyrosine. The peptides contained either the YMTM motif found in insulin receptor substrate-1 (IRS-1) or the YGEF motif identified from peptide library experiments to be the optimal sequence for Src. Mutations at positions Leu-472 or Thr-429 caused changes in substrate specificity at positions P+1 and P+3 (i.e. one or three residues C-terminal to tyrosine). This was particularly evident in the case of the L-472W mutant, which had pronounced alterations in its preferences at the P+1 position. The results suggest that residue Leu-472 plays a role in P+1 substrate recognition by Src. We discuss the results in the light of recent work on the roles of the SH2, SH3 and catalytic domains of Src in substrate specificity.

The P190, P210, and P230 forms of the BCR/ABL oncogene induce a similar chronic myeloid leukemia-like syndrome in mice but have different lymphoid leukemogenic activity

The product of the Philadelphia chromosome (Ph) translocation, the BCR/ABL oncogene, exists in three principal forms (P190, P210, and P230 BCR/ABL) that are found in distinct forms of Ph-positive leukemia, suggesting the three proteins have different leukemogenic activity. We have directly compared the tyrosine kinase activity, in vitro transformation properties, and in vivo leukemogenic activity of the P190, P210, and P230 forms of BCR/ABL. P230 exhibited lower intrinsic tyrosine kinase activity than P210 and P190. Although all three oncogenes transformed both myeloid (32D cl3) and lymphoid (Ba/F3) interleukin (IL)-3-dependent cell lines to become independent of IL-3 for survival and growth, their ability to stimulate proliferation of Ba/F3 lymphoid cells differed and correlated directly with tyrosine kinase activity. In a murine bone marrow transduction/transplantation model, the three forms of BCR/ABL were equally potent in the induction of a chronic myeloid leukemia (CML)-like myeloproliferative syndrome in recipient mice when 5-fluorouracil (5-FU)-treated donors were used. Analysis of proviral integration showed the CML-like disease to be polyclonal and to involve multiple myeloid and B lymphoid lineages, implicating a primitive multipotential target cell. Secondary transplantation revealed that only certain minor clones gave rise to day 12 spleen colonies and induced disease in secondary recipients, suggesting heterogeneity among the target cell population. In contrast, when marrow from non- 5-FU-treated donors was used, a mixture of CML-like disease, B lymphoid acute leukemia, and macrophage tumors was observed in recipients. P190 BCR/ABL induced lymphoid leukemia with shorter latency than P210 or P230. The lymphoid leukemias and macrophage tumors had provirus integration patterns that were oligo- or monoclonal and limited to the tumor cells, suggesting a lineage-restricted target cell with a requirement for additional events in addition to BCR/ABL transduction for full malignant transformation. These results do not support the hypothesis that P230 BCR/ABL induces a distinct and less aggressive form of CML in humans, and suggest that the rarity of P190 BCR/ABL in human CML may reflect infrequent BCR intron 1 breakpoints during the genesis of the Ph chromosome in stem cells, rather than intrinsic differences in myeloid leukemogenicity between P190 and P210.

Cycling, stressed-out and nervous: cellular functions of c-Abl

c-Abl, the product of the cellular homologue of the transforming gene of Abelson murine leukaemia virus, has been a protein in search of a purpose for over two decades. Because c-Abl is implicated in the pathogenesis of several human leukaemias, understanding the functions of Abl is an important goal. Recently, biochemical and genetic approaches have converged to shed new light on the mechanism of regulation of c-Abl kinase activity and the multiple roles of c-Abl in cellular physiology. This review summarizes our current understanding of the many facets of c-Abl biology, emphasizing recent studies on Drosophila and mammalian Abl.

Comparison of SH3 and SH2 domain dynamics when expressed alone or in an SH(3+2) construct: the role of protein dynamics in functional regulation

Protein dynamics play an important role in protein function and regulation of enzymatic activity. To determine how additional interactions with surrounding structure affects local protein dynamics, we have used hydrogen exchange and mass spectrometry to investigate the SH2 and SH3 domains of the protein tyrosine kinase Hck. Exchange rates of isolated Hck SH3 and SH2 domains were compared with rates for the same domains when part of a larger SH(3+2) construct. Increased deuterium incorporation was observed for the SH3 domain in the joint construct, particularly near the SH2 interface and the short sequence that connects SH3 to SH2, implying greater flexibility of SH3 when it is part of SH(3+2). Slow cooperative unfolding of the SH3 domain occurred at the same rate in isolated SH3 as in the SH(3+2) construct, suggesting a functional significance for this unfolding. The SH2 domain displayed relatively smaller changes in flexibility when part of the SH(3+2) construct. These results suggest that the domains influence each other. Further, our results imply a link between functional regulation and structural dynamics of SH3 and SH2 domains.

Regulation of human c-Abl tyrosine kinase activity in Xenopus oocytes and acceleration of progesterone-induced G2/M transition by oncogenic forms

Deregulated activity of the Abl protein tyrosine kinase is oncogenic in humans and in animals. The normal cellular form of the enzyme is maintained at a low state of activity by mechanisms that have not yet been entirely elucidated. In particular, little is known about the trans-acting cellular factors involved. We have tested the activity of human c-Abl microinjected into oocytes of Xenopus laevis. In contrast to versions of Abl capable of transforming mammalian cells, which were highly active when introduced into oocytes, the activity of wild type c-Abl was inhibited. Oncogenic forms of Abl efficiently enhanced the ability of Xenopus oocytes to enter M phase following stimulation by progesterone. Abl-enhanced maturation was normal as judged by accumulation of Mos as well as activation of MAP kinase and Cdc2/CyclinB (MPF). Concomitant with maturation and activation of these kinases, Abl became extensively phosphorylated. Altogether, this suggests that an SH3 domain-dependent Abl regulation mechanism similar to the one observed in mammalian cells operates in Xenopus oocytes. Maturation enhancement by microinjection into Xenopus oocytes represents a useful novel assay for analyzing Abl activity. Moreover, the Xenopus oocyte may be a convenient source of trans-acting Abl regulators for biochemical studies.

Essential roles for the Abl and Arg tyrosine kinases in neurulation

The Abl and Arg tyrosine kinases play fundamental roles in the development and function of the central nervous system. Arg is most abundant in adult mouse brain, especially in synapse-rich regions. arg(-/-) mice develop normally but exhibit multiple behavioral abnormalities, suggesting that arg(-/-) brains suffer from defects in neuronal function. Embryos deficient in both Abl and Arg suffer from defects in neurulation and die before 11 days postcoitum (dpc). Although they divide normally, abl(-/-)arg(-/-) neuroepithelial cells display gross alterations in their actin cytoskeleton. We find that Abl and Arg colocalize with each other and with actin microfilaments at the apical surface of the developing neuroepithelium. Thus, Abl and Arg play essential roles in neurulation and can regulate the structure of the actin cytoskeleton.

Intramolecular regulatory interactions in the Src family kinase Hck probed by mutagenesis of a conserved tryptophan residue

Intramolecular interactions between the Src homology domains (SH2 and SH3) and the catalytic domains of Src family kinases result in repression of catalytic activity. The crystal structure of the Src family kinase Hck, with its regulatory domains intact, has been solved. It predicts that a conserved residue, Trp260, at the end of the linker between the SH2 and the catalytic domains plays an important role in regulation by the SH3 and SH2 domains. We have mutated this residue and compared the activities of C-terminally phosphorylated wild type Hck and W260A Hck. The W260A mutant has a higher specific activity than wild type Hck. The W260A mutant requires autophosphorylation at Tyr416 for full activity, but it is not activated by ligand binding to the SH3 or SH2 domains. This mutation also changes the accessibility of the SH2 and SH3 domains to their cognate peptide ligands. Our results indicate that Trp260 plays a critical role in the coupling of the regulatory domains to the catalytic domain, as well as in positioning the ligand binding surfaces.

Lasp-1, a novel type of actin-binding protein accumulating in cell membrane extensions

The Lasp-1 gene, which has been localized to the q12-q21 region of human chromosome 17, is amplified and overexpressed in human breast cancers. In addition to the previously reported LIM and SH3 domains of Lasp-1, we report here the identification of an actin-binding domain in the core of the protein. This domain is functional as we demonstrate that Lasp-1 binds actin in vivo and in vitro. In addition, confocal analysis of the Lasp-1 subcellular distribution shows that the protein is colocalized with actin at peripheral cell extensions in individual epithelial cancer cells and in transformed fibroblastic cells. Moreover, Lasp-1 is tyrosine phosphorylated in fibroblast cell lines transformed by a constitutively active form of c-Src (c-SrcY527F). Altogether, our results show that Lasp-1 defines a new type of actin-binding protein and suggest that the protein may play a role in a signaling pathway involved in the organization of the cytoskeleton.

Comparison of the intrinsic kinase activity and substrate specificity of c-Abl and Bcr-Abl

We studied the intrinsic tyrosine kinase activity and substrate specificity of c-Abl and Bcr-Abl protein tyrosine kinases (PTKs) using the peptide substrates discovered from a synthetic combinatorial peptide library. Our data indicate that the phosphorylation of these peptides by Bcr-Abl was consistently stronger than that by c-Abl. Bcr-Abl also showed substrate preference towards those peptides with one or more positive charges.

Enhanced phosphorylation of Src family kinase substrates containing SH2 domain binding sites

Src family protein-tyrosine kinases possess several modular domains important for regulation of catalytic activity and interaction with potential substrates. Here, we explore interactions between the SH2 domain of Hck, a Src family kinase, and substrates containing SH2 domain-binding sites. We have synthesized a series of peptide substrates containing a high affinity SH2 domain binding site, (phospho)Tyr-Glu-Glu-Ile. We show that the presence of this sequence in a peptide results in a dramatic increase in the phosphorylation rate of a second tyrosine located at the N terminus. Enhanced phosphorylation is not a consequence of stimulation of enzymatic activity by C-terminal tail displacement but is imparted instead by a 10-fold reduction in the Km of the phosphotyrosine-containing peptide when compared with a control. The isolated catalytic domain of the non-receptor tyrosine kinase Abl does not show a preference for the pYEEI motif-containing peptide; however, the preference is restored when the SH2 domain of Src is introduced into Abl. Furthermore, enhanced phosphorylation is dependent on the distance between SH2 domain-binding site and phosphorylatable tyrosine, with the minimum distance requirement being seven amino acids. Reversing the orientation of the pYEEI motif with respect to the substrate sequence decreases phosphorylation by down-regulated Hck, but both orientations are utilized equally well by activated Hck. We discuss the possible implications of these results for processive phosphorylation of substrates in vivo by Src family kinases.

Identification and characterization of a novel SH3-domain binding protein, Sab, which preferentially associates with Bruton's tyrosine kinase (BtK)

Protein interaction cloning method was used to identify a novel molecule, Sab, which binds to the SH3 domain of Bruton's tyrosine kinase (Btk), the deficient cytoplasmic tyrosine kinase in human X-linked agammaglobulinemia and murine X-linked immunodeficiency. Immunoprecipitation using the anti-Sab antibody identified the protein product of the gene as a 70 kDa molecule. While Sab does not have a proline-rich sequence, it was shown to bind to Btk through the commonly conserved structure among SH3 domains. Remarkably, Sab exhibited a high preference for binding to Btk rather than to other cytoplasmic tyrosine kinases, which suggests a unique role of Sab in the Btk signal transduction pathway.

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.

Regulation of Rad51 function by c-Abl in response to DNA damage

The Rad51 protein, a homolog of bacterial RecA, functions in DNA double-strand break repair and genetic recombination. Whereas Rad51 catalyzes ATP-dependent pairing and strand exchange between homologous DNA molecules, regulation of this function is unknown. The c-Abl tyrosine kinase is activated by ionizing radiation and certain other DNA-damaging agents. Here we demonstrate that c-Abl interacts constitutively with Rad51. We show that c-Abl phosphorylates Rad51 on Tyr-54 in vitro. The results also show that treatment of cells with ionizing radiation induces c-Abl-dependent phosphorylation of Rad51. Phosphorylation of Rad51 by c-Abl inhibits the binding of Rad51 to DNA and the function of Rad51 in ATP-dependent DNA strand exchange reactions. These findings represent the first demonstration that Rad51 is regulated by phosphorylation and support a functional role for c-Abl in regulating Rad51-dependent recombination in the response to DNA damage.

Biology of chronic myelogenous leukemia

This article reviews the biology of chronic myelogenous leukemia (CML) and its effect on the process of hematopoiesis. The relevance of the BCR-ABL fusion protein as well as murine models are also discussed. CML has been studied more extensively than any other malignancy, yet the correlation between the clinical symptoms of chronic phase CML and the BCR-ABL oncoprotein is poorly understood. Insights from recent efforts both to develop a good in vivo animal model and to characterize the effect of the BCR-ABL oncoprotein on relevant signal molecules may lead to a better understanding of the pathophysiology of chronic phase CML and, thereby, to the development of targeted therapeutic approaches.

The Src family kinase Hck interacts with Bcr-Abl by a kinase-independent mechanism and phosphorylates the Grb2-binding site of Bcr

bcr-abl, the oncogene causing chronic myeloid leukemia, encodes a fusion protein with constitutively active tyrosine kinase and transforming capacity in hematopoietic cells. Various intracellular signaling intermediates become activated and/or associate by/with Bcr-Abl, including the Src family kinase Hck. To elucidate some of the structural requirements and functional consequences of the association of Bcr-Abl with Hck, their interaction was investigated in transiently transfected COS7 cells. Neither the complex formation of Hck kinase with Bcr-Abl nor the activation of Hck by Bcr-Abl was dependent on the Abl kinase activity. Both inactivating point mutations of Hck and dephosphorylation of Hck enhanced its complex formation with Bcr-Abl, indicating that their physical interaction was negatively regulated by Hck (auto)phosphorylation. Finally, experiments with a series of kinase negative Bcr-Abl mutants showed that Hck phosphorylated Bcr-Abl and induced the binding of Grb2 to Tyr177 of Bcr-Abl. Taken together, our results suggest that Bcr-Abl preferentially binds inactive forms of Hck by an Abl kinase-independent mechanism. This physical interaction stimulates the Hck tyrosine kinase, which may then phosphorylate the Grb2-binding site in Bcr-Abl.

BCR/ABL-induced leukemogenesis causes phosphorylation of Hef1 and its association with Crkl

BCR/ABL is considered responsible for the development of Philadelphia chromosome-positive leukemia. Experimental animal models, such as transgenic mice, have demonstrated unambiguously that Bcr/Abl is capable of inducing leukemogenesis. The adaptor molecule Crkl is a major in vivo substrate of the deregulated Bcr/Abl tyrosine kinase and functions as a molecular link with other signaling proteins. While associated in vivo with Bcr/Abl through its SH3 domain, Crkl can interact simultaneously via its SH2 domain with other tyrosine-phosphorylated proteins. Here we report the identification of prominently tyrosine-phosphorylated proteins with a molecular mass of approximately 110 kDa, which bind specifically to the Crkl SH2 domain in leukemic tissues of P190BCR/ABL transgenic mice. We demonstrate that these proteins are identical to Hef1/Cas-L, which is related to p130(Cas). The proto-oncoprotein p120(Cbl) and Hef1, but not p130(Cas), were detectably phosphorylated on tyrosine in P190Bcr/Abl-expressing leukemic cells and were found in complex with Crkl, showing the existence of protein complexes in P190Bcr/Abl leukemic cells, consisting of P190Bcr/Abl, Crkl, and Hef1 or p120(Cbl). This supports a model in which Crkl acts as mediator between Bcr/Abl and downstream effectors. Since Hef1 is involved in the beta1-integrin signaling pathway, our study demonstrates that Bcr/Abl could specifically interfere with normal beta1-integrin signaling.

Nonphosphorylated peptide ligands for the Grb2 Src homology 2 domain

Critical intracellular signals in normal and malignant cells are transmitted by the adaptor protein Grb2 by means of its Src homology 2 (SH2) domain, which binds to phosphotyrosyl (pTyr) residues generated by the activation of tyrosine kinases. To understand this important control point and to design inhibitors, previous investigations have focused on the molecular mechanisms by which the Grb2 SH2 domain selectively binds pTyr containing peptides. In the current study, we demonstrate that the Grb2 SH2 domain can also bind in a pTyr independent manner. Using phage display, an 11-amino acid cyclic peptide, G1, has been identified that binds to the Grb2 SH2 domain but not the src SH2 domain. Synthetic G1 peptide blocks Grb2 SH2 domain association (IC50 10-25 microM) with a 9-amino acid pTyr-containing peptide derived from the SHC protein (pTyr317). These data and amino acid substitution analysis indicate that G1 interacts in the phosphopeptide binding site. G1 peptide requires a YXN sequence similar to that found in natural pTyr-containing ligands, and phosphorylation of the tyrosine increases G1 inhibitory activity. G1 also requires an internal disulfide bond to maintain the active binding conformation. Since the G1 peptide does not contain pTyr, it defines a new type of SH2 domain binding motif that may advance the design of Grb2 antagonists.

Binding of Ku and c-Abl at the kinase homology region of DNA-dependent protein kinase catalytic subunit

The DNA-dependent protein kinase (DNA-PK) controls the repair of double-stranded DNA breaks in mammalian cells. The protein kinase subunit of DNA-PK (DNA-PKcs) is targeted to DNA breaks by association with the Ku DNA-binding heterodimer. Here we show that a Ku association site is present at the carboxyl terminus of DNA-PKcs (amino acids 3002-3850) near the protein kinase domain. Correspondingly, the nuclear c-Abl tyrosine kinase that associates with DNA-PK also binds to the kinase homology domain. The c-Abl SH3 domain binds to amino acids 3414-3850 of DNA-PKcs. c-Abl phosphorylates C-terminal fragments of DNA-PKcs, particularly amino acids 3414-3850. c-Abl phosphorylation of DNA-PKcs disassociates the DNA-PKcs.Ku complex. Thus, Ku and c-Abl provide opposing functions with regard to DNA-PK activity.

Inhibition of phosphatidylinositol 3-kinase by c-Abl in the genotoxic stress response

Activation of phosphatidylinositol (PI) 3-kinase by growth factors results in phosphorylation of phosphatidylinositol lipids at the D3 position. Although PI 3-kinase is essential to cell survival, little is known about mechanisms that negatively regulate this activity. Here we show that the c-Abl tyrosine kinase interacts directly with the p85 subunit of PI 3-kinase. Activation of c-Abl by ionizing radiation exposure is associated with c-Abl-dependent phosphorylation of PI 3-kinase. We also show that phosphorylation of p85 by c-Abl inhibits PI 3-kinase activity in vitro and in irradiated cells. These findings indicate that c-Abl negatively regulates PI 3-kinase in the stress response to DNA damage.

Modified phage peptide libraries as a tool to study specificity of phosphorylation and recognition of tyrosine containing peptides

Tyrosine phosphorylation and protein recognition, mediated by phosphotyrosine containing peptides, play an important role in determining the specific response of a cell, when stimulated by external signals. We have used peptide repertoires displayed by filamentous phage as a tool to study the substrate specificity of the protein tyrosine kinase (PTK) p55(fyn) (Fyn). Peptide libraries were incubated for a short time in the presence of Fyn and phages displaying efficiently phosphorylated peptides were selected by panning over anti-phosphotyrosine antibodies. The characterization of the peptides enriched after three phosphorylation/selection rounds allowed us to define a canonical substrate sequence for the kinase Fyn, E-(phi/T)YGx phi, where phi represents any hydrophobic residue. A peptide conforming to this sequence is a better substrate than a second peptide designed to be in accord with the consensus sequence recognised by the Fyn SH2 domain. When the library phosphorylation reaction is carried out in saturation conditions, practically all the tyrosine containing peptides are phosphorylated, irrespective of their context. These "fully modified" peptide libraries are a valuable tool to study the specificity of phosphotyrosine mediated protein recognition. We have used this new tool to identify a family of peptides that bind the PTB domain of the adapter protein Shc. Comparison of the peptide sequences permits us to confirm the essential role of N at position -3, while P often found at position -2 in natural targets is not absolutely required. Furthermore, our approach permits us to reveal an "extended" consensus indicating that residues that do not seem to influence binding in natural peptides can make productive contacts, at least in linear peptides.

Signal transduction pathways involved in BCR-ABL transformation

BCR-ABL is an oncogenic fusion gene found in patients with chronic myelogenous leukaemia (CML) and acute lymphocytic leukaemia whose oncogenic potential has been demonstrated using in vitro and in vivo model systems. Current research efforts are focused on defining the mechanism by which BCR-ABL transforms cells, with a view toward applying insights from these studies to the treatment of CML patients. BCR-ABL contains tyrosine residues, an SH2 domain, an SH3 domain, and proline-rich sequences. The presence of so many protein-protein interaction domains raises the possibility of multiple contacts with cellular signal transduction pathways. Indeed, BCR-ABL is reported to bind and/or phosphorylate more than 20 proteins. Many of these can be directly linked to signal transduction pathways based on defined roles in other systems, but others have no known function. As the list of such proteins grows, it is critical to define the role of each in the leukaemogenic activity of BCR-ABL. This review summarizes current views of the mechanism of BCR-ABL transformation with emphasis on the substrates and signal transduction pathways affected by its tyrosine kinase activity.

Functional interaction between DNA-PK and c-Abl in response to DNA damage

How DNA damage is converted into intracellular signals that can control cell behaviour is unknown. The c-Abl protein tyrosine kinase is activated by ionizing radiation and certain other DNA-damaging agents, whereas the DNA-dependent protein kinase (DNA-PK), consisting of a serine/threonine kinase and Ku DNA-binding subunits, requires DNA double-strand breaks or other DNA lesions for activation. Here we demonstrate that c-Abl interacts constitutively with DNA-PK. Ionizing radiation stimulates binding of c-Abl to DNA-PK and induces an association of c-Abl with Ku antigen. We show that DNA-PK phosphorylates and activates c-Abl in vitro. Cells deficient in DNA-PK are defective in c-Abl activation induced by ionizing radiation. In a potential feedback mechanism, c-Abl phosphorylates DNA-PK, but not Ku, in vitro. Phosphorylation of DNA-PK by c-Abl inhibits the ability of DNA-PK to form a complex with DNA. We also show that treatment of cells with ionizing radiation results in phosphorylation of DNA-PK that is dependent on c-Abl. Our results support the hypothesis that there are functional interactions between c-Abl and DNA-PK in the response to DNA damage.

The p120-v-Abl protein interacts with E2F-1 and regulates E2F-1 transcriptional activity

The E2F family of transcription factors regulates cell cycle progression, and deregulated expression of E2F-1 can lead to neoplastic transformation. In myeloid cells, introduction and expression of the Abelson leukemia virus causes growth factor independence. Here, the p120 v-Abl protein activates E2F-1-mediated transcription through a physical interaction with the E2F-1 transcription factor. BCR-Abl and c-Abl also stimulate E2F-1-mediated transcription. Our results suggest a new mechanism by which v-Abl leads to factor-independent myeloid cell proliferation: the activation of E2F-1-mediated transcription.

Experimental plasmacytomagenesis in mice

This article discusses some of the mechanistic aspects of plasma cell tumor development. Plasmacytomagenesis, much like other forms of neoplastic development, is a highly complex process that develops in the B cell differentiation lineage. As more is learned about the molecular genetics of multiple myeloma and PCTs in mice, a unifying concept will emerge that possibly can explain the phenotypic differences in the two neoplastic cell processes as variants of a common process.

Activation of Pak by membrane localization mediated by an SH3 domain from the adaptor protein Nck

BACKGROUND

The adaptor protein Nck consists of three Src homology 3 (SH3) domains followed by one SH2 domain. Like the Grb2 adaptor protein, which is known to couple receptor tyrosine kinases to the small GTPase Ras, Nck is presumed to bind to tyrosine-phosphorylated proteins using its SH2 domain and to downstream effector proteins using its SH3 domain. Little is known, however, about the specific biological function of Nck. The Pak family of serine/threonine kinases are known to be activated by binding to the GTP-bound form of Cdc42 or Rac1, which are small GTPases of the Rho family that are involved in regulating the organization of the actin cytoskeleton.

RESULTS

We present evidence that Nck can mediate the relocalization and subsequent activation of the Pak1 kinases. We show that Nck associates in vivo with Pak using the second of its three SH3 domains, and that localization of this individual Nck SH3 domain, or of Pak kinase itself, to the membrane results in activation of Pak and stimulation of downstream mitogen activated protein kinase cascades. Activation of downstream signaling by the membrane-localized Nck SH3 domain is blocked by a kinase-inactive mutant form of Pak1.

CONCLUSION

These results demonstrate that localization of Pak1 to the membrane in the absence of other signals is sufficient for its activation, and imply that the Nck adaptor protein could function to link changes in tyrosine phosphorylation of cellular proteins to the Cdc42/Pak signaling pathway.

Deletion of Src homology 3 domain results in constitutive activation of Tec protein-tyrosine kinase

Tec protein-tyrosine kinase (PTK) is the prototype of a new subfamily of non-receptor type PTKs, and is abundantly expressed in hematopoietic tissues. We have revealed that Tec is inducibly tyrosine-phosphorylated and activated by stimulation with a wide range of cytokines. To get more insight into the signaling mechanism through Tec, we have generated a constitutively active form of Tec PTK. Deletion of the Src homology (SH) 3 domain gave rise to a hyperphosphorylated and activated Tec kinase (Tec deltaSH3). The activity of Tec deltaSH3 was confirmed in 293 cells, as well as in cytokine-dependent hematopoietic cells (BA/F3). Tec deltaSH3 should be a useful tool to study the in vivo substrates of Tec PTK.

Involvement of a polyproline helix-like structure in the interaction of 7B2 with prohormone convertase 2

The neuroendocrine protein 7B2 is known to be involved in the biosynthesis and activity of prohormone convertase 2 (PC2). Previous studies have demonstrated that while the carboxyl-terminal portion of 7B2 (residues 155-186) regulates the enzymatic activity of PC2, the amino terminus of the molecule (residues 1-151) is required for maturation of proPC2. In this study we employed four different experimental approaches (co-immunoprecipitation with proPC2, facilitation of pro-PC2 maturation, acquisition of enzymatic activity, and thermal protection assays) to identify structural elements of 7B2 important for bioactivity. Inspection of the sequence of 7B2 indicated potential involvement of a polyproline helix-like (PPII) structure, with similarities to those present within SH3 domain ligands, in the interaction of 7B2 with proPC2. Site-directed point mutagenesis of this proline-rich region confirmed the involvement of this area. Replacement of prolines in positions critical to helix formation (Pro90, Pro91, Pro93, and Pro95) either severely impaired or totally abolished 7B2 bioactivity, as gauged by the four assays described. In addition, constructs longer than residues 1-121 were still functional, whereas those shorter than residues 1-109 were not. Computer-assisted analysis predicts the presence of an alpha-helix structure between residues 107 and 123. We conclude that both the proline-rich region and the alpha-helix contribute to 7B2 activity. Polyproline-containing peptides have been shown to be involved in cytoplasmic protein-protein interactions; our results suggest that the polyproline helix motif may also be used to mediate protein-protein interactions within the secretory pathway.

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.

Growth inhibition by Abl requires an interplay of its SH2 and tyrosine kinase domains

Overexpression of c-Abl tyrosine kinase can be growth inhibitory in certain fibroblast cell lines. Using a series of conditional chimeras between Abl and Src, we have now further dissected the Abl protein to determine which domains are required for this function. We found that growth inhibition, unlike transformation by oncogenic forms of Abl, is dependent on the presence of the cognate SH2 and tyrosine kinase domains. Since growth inhibition correlates with low tyrosine kinase activity, it may involve highly specific interactions of target proteins with both domains without the processivity of phosphorylation associated with oncogenic Abl.

Regulation of Btk function by a major autophosphorylation site within the SH3 domain

Bruton's tyrosine kinase (Btk) plays a crucial role in B cell development. Overexpression of Btk with a Src family kinase increases tyrosine phosphorylation and catalytic activity of Btk. This occurs by transphosphorylation at Y551 in the Btk catalytic domain and the enhancement of Btk autophosphorylation at a second site. A gain-of-function mutant called Btk* containing E41 to K change within the pleckstrin homology domain induces fibroblast transformation. Btk* enhances the transphosphorylation of Y551 by endogenous Src family tyrosine kinases and autophosphorylation at the second site. We mapped the major Btk autophosphorylation site to Y223 within the SH3 domain. Mutation of Y223 to F blocks Btk autophosphorylation and dramatically potentiates the transforming activity of Btk* in fibroblasts. The location of Y223 in a potential ligand-binding pocket suggests that autophosphorylation regulates SH3-mediated signaling by Btk.

c-Abl activation regulates induction of the SEK1/stress-activated protein kinase pathway in the cellular response to 1-beta-D-arabinofuranosylcytosine

Previous work has shown that treatment of cells with the antimetabolite 1-beta-D-arabinofuranosylcytosine (ara-C) is associated with induction of the c-jun gene. The present studies demonstrate that ara-C activates the c-Abl non-receptor tyrosine kinase. We also demonstrate that activity of the stress-activated protein kinase (SAP kinase/JNK) is increased in ara-C-treated cells. Using cells deficient in c-Abl (Abl-/-) and after introduction of the c-abl gene, we show that ara-C-induced c-Abl activity is necessary for the stimulation of SAP kinase. Other studies using cells transfected with a SEK1 dominant negative demonstrate that ara-C-induced SAP kinase activity is SEK1-dependent. Furthermore, we show that overexpression of truncated c-Abl results in activation of the SEK1/SAP kinase cascade.

The solution structure of Abl SH3, and its relationship to SH2 in the SH(32) construct

BACKGROUND

The Src homology domains, SH3 and SH2, of Abl protein tyrosine kinase regulate enzymatic activity in vivo. Abl SH3 suppresses kinase activity, whereas Abl SH2 is required for the transforming activity of the activated form of Abl. We expect that the solution structures of Abl SH3, Abl SH2 and Abl SH(32) (a dual domain comprising SH3 and SH2 subdomains) will contribute to a structural basis for understanding the mechanism of the Abl 'regulatory apparatus'.

RESULTS

We present the solution structure of the free Abl SH3 domain and a structural characterization of the Abl regulatory apparatus, the SH(32) dual domain. The solution structure of Abl SH3 was determined using multidimensional double resonance NMR spectroscopy. It consists of two antiparallel beta sheets packed orthogonally, an arrangement first shown in spectrin SH3. Compared with the crystal structure of the Abl SH3 complexed with a natural ligand, there is no significant difference in overall folding pattern. The structure of the Abl SH(32) dual domain was characterized by NMR spectroscopy using the 1H and 15N resonance assignment of Abl SH3 and Abl SH2. On the basis of the high degree of similarity in chemical shifts and hydrogen/deuterium exchange pattern for the individual domains of SH3 and SH2 compared with those of the SH(32) dual domain, a structural model of the Abl SH(32) regulatory apparatus is suggested. This model is in good agreement with the ligand-binding characteristics of Abl SH3, SH2 and SH(32). The binding constants for isolated SH3 and SH2 domains when binding to natural ligands, measured by intrinsic fluorescence quenching, do not differ significantly from the constants of these domains within SH(32).

CONCLUSION

The solution structures of free Abl SH3 and Abl SH2, and the structural model of Abl SH(32), provide information about the overall topology of these modular domains. The structural model of Abl SH(32), a monomer, consists of the SH3 and SH2 domains connected by a flexible linker. Sites of ligand binding for the two subdomains are independent.

Tyrosine kinases: modular signaling enzymes with tunable specificities

Cytoplasmic tyrosine kinases are composed of modular domains; one (SH1) has catalytic activity, the other two (SH2 and SH3) do not. Kinase specificity is largely determined by the binding preferences of the SH2 domain. Attaching the SH1 domain to a new SH2 domain, via protein-protein association or mutation, can thus dramatically change kinase function.

The SH2 domains of Src family kinases associate with Syk

Src family kinases (Lyn, Fyn, Lck, and Blk) and Syk, a tandem SH2 domain containing tyrosine kinase, have been demonstrated to be associated with the antigen receptor in B cells. Both of these categories of tyrosine kinases are presumed to be critical players in the process of antigen-mediated signal transduction. Cross-linking of membrane immunoglobulin on the surface of B cells leads to the activation of Lyn, Fyn, and Blk, which presumably associate with the cytoplasmic tails of the membrane immunoglobulin-associated Ig alpha/beta heterodimer. Receptor ligation also leads to the tyrosine phosphorylation and catalytic activation of Syk, but the mechanism of association of this kinase with the antigen receptor remains to be established. A number of phosphoproteins that can associate with the SH2 domains of Blk, Lyn, and Fyn have been described in activated B cells. We demonstrate here that Syk is one of the proteins in the lysates of activated B cells which bind to the SH2 domains of Src family kinases. Syk binds directly to the SH2 domain of Blk and complexes in vivo with Lyn and Blk in activated B cells.

Structure-function relationships in Src family and related protein tyrosine kinases

There is increasing evidence to suggest that cytoplasmic tyrosine kinases of the Src family have a pivotal role in the regulation of a number of cellular processes. Members of this family have been implicated in cellular responses to a variety of extracellular signals, such as those arising from growth factors and cell-cell interactions, as well as in differentiative and developmental processes in both vertebrates and invertebrates. A better understanding of the regulation and of the structure-function relationships of these enzymes might aid in the development of specific ways to interfere with their action, as well as serving as a paradigm for regulation of other protein tyrosine kinases that have SH2 and SH3 domains. In this review we will first discuss the regulation of Src family protein tyrosine kinases, with particular emphasis on their SH2 and SH3 domains. We will then briefly review other non-receptor protein tyrosine kinases that have SH2 and SH3 domains.

Evidence that SH2 domains promote processive phosphorylation by protein-tyrosine kinases

BACKGROUND

Non-receptor protein-tyrosine kinases often contain at least one Src homology 2 (SH2) domain, a protein module that binds with high affinity to tyrosine-phosphorylated peptides. Because SH2 domains would be predicted to bind with high affinity to proteins phosphorylated by the kinase, but not to the unphosphorylated substrate, their presence in tyrosine kinases has been puzzling. An important role for the SH2 domain of the Abl tyrosine kinase was suggested by work showing that Abl requires an intact SH2 domain in order to malignantly transform cells, and that replacement of the Abl SH2 domain with heterologous SH2 domains alters the spectrum of proteins phosphorylated detectably by Abl in vivo.

RESULTS

We have used purified wild-type and mutant Abl kinases to examine the roles of the Abl's SH2 and catalytic domains in phosphorylation of p130CAS, a model substrate that has multiple potential phosphorylation sites. We find that an SH2 domain is required for efficient hyperphosphorylation of p130 in vitro. We use chimeric mutants with heterologous SH2 domains to demonstrate that the SH2 domain of the oncogenically transforming adaptor protein Crk, which is the SH2 domain predicted to bind with highest affinity (of those tested) to potential phosphorylation sites in p130, is best able to facilitate hyperphosphorylation. This is the case whether the catalytic domain of the kinase is derived from Abl or from its distant relative, Src. These studies also reveal a role for binding of Crk to Abl in mediating phosphorylation by the kinase. Using purified proteins, we demonstrate that association with Crk strikingly enhances the ability of Abl to hyperphosphorylate p130. There is an excellent correlation between the ability of mutant Crk proteins to promote hyperphosphorylation of p130 by Abl and their ability to transform rodent fibroblasts.

CONCLUSION

Our data suggest that, ultimately, the substrate specificity of a non-receptor tyrosine kinase is dependent on the binding specificity of its associated SH2 domain. The SH2 domain binds tightly to a subset of proteins phosphorylated by the catalytic domain, leading to processive phosphorylation of those proteins. Substrate specificity can be broadened by an association between the kinase and proteins, such as Crk, that contain additional SH2 domains; this may play a role in malignant transformation by Crk.

Catalytic specificity of protein-tyrosine kinases is critical for selective signalling

How do distinct protein-tyrosine kinases activate specific down-stream events? Src-homology-2 (SH2) domains on tyrosine kinases or targets of tyrosine kinases recognize phosphotyrosine in a specific sequence context and thereby provide some specificity. The role of the catalytic site of tyrosine kinases in determining target specificity has not been fully investigated. Here we use a degenerate peptide library to show that each of nine tyrosine kinases investigated has a unique optimal peptide substrate. We find that the cytosolic tyrosine kinases preferentially phosphorylate peptides recognized by their own SH2 domains or closely related SH2 domains (group I; ref. 3), whereas receptor tyrosine kinases preferentially phosphorylate peptides recognized by subsets of group III SH2 domains. The importance of these findings for human disease is underscored by our observation that a point mutation in the RET receptor-type tyrosine kinase, which causes multiple endocrine neoplasia type 2B, results in a shift in peptide substrate specificity.