Recognition and specificity in protein tyrosine kinase-mediated signalling

Association of protein kinase A type I with detergent-resistant structures of mammalian sperm cells

The finding that flagellar movement in detergent-permeabilized sperm cells is restored when Mg ATP and cAMP are added implicated detergent-resistant protein kinase A (PKA) in the regulation of sperm motility. It is widely believed that only the PKA regulatory subunit RII can associate with the cytoskeleton and/or organelles. In this paper we used monoclonal antibodies against the PKA catalytic subunit and RI subunit and demonstrated that PKA type I is also associated with the sperm cytoskeleton. To our knowledge, this is the first report showing anchored PKA type I. This association was found in sperm of nonrodent mammalian species and, to a lesser extent, also in mouse sperm. The PKA catalytic subunit is bound to the cytoskeleton secondarily via its complex with the regulatory subunit. The detergent-resistant complexes of RI and catalytic subunits localize predominantly to the flagellum. Ultrastructural immunogold labeling revealed the association of detergent-resistant PKA type I with outer dense fibers (ODF) and the fibrous sheath (FS) but not with microtubules. This location is consistent with a proposed role of PKA in regulation of FS sliding on underlying ODF.

Tyrosine phosphorylation regulates the SH3-mediated binding of the Wiskott-Aldrich syndrome protein to PSTPIP, a cytoskeletal-associated protein

Wiskott-Aldrich syndrome is an X-linked hematopoietic disease that manifests itself in platelet deficiency and a compromised immune system. Analysis of hematopoietic cells from affected individuals reveals that mutations in the Wiskott-Aldrich syndrome protein (WASP) result in structural and functional abnormalities in the cell cortex, consistent with the suggestion that WASP is involved with regulation of the actin-rich cortical cytoskeleton. Here we report that WASP interacts with a recently described cytoskeletal-associated protein, PSTPIP, a molecule that is related to the Schizosaccharomyces pombe cleavage furrow regulatory protein, CDC15p. This association is mediated by an interaction between the PSTPIP SH3 domain and two polyproline-rich regions in WASP. Co-expression of PSTPIP with WASP in vivo results in a loss of WASP-induced actin bundling activity and co-localization of the two proteins, which requires the PSTPIP SH3 domain. Analysis of tyrosine phosphorylation of PSTPIP reveals that two sites are modified in response to v-Src co-transfection or pervanadate incubation. One of these tyrosines is found in the SH3 domain poly-proline recognition site, and mutation of this tyrosine to aspartate or glutamate to mimic this phosphorylation state results in a loss of WASP binding in vitro and a dissolution of co-localization in vivo. In addition, PSTPIP that is tyrosine phosphorylated in the SH3 domain interacts poorly with WASP in vitro. These data suggest that the PSTPIP and WASP interaction is regulated by tyrosine phosphorylation of the PSTPIP SH3 domain, and this binding event may control aspects of the actin cytoskeleton.

PDZ motifs in PTP-BL and RIL bind to internal protein segments in the LIM domain protein RIL

The specificity of protein-protein interactions in cellular signaling cascades is dependent on the sequence and intramolecular location of distinct amino acid motifs. We used the two-hybrid interaction trap to identify proteins that can associate with the PDZ motif-rich segment in the protein tyrosine phosphatase PTP-BL. A specific interaction was found with the Lin-11, Isl-1, Mec-3 (LIM) domain containing protein RIL. More detailed analysis demonstrated that the binding specificity resides in the second and fourth PDZ motif of PTP-BL and the LIM domain in RIL. Immunohistochemistry on various mouse tissues revealed a submembranous colocalization of PTP-BL and RIL in epithelial cells. Remarkably, there is also an N-terminal PDZ motif in RIL itself that can bind to the RIL-LIM domain. We demonstrate here that the RIL-LIM domain can be phosphorylated on tyrosine in vitro and in vivo and can be dephosphorylated in vitro by the PTPase domain of PTP-BL. Our data point to the presence of a double PDZ-binding interface on the RIL-LIM domain and suggest tyrosine phosphorylation as a regulatory mechanism for LIM-PDZ associations in the assembly of multiprotein complexes. These findings are in line with an important role of PDZ-mediated interactions in the shaping and organization of submembranous microenvironments of polarized cells.

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.

Cloning and tyrosine phosphorylation of a novel invertebrate immunocyte protein containing immunoreceptor tyrosine-based activation motifs

Immunoreceptor tyrosine-based activation motif (ITAM) plays an important role in signal transduction through mammalian T-cell and B-cell antigen receptors and Fc receptors. The ITAM has been found only in vertebrate immunocytes. Ascidians are intriguing invertebrates from the viewpoint of the evolution of immune systems because they are considered to be ancestors of the vertebrates. We have previously shown that the monoclonal antibody A74 inhibits cellular defense reactions of the ascidian. In the present studies, we found that the A74 antigen protein has two ITAMs and several motifs that are proposed to function in signal transduction. The A74 protein is tyrosine-phosphorylated and associated with other proteins in the initial stages of cellular defense reactions. The ITAMs of the A74 protein are tyrosine-phosphorylated by a c-Src kinase in vitro. The A74 protein provides a key to the understanding of the origin of vertebrate immune systems.

Cross-talk between phorbol ester-mediated signaling and tyrosine kinase proto-oncogenes. I. Activation of protein kinase C stimulates tyrosine phosphorylation and activation of ErbB2 and ErbB3

The tumor-promoting phorbol ester, phorbol 12-myristate 13-acetate (PMA), acutely stimulates the tyrosine phosphorylation of proteins of approximately 190, 120, and 70 kDa in the well differentiated Fao rat hepatoma cell line. This phosphorylation is dependent on protein kinase C (PKC) and is abolished by down-regulation of PKC or pretreatment with a PKC inhibitor. Purification of the 190-kDa tyrosine-phosphorylated protein revealed that it consists of both ErbB2 and ErbB3. Following PMA-induced tyrosine phosphorylation, ErbB2 and ErbB3 were able to associate with the SH2 domains of several signaling proteins including the p85alpha subunit of phosphatidylinositol 3-kinase, Syp, and Grb2. The 120-kDa protein phosphorylated in response to PMA consists of at least two proteins: focal adhesion kinase that exhibits a minor increase in tyrosine phosphorylation following treatment with PMA, and a major 120-kDa tyrosine-phosphorylated species in PMA-stimulated Fao cells which as yet is unidentified. Similarly, the 70-kDa tyrosine-phosphorylated protein also appears to represent more than one protein, including paxillin and a second protein of similar mobility which appears to be the major tyrosine phosphorylation in response to PMA. Both ErbB2 and paxillin also exhibit reduced migration on SDS-polyacrylamide gel electrophoresis following PMA treatment, suggesting that they are also phosphorylated on serine/threonine residues. The mobility shift of both of these proteins is abolished by treatment with inhibitors of PKC or mitogen-activated protein kinase/extracellular signal-related kinase kinase. These results suggest a novel mechanism of cross-talk between the serine/threonine kinase PKC and tyrosine phosphorylation pathways. The activation of ErbB2 and ErbB3 that is initiated by PMA may contribute to the tumor promoting activity of these compounds.

Insulin dependent tyrosine phosphorylation of the tyrosine internalisation motif of TGN38 creates a specific SH2 domain binding site

Tyrosine-based motifs are involved in both protein targeting and, via SH2 domain binding, intracellular signalling. To date there has only been one example of such a motif acting as both an intracellular sorting signal and SH2 binding determinant, namely that of the T cell costimulation receptor, CTLA-4. We show that insulin stimulation of cultured rat hepatoma cells results in increased cell surface expression of TGN38. Furthermore, the cytosolic domain of TGN38 can be phosphorylated by the insulin receptor in vitro and tyrosine phosphorylated TGN38 can specifically bind to the SH2 domains of the spleen tyrosine kinase Syk. These data imply that tyrosine-based motifs may play a broader role than has previously been accepted and could help to integrate trafficking and signalling events.

daf-2, an insulin receptor-like gene that regulates longevity and diapause in Caenorhabditis elegans

A C. elegans neurosecretory signaling system regulates whether animals enter the reproductive life cycle or arrest development at the long-lived dauer diapause stage. daf-2, a key gene in the genetic pathway that mediates this endocrine signaling, encodes an insulin receptor family member. Decreases in DAF-2 signaling induce metabolic and developmental changes, as in mammalian metabolic control by the insulin receptor. Decreased DAF-2 signaling also causes an increase in life-span. Life-span regulation by insulin-like metabolic control is analogous to mammalian longevity enhancement induced by caloric restriction, suggesting a general link between metabolism, diapause, and longevity.

Ezrin is an effector of hepatocyte growth factor-mediated migration and morphogenesis in epithelial cells

The dissociation, migration, and remodeling of epithelial monolayers induced by hepatocyte growth factor (HGF) entail modifications in cell adhesion and in the actin cytoskeleton through unknown mechanisms. Here we report that ezrin, a membrane-cytoskeleton linker, is crucial to HGF-mediated morphogenesis in a polarized kidney-derived epithelial cell line, LLC-PK1. Ezrin is a substrate for the tyrosine kinase HGF receptor both in vitro and in vivo. HGF stimulation causes enrichment of ezrin recovered in the detergent-insoluble cytoskeleton fraction. Overproduction of wild-type ezrin, by stable transfection in LLC-PK1 cells, enhances cell migration and tubulogenesis induced by HGF stimulation. Overproduction of a truncated variant of ezrin causes mislocalization of endogenous ezrin from microvilli into lateral surfaces. This is concomitant with altered cell shape, characterized by loss of microvilli and cell flattening. Moreover, the truncated variant of ezrin impairs the morphogenic and motogenic response to HGF, thus suggesting a dominant-negative mechanism of action. Site-directed mutagenesis of ezrin codons Y145 and Y353 to phenylalanine does not affect the localization of ezrin at microvilli, but perturbs the motogenic and morphogenic responses to HGF. These results provide evidence that ezrin displays activities that can control cell shape and signaling.

Loss of expression of receptor tyrosine kinase family genes PTK7 and SEK in metastatic melanoma

Protein tyrosine kinases (PTKs) have been implicated in the development of many common human tumours including melanoma. Previously we isolated PTK gene sequences expressed in normal melanocytes. Here we examined expression of 9 of these genes in cell lines derived from defined stages of melanoma progression, by Northern blotting and in some cases immunoblotting. We also tested cells from 2 animal models of particular stages in progression, as well as uncultured biopsies of metastatic melanoma. The expression of 2 receptor kinase family members found in melanocytes, PTK7/CCK-4 and SEK/TYRO1, was decreased or lost in advanced melanomas. PTK7 mRNA was found in only 54% of melanoma cell lines and 20% of melanoma biopsies. Similarly, expression was lost in 2 advanced cell lines selected from an early melanoma line that did express PTK7 mRNA. SEK/TYRO1 expression was observed in 75% and 17% of cell lines from primary and metastastic melanomas, respectively. Conversely, mRNA for the non-receptor kinase PTK6/BRK was not detected in normal melanocytes or primary melanoma lines, but was found in 9% of metastatic melanoma cell lines.

The complexity of signaling pathways activated by the BCR

Cross-linking of the B cell antigen receptor (BCR) leads to the activation of three types of intracellular protein tyrosine kinases. These tyrosine kinases then phosphorylate signaling components to activate a variety of signaling reactions, including phosphatidylinositol 4,5-bisphosphate hydrolysis, Ras activation, and phosphatidylinositol 3-kinase activation. Each of these signaling reactions, and also the signaling molecules Vav and HS1, appears to be important for at least some of the many types of B cell responses to antigen. The complexity of BCR signaling reactions may be required to allow the B cell to respond in a number of distinct ways to antigen (proliferation, survival, apoptosis, maturational arrest, etc.) depending on the maturation state of the B cell, the location in the body, the physical nature of the antigen, and the possible presence of the antigen in complex with antibody or complement components.

Sequence specificity of C-terminal Src kinase (CSK)--a comparison with Src-related kinases c-Fgr and Lyn

An eicosapeptide encompassing the C-terminal tail of c-Src (Tyr527) which is conserved in most Src-related protein kinases, is phosphorylated by C-terminal Src kinase (CSK) and by the two Src-related protein kinases c-Fgr and Lyn, with similar kinetic constants. Two related peptides reproducing the C-terminal segments of c-Src mutants defective in CSK phosphorylation [MacAuley, A., Okada, M., Nada, S., Nakagawa, H. & Cooper, J. A. (1993) Oncogene 8, 117-124] AFLEDSCTGTEPLYQRGENL (mutant number 28) and AFLEDNFTGTKPQYHPGENL (mutant number 29), proved a better and a much worse substrates, respectively than the wild-type peptide, with either CSK or the two Src kinases. By changing individual residues in the best peptide substrate, it was shown that the main element responsible for its improved phosphorylation is leucine at position -1 (instead of glutamine), while lysine at position -3 (instead of glutamate) has a detrimental effect, possibly accounting for the negligible phosphorylation of peptide derived from mutant number 29. By contrast to most peptide substrates, including the Src C-terminal peptides, which exhibit relatively high K(m) values, a polyoma-virus-middle-T-antigen-(mT)-derived peptide with tyrosine embedded in a highly hydrophobic sequence (EEEPQFEEIPIYLELLP) exhibits with CSK a quite low K(m) value (63 microM). Consistent with this, the optimal sequence selected by CSK in an oriented peptide library is XXXIYMFFF. This is different from sequences selected by Lyn (DEEIYEELX) and c-Fgr (XEEIYGIFF), although they all share a high selection for a hydrophobic residue at n-1. In sharp contrast, TPKIIB/p38syk, related to the catalytic domain of p72syk, selects acidic residues at nearly all positions, n-1 included. These data support the notion that the features determining the specific phosphorylation of the C-terminal tyrosine residue of Src do not reside in the primary structure surrounding the target tyrosine. They also show that this site does not entirely fulfil the optimal consensus sequence recognized by CSK, disclosing the possibility that as yet unrecognized CSK targets structurally unrelated to the C-terminal tyrosine residue of Src kinases may exist.

Molecular cloning of SLAP-130, an SLP-76-associated substrate of the T cell antigen receptor-stimulated protein tyrosine kinases

Previous work has demonstrated that SLP-76, a Grb2-associated tyrosine-phosphorylated protein, augments Interleukin-2 promoter activity when overexpressed in the Jurkat T cell line. This activity requires regions of SLP-76 that mediate protein-protein interactions with other molecules in T cells, suggesting that SLP-76-associated proteins also function to regulate signal transduction. Here we describe the molecular cloning of SLAP-130, a SLP-76-associated phosphoprotein of 130 kDa. We demonstrate that SLAP-130 is hematopoietic cell-specific and associates with the SH2 domain of SLP-76. Additionally, we show that SLAP-130 is a substrate of the T cell antigen receptor-induced protein tyrosine kinases. Interestingly, we find that in contrast to SLP-76, overexpression of SLAP-130 diminishes T cell antigen receptor-induced activation of the interleukin-2 promoter in Jurkat T cells and interferes with the augmentation of interleukin-2 promoter activity seen when SLP-76 is overexpressed in these cells. These data suggest that SLP-76 recruits a negative regulator, SLAP-130, as well as positive regulators of signal transduction in T cells.

A novel Drosophila receptor tyrosine kinase expressed specifically in the nervous system. Unique structural features and implication in developmental signaling

We report the identification and characterization of Dnrk (Drosophila neurospecific receptor kinase), a Drosophila gene encoding a putative receptor tyrosine kinase (RTK) highly related to the Trk and Ror families of RTKs. During Drosophila embryogenesis, the Dnrk gene is expressed specifically in the developing nervous system. The Dnrk protein possesses two conserved cysteine-containing domains and a kringle domain within its extracellular domain, resembling those observed in Ror family RTKs (Ror1, Ror2, and a Drosophila Ror, Dror). This protein contains the catalytic tyrosine kinase (TK) domain with two putative ATP-binding motifs, resembling those observed in another Drosophila RTK (Dtrk) that mediates homophilic cell adhesion. The TK domain of Dnrk, expressed in bacteria or mammalian cells, exhibits apparent autophosphorylation activities in vitro. The TK domain lacking the distal ATP-binding motif also exhibits autophosphorylation activity, yet to a lesser extent. In addition to its TK activity, there are several putative tyrosine-containing motifs that upon phosphorylation may interact with Src homology 2 regions of other signaling molecules. Collectively, these results suggest that Dnrk may play an important role in neural development during Drosophila embryogenesis.

Dynamic glycosylation of nuclear and cytosolic proteins. Cloning and characterization of a unique O-GlcNAc transferase with multiple tetratricopeptide repeats

O-Linked N-acetylglucosamine (O-GlcNAc) glycosylation is a dynamic modification of eukaryotic nuclear and cytosolic proteins analogous to protein phosphorylation. We have cloned and characterized a novel gene for an O-GlcNAc transferase (OGT) that shares no sequence homology or structural similarities with other glycosyltransferases. The OGT gene is highly conserved (up to 80% identity) in all eukaryotes examined. Unlike previously described glycosyltransferases, OGT is localized to the cytosol and nucleus. The OGT protein contains multiple tandem repeats of the tetratricopeptide repeat motif. The presence of tetratricopeptide repeats, which can mediate protein-protein interactions, suggests that OGT may be regulated by protein interactions that are independent of the enzyme's catalytic site. The OGT is also modified by tyrosine phosphorylation, indicating that tyrosine kinase signal transduction cascades may play a role in modulating OGT activity.

Tyrosine phosphorylation of transmembrane ligands for Eph receptors

Axonal pathfinding in the nervous system is mediated in part by cell-to-cell signaling events involving members of the Eph receptor tyrosine kinase (RTK) family and their membrane-bound ligands. Genetic evidence suggests that transmembrane ligands may transduce signals in the developing embryo. The cytoplasmic domain of the transmembrane ligand Lerk2 became phosphorylated on tyrosine residues after contact with the Nuk/Cek5 receptor ectodomain, which suggests that Lerk2 has receptorlike intrinsic signaling potential. Moreover, Lerk2 is an in vivo substrate for the platelet-derived growth factor receptor, which suggests crosstalk between Lerk2 signaling and signaling cascades activated by tyrosine kinases. It is proposed that transmembrane ligands of Eph receptors act not only as conventional RTK ligands but also as receptorlike signaling molecules.

Growth factor-dependent phosphoinositide signalling

A wide variety of messages, in the form of diffusible growth factors, hormones and cytokines, are carried throughout multicellular organisms to coordinate important physiological properties of target cells, such as proliferation, differentiation, migration, apoptosis and metabolism. Most messengers bind to cognate receptors on target cells, which initiate a characteristic cascade of reactions within the cell, ultimately leading to the desired response. The cellular response is defined by the combination of signalling components whose individual activity depends upon the number and type of surface receptors. Consequently the responses of different cell types to one or more stimuli can be quite disparate. A molecular understanding of the signalling pathways employed by each type of receptor therefore underlies the ability to rationalize many cellular functions and to correct disfunctions. As a well studied example of the primary signalling events that take place on the cytoplasmic leaflet of the plasma membrane following receptor activation, we will discuss how the widely expressed receptor for epidermal growth factor (EGF) causes the phosphorylation and hydrolysis of a signalling precursor, the membrane lipid phosphatidylinositol. This paradigm will be used to illustrate certain general principles of signalling, including formation of multienzyme complexes, compartmentation of second messengers and intermediates, and cross-talk between different signalling pathways.

Modular peptide recognition domains in eukaryotic signaling

A characteristic feature of cellular signal transduction pathways in eukaryotes is the separation of catalysis from target recognition. Several modular domains that recognize short peptide sequences and target signaling proteins to these sequences have been identified. The structural bases of the specificities of recognition by SH2, SH3, and PTB domains have been elucidated by X-ray crystallography and NMR, and these results are reviewed here. In addition, the mechanism of cooperative interactions between these domains is discussed.

Direct association with and dephosphorylation of Jak2 kinase by the SH2-domain-containing protein tyrosine phosphatase SHP-1

SHP-1 is an SH2-containing cytoplasmic tyrosine phosphatase that is widely distributed in cells of the hematopoietic system. SHP-1 plays an important role in the signal transduction of many cytokine receptors, including the receptor for erythropoietin, by associating via its SH2 domains to the receptors and dephosphorylating key substrates. Recent studies have suggested that SHP-1 regulates the function of Jak family tyrosine kinases, as shown by its constitutive association with the Tyk2 kinase and the hyperphosphorylation of Jak kinases in the motheaten cells that lack functional SHP-1. We have examined the interactions of SHP-1 with two tyrosine kinases activated during engagement of the erythropoietin receptor, the Janus family kinase Jak-2 and the c-fps/fes kinase. Immunoblotting studies with extracts from mouse hematopoietic cells demonstrated that Jak2, but not c-fes, was present in anti-SHP-1 immunoprecipitates, suggesting that SHP-1 selectively associates with Jak2 in vivo. Consistent with this, when SHP-1 was coexpressed with these kinases in Cos-7 cells, it associated with and dephosphorylated Jak2 but not c-fes. Transient cotransfection of truncated forms of SHP-1 with Jak2 demonstrated that the SHP-1-Jak2 interaction is direct and is mediated by a novel binding activity present in the N terminus of SHP-1, independently of SH2 domain-phosphotyrosine interaction. Such SHP-1-Jak2 interaction resulted in induction of the enzymatic activity of the phosphatase in in vitro protein tyrosine phosphatase assays. Interestingly, association of the SH2n domain of SHP-1 with the tyrosine phosphorylated erythropoietin receptor modestly potentiated but was not essential for SHP-1-mediated dephosphorylation of Jak2 and had no effect on c-fes phosphorylation. These data indicate that the main mechanism for regulation of Jak2 phosphorylation by SHP-1 involves a direct, SH2-independent interaction with Jak2 and suggest the existence of similar mechanisms for other members of the Jak family of kinases. They also suggest that such interactions may provide one of the mechanisms that control SHP-1 substrate specificity.

Constitutive activation of the RON gene promotes invasive growth but not transformation

MET, RON, and SEA are members of a gene family encoding tyrosine kinase receptors with distinctive properties. Besides mediating growth, they control cell dissociation, motility ("scattering"), and formation of branching tubules. While there are transforming counterparts of MET and SEA, no oncogenic forms of RON have yet been identified. A chimeric Tpr-Ron, mimicking the oncogenic form of Met (Tpr-Met) was generated to investigate its transforming potential. For comparison, a chimeric Tpr-Sea was also constructed. Fusion with Tpr induced constitutive activation of the Ron and Sea kinases. While Tpr-Sea was more efficient than Tpr-Met in transformation, Tpr-Ron did not transform NIH 3T3 cells. The differences in the transforming abilities of Tpr-Met and Tpr-Ron were linked to the functional features of the respective tyrosine kinases using the approach of swapping subdomains. Kinetic analysis showed that the catalytic efficiency of Tpr-Ron is five times lower than that of Tpr-Met. Moreover, constitutive activation of Ron resulted in activation of the MAP kinase signaling cascade approximately three times lower than that attained by Tpr-Met. However, constitutive activation of Ron did induce a mitogenic-invasive response, causing cell dissociation, motility, and invasion of extracellular matrices. Tpr-Ron also induced formation of long, unbranched tubules in tridimensional collagen gels. These data show that RON has the potential to elicit a motile-invasive rather than a transformed phenotype.

c-Jun can recruit JNK to phosphorylate dimerization partners via specific docking interactions

Structurally related serine/threonine kinases recognize similar phosphoacceptor peptides in vitro yet in vivo, they phosphorylate distinct substrates. To understand the basis for this specificity, we studied the interaction between the Jun kinases (JNKs) and Jun proteins. JNKs phosphorylate c-Jun very efficiently, JunD less efficiently, but they do not phosphorylate JunB. Effective JNK substrates require a separate docking site and specificity-conferring residues flanking the phosphoacceptor. The docking site increases the efficiency and specificity of the phosphorylation reaction. JunB has a functional JNK docking site but lacks specificity-conferring residues. Insertion of such residues brings JunB under JNK control. JunD, by contrast, lacks a JNK docking site, but its phosphoacceptor peptide is identical to that of c-Jun. Substrates such as JunD can be phosphorylated by JNK through heterodimerization with docking competent partners. Therefore, heterodimerization can affect the recognition of transcription factors by signal-regulated protein kinases.

Structural determinants of interaction of tyrosine-based sorting signals with the adaptor medium chains

Many integral membrane proteins contain tyrosine-based signals within their cytoplasmic domains that mediate internalization from the cell surface and targeting to lysosomal compartments. Internalization depends on an interaction of the tyrosine-based signals with the clathrin-associated adaptor complex AP-2 at the plasma membrane, whereas lysosomal targeting involves interaction of the signals with an analogous complex, AP-1, at the trans-Golgi network. Recent studies have identified the medium chains mu2 of AP-2 and mu1 of AP-1 as the recognition molecules for tyrosine-based signals. We have now investigated the structural determinants for interaction of the signals with mu2 and mu1. The position of the signals was found to be an important determinant of interactions with mu2 and mu1; signals were most effective when present at the carboxyl terminus of a polypeptide sequence. Another important determinant of interactions was the identity of residues surrounding the critical tyrosine residue. Mutation of some residues affected interactions with mu2 and mu1 similarly, whereas other mutations had differential effects. These observations suggest that both the position and the exact sequence of tyrosine-based sorting signals are major determinants of selectivity in their interaction with clathrin-associated adaptor complexes.

Protein targeting by tyrosine- and di-leucine-based signals: evidence for distinct saturable components

Targeting of transmembrane proteins to lysosomes, endosomal compartments, or the trans-Golgi network is largely dependent upon cytoplasmically exposed sorting signals. Among the most widely used signals are those that conform to the tyrosine-based motif, YXXO (where Y is tyrosine, X is any amino acid, and O is an amino acid with a bulky hydrophobic group), and to the di-leucine (or LL) motif. Signals conforming to both motifs have been implicated in protein localization to similar post-Golgi compartments. We have exploited the saturability of sorting to ask whether different YXXO or LL signals use shared components of the targeting machinery. Chimeric proteins containing various cytoplasmic domains and/or targeting signals were overexpressed in HeLa cells by transient transfection. Endogenous transferrin receptor and lysosomal proteins accumulated at the cell surface upon overexpression of chimeric proteins containing functional YXXO targeting signals, regardless of the compartmental destination imparted by the signal. Furthermore, overexpression of these chimeric proteins compromised YXXO-mediated endocytosis and lysosomal delivery. These activities were ablated by mutating the signals or by appending sequences that conformed to the YXXO motif but lacked targeting activity. Interestingly, overexpression of chimeric proteins containing cytoplasmic LL signals failed to induce surface displacement of endogenous YXXO-containing proteins, but did displace other proteins containing LL motifs. Our data demonstrate that: (a) Protein targeting and internalization mediated by either YXXO or LL motifs are saturable processes; (b) common saturable components are used in YXXO-mediated protein internalization and targeting to different post-Golgi compartments; and (c) YXXO- and LL-mediated targeting mechanisms use distinct saturable components.

Signal-transducing protein phosphorylation cascades mediated by Ras/Rho proteins in the mammalian cell: the potential for multiplex signalling

The features of three distinct protein phosphorylation cascades in mammalian cells are becoming clear. These signalling pathways link receptor-mediated events at the cell surface or intracellular perturbations such as DNA damage to changes in cytoskeletal structure, vesicle transport and altered transcription factor activity. The best known pathway, the Ras-->Raf-->MEK-->ERK cascade [where ERK is extracellular-signal-regulated kinase and MEK is mitogen-activated protein (MAP) kinase/ERK kinase], is typically stimulated strongly by mitogens and growth factors. The other two pathways, stimulated primarily by assorted cytokines, hormones and various forms of stress, predominantly utilize p21 proteins of the Rho family (Rho, Rac and CDC42), although Ras can also participate. Diagnostic of each pathway is the MAP kinase component, which is phosphorylated by a unique dual-specificity kinase on both tyrosine and threonine in one of three motifs (Thr-Glu-Tyr, Thr-Phe-Tyr or Thr-Gly-Tyr), depending upon the pathway. In addition to activating one or more protein phosphorylation cascades, the initiating stimulus may also mobilize a variety of other signalling molecules (e.g. protein kinase C isoforms, phospholipid kinases, G-protein alpha and beta gamma subunits, phospholipases, intracellular Ca2+). These various signals impact to a greater or lesser extent on multiple downstream effectors. Important concepts are that signal transmission often entails the targeted relocation of specific proteins in the cell, and the reversible formation of protein complexes by means of regulated protein phosphorylation. The signalling circuits may be completed by the phosphorylation of upstream effectors by downstream kinases, resulting in a modulation of the signal. Signalling is terminated and the components returned to the ground state largely by dephosphorylation. There is an indeterminant amount of cross-talk among the pathways, and many of the proteins in the pathways belong to families of closely related proteins. The potential for more than one signal to be conveyed down a pathway simultaneously (multiplex signalling) is discussed. The net effect of a given stimulus on the cell is the result of a complex intracellular integration of the intensity and duration of activation of the individual pathways. The specific outcome depends on the particular signalling molecules expressed by the target cells and on the dynamic balance among the pathways.

The purification and characterization of the catalytic domain of Src expressed in Schizosaccharomyces pombe. Comparison of unphosphorylated and tyrosine phosphorylated species

The catalytic domain of chicken Src including the C-terminal tail (Src-CD), has been expressed in Schizosaccharomyces pombe and purified to homogeneity. The expressed protein is a mixture of unphosphorylated (80%) and mono-phosphorylated (20%) species, that can be separated from each other by Mono Q chromatography. By a novel mass spectrometric method that utilizes parent ion scans of unseparated peptide mixtures, we found that the mono-phosphorylated form is phosphorylated either at Tyr416 or at Tyr436. The stability of Src-CD is comparable to the wild-type protein. Src-CD auto-phosphorylates and efficiently phosphorylates substrate peptides and proteins. Auto-phosphorylation occurs by an intermolecular mechanism and is completely inhibited by an excess of substrate peptide. Kinetic measurements for two exogenous substrates, the Src substrate peptide (AEEEIYGEFEAKKKK) and denatured enolase, showed that the overall activity (kcat) of the Src-CD molecule is about 10 times higher than that of wild-type Src. The kcat values for phosphorylation of the Src substrate peptide are similar for the unphosphorylated and monophosphorylated Src-CD (50 min-1), but the apparent K(m) values differ significantly (approximately 3 microM and 10 microM, respectively). Therefore, at low substrate concentrations in vitro the mono-phosphorylated form is more active, in agreement with the importance of Tyr416 for in vivo activity. The apparent K(m) values of the mono-phosphorylated Src-CD and wild-type Src for the Src substrate peptide and enolase are similar, indicating that, under these conditions, the kinase domain is mainly responsible for substrate binding.

p150Ship, a signal transduction molecule with inositol polyphosphate-5-phosphatase activity

The production, survival, and function of monocytes and macrophages is regulated by the macrophage colony-stimulating factor (M-CSF or CSF-1) through its tyrosine kinase receptor Fms. Binding of M-CSF to Fms induces the tyrosine phosphorylation and association of a 150-kD protein with the phosphotyrosine-binding (PTB) domain of Shc. We have cloned p150 using a modified yeast two-hybrid screen. p150 contains one SH2 domain, two potential PTB-binding sites, an ATP/GTP-binding domain, several potential SH3-binding sites, and a domain with homology to inositol polyphosphate-5-phosphatases. p150 antibodies detect this protein in FDC-P1 myeloid cells, but the same protein is not detectable in fibroblasts. The antibodies immunoprecipitate a 150-kD protein from quiescent or M-CSF-stimulated FDC-P1 cells that hydrolyzes PtdIns(3,4,5)P3, to PtdIns(3,4)P2. This activity is observed in Shc immunoprecipitates only after M-CSF stimulation. Retroviral expression of p15O in FD-Fms cells results in strong inhibition of cell growth in M-CSF and a lesser inhibition in IL-3. Ectopic expression of p150 in fibroblasts does not inhibit growth. This novel protein, p150(ship) (SH2-containing inositol phosphatase), identifies a component of a new growth factor-receptor signaling pathway in hematopoietic cells.

Solution studies of the SH2 domain from the fyn tyrosine kinase: secondary structure, backbone dynamics and protein association

The SH2 domain from Fyn tyrosine kinase, corresponding to residues 155-270 of the human enzyme, was expressed as a GST-fusion protein in a pGEX-E. coli system. After thrombin cleavage and removal of GST, the protein was studied by heteronuclear NMR. Two different phosphotyrosyl-peptides were synthesized and added to the SH2 domain. One peptide corresponded to the regulatory C-terminal tail region of Fyn. Sequence-specific assignment of NMR spectra was achieved using a combination of 1H-15N-correlated 2D HSQC, 15N-edited 3D TOCSY-HMQC, and 15N-edited 3D NOESY-HMQC spectra. By analysis of the alpha-proton chemical shifts and NOE intensities, the positions of secondary structural elements were determined and found to correspond closely to that seen in the crystal structure of the, homologous, Src-SH2 domain. To investigate the internal dynamics of the protein backbone, T1 and T2 relaxation parameters were measured on the free protein, as well as on both peptide complexes. Analytical ultracentrifugation and dynamic light scattering were employed to measure the effect of concentration and peptide-binding on self-association. The results suggest that, at NMR-sample concentrations, the free protein is present in at least dimeric form. Phosphopeptide binding and lower concentration significantly, but not completely, shift the equilibrium towards monomers. The possible role of this protein association in the regulation of the Src-family tyrosine kinases is discussed.