A region in Shc distinct from the SH2 domain can bind tyrosine-phosphorylated growth factor receptors

Hypoxic induction of the hypoxia-inducible factor is mediated via the adaptor protein Shc in endothelial cells

Tyrosine kinase cascades may play a role in the hypoxic regulation of hypoxia-inducible factor (HIF)-1. We investigated the role of tyrosine kinase phosphorylation and of the Shc/Ras cascade on hypoxic HIF-1 stabilization. Exposure of human umbilical vein endothelial cells to hypoxia results in HIF protein stabilization as early as 10 minutes, with a maximum at 3 hours, and also in Shc tyrosine phosphorylation, with a maximum at 10 minutes. To test whether Shc directly mediates hypoxia-induced HIF stabilization, human umbilical vein endothelial cells were transfected with a dominant-negative Shc mutant (dnShc), resulting in significantly reduced HIF protein levels compared with control. Similar results were obtained with cells transfected with dominant-negative Ras, a known downstream effector of Shc. Hypoxia-induced Ras activity was significantly reduced in cells transfected with dnShc compared with control levels, indicating that Ras indeed acts downstream from Shc. Moreover, cells pretreated with a specific Raf-1 kinase inhibitor, a known downstream effector of Ras, exhibited reduced HIF protein levels. To examine the functional consequences of Shc in hypoxic signaling, HIF-1 ubiquitination, protein stabilization, and endothelial cell migration were assessed. Overexpression of dnShc increased ubiquitination of HIF-1 and reduced the half-life of the protein. Moreover, dnShc, dominant-negative Ras, or the Raf-1 kinase inhibitor significantly inhibited migration under hypoxia. Thus, Shc in concert with Ras and Raf-1 contributes to hypoxia-induced HIF-1alpha protein stabilization and endothelial cell migration.

Evidence for a role of the nerve growth factor receptor TrkA in tyrosine phosphorylation and processing of beta-APP

The cytoplasmic tail of the beta-amyloid precursor protein (APP) contains a Y(682)ENPTY(687) sequence through which APP associates with phosphotyrosine binding (PTB) domain containing proteins in a tyrosine phosphorylation-independent manner. We have recently found that tyrosine phosphorylation of APP-Y(682) promotes docking of Shc proteins that modulate growth factor signaling to the ERK and PI3K/Akt pathways. We have also shown that APP is phosphorylated on Y(682) in cells that overexpress a constitutively active form of the tyrosine kinase abl. Here we present evidence that the nerve growth factor receptor TrkA may also promote phosphorylation of APP. Overexpression of TrkA, but not of mutated, kinase inactive TrkA resulted in tyrosine phosphorylation of APP. Site-directed mutagenesis studies showed that TrkA overexpression was associated with phosphorylation of APP-Y(682). Moreover, overexpression of TrkA also affected APP processing reducing the generation of the APP intracellular domain (AID). Thus, tyrosine phosphorylation of APP may functionally link APP processing and neurotrophic signaling to intracellular pathways associated with cellular differentiation and survival.

JAKs, STATs and Src kinases in hematopoiesis

Hematopoiesis is the cumulative result of intricately regulated signal transduction cascades that are mediated by cytokines and their cognate receptors. Proper culmination of these diverse signaling pathways forms the basis for an orderly generation of different cell types and aberrations in these pathways is an underlying cause for diseases such as leukemias and other myeloproliferative and lymphoproliferative disorders. Over the past decade, downstream signal transduction events initiated upon cytokine/growth factor stimulation have been a major focus of basic and applied biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of transcription factors, termed signal transducers and activators of transcription (STATs) appear to orchestrate the downstream events propagated by cytokine/growth factor interactions with their cognate receptors. Similarly, cytoplasmic Janus protein tyrosine kinases (JAKs) and Src family of kinases seem to play a critical role in diverse signal transduction pathways that govern cellular survival, proliferation, differentiation and apoptosis. Accumulating evidence suggests that STAT protein activation may be mediated by members of both JAK and Src family members following cytokine/growth factor stimulation. In addition, JAK kinases appear to be essential for the phosphorylation of the cytokine receptors which results in the creation of docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Cell and tissue-specificity of cytokine action appears to be determined by the nature of signal transduction pathways activated by cytokine/receptor interactions. The integration of these diverse signaling cues from active JAK kinases, members of the Src-family kinases and STAT proteins, leads to cell proliferation, cell survival and differentiation, the end-point of the cytokine/growth factor stimulus.

FRS2 PTB domain conformation regulates interactions with divergent neurotrophic receptors

Membrane-anchored adaptor proteins FRS2alpha/beta (also known as SNT-1/2) mediate signaling of fibroblast growth factor receptors (FGFRs) and neurotrophin receptors (TRKs) through their N-terminal phosphotyrosine binding (PTB) domains. The FRS2 PTB domain recognizes tyrosine-phosphorylated TRKs at an NPXpY (where pY is phosphotyrosine) motif, whereas its constitutive association with FGFR involves a receptor juxtamembrane region lacking Tyr and Asn residues. Here we show by isothermal titration calorimetry that the FRS2alpha PTB domain binding to peptides derived from TRKs or FGFR is thermodynamically different. TRK binding is largely enthalpy-driven, whereas the FGFR interaction is governed by a favorable entropic contribution to the free energy of binding. Furthermore, our NMR spectral analysis suggests that disruption of an unstructured region C-terminal to the PTB domain alters local conformation and dynamics of the residues at the ligand-binding site, and that structural disruption of the beta8-strand directly weakens the PTB domain association with the FGFR ligand. Together, our new findings support a molecular mechanism by which conformational dynamics of the FRS2alpha PTB domain dictates its association with either fibroblast growth factor or neurotrophin receptors in neuronal development.

Tyrosine phosphorylation of the beta-amyloid precursor protein cytoplasmic tail promotes interaction with Shc

beta-Amyloid precursor protein (APP) is a widely expressed transmembrane protein of unknown function that is involved in the pathogenesis of Alzheimer's disease. The cytoplasmic tail of APP interacts with phosphotyrosine binding (PTB) domain containing proteins (Fe65, X11, mDab-1, and JIP-1) and may modulate gene expression and apoptosis. We now identify Shc A and Shc C, PTB-containing adapter proteins that signal to cellular differentiation and survival pathways, as novel APP-interacting proteins. The APP cytoplasmic tail contains a PTB-binding motif (Y(682)ENPTY(687)) that, when phosphorylated on Tyr(682), precipitated the PTB domain of Shc A and Shc C, as well as endogenous full-length Shc A. APP and Shc C were physically associated in adult mouse brain homogenates. Increase in phosphorylation of APP by overexpression of the nerve growth factor receptor Trk A in 293T cells promoted the interaction of transfected APP and endogenous Shc A. Pervanadate treatment of N2a neuroblastoma cells resulted in tyrosine phosphorylation and association of endogenous APP and Shc A. Thus, APP and Shc proteins interact in vitro, in cells, and in the mouse brain. Tyrosine phosphorylation of APP may promote the interaction with Shc proteins.

Phosphotyrosine-binding domains in signal transduction

Protein phosphorylation provides molecular control of complex physiological events within cells. In many cases, phosphorylation on specific amino acids directly controls the assembly of multi-protein complexes by recruiting phospho-specific binding modules. Here, the function, structure, and cell biology of phosphotyrosine-binding domains is discussed.

Interaction domains: from simple binding events to complex cellular behavior

Many of the signaling pathways and regulatory systems in eukaryotic cells are controlled by proteins with multiple interaction domains that mediate specific protein-protein and protein-phospholipid interactions, and thereby determine the biological output of receptors for external and intrinsic signals. Here, we discuss the basic features of interaction domains, and suggest that rather simple binary interactions can be used in sophisticated ways to generate complex cellular responses.

PTB or not PTB -- that is the question

Phosphotyrosine binding (PTB) domains are structurally conserved modules found in proteins involved in numerous biological processes including signaling through cell-surface receptors and protein trafficking. While their original discovery is attributed to the recognition of phosphotyrosine in the context of NPXpY sequences -- a function distinct from that of the classical src homology 2 (SH2) domain -- recent studies show that these protein modules have much broader ligand binding specificities. These studies highlight the functional diversity of the PTB domain family as generalized protein interaction domains, and reinforce the concept that evolutionary changes of structural elements around the ligand binding site on a conserved structural core may endow these protein modules with the structural plasticity necessary for functional versatility.

Role of integrins in cell invasion and migration

As cancer cells undergo metastasis--invasion and migration of a new tissue--they penetrate and attach to the target tissue's basal matrix. This allows the cancer cell to pull itself forward into the tissue. The attachment is mediated by cell-surface receptors known as integrins, which bind to components of the extracellular matrix. Integrins are crucial for cell invasion and migration, not only for physically tethering cells to the matrix, but also for sending and receiving molecular signals that regulate these processes.

Dockers at the crossroads

The family of docker proteins containing phosphotyrosine-binding (PTB) domains appears to represent a family of critically positioned and exquisitely controlled signalling proteins that relay signals from the activated receptors to downstream pathways. These proteins all have a membrane attachment domain, a PTB domain that targets the protein to a subset of receptors and a number of phosphorylatable tyrosines that dock other signalling proteins. Evidence is accruing that suggests that the PTB domain has evolved from a pleckstrin homology (PH) domain to bind to a range of sequences that, while bestowing specificity, allows switching of the docker protein between receptors or signalling systems. The history of the PTB domain and how it influences the participation of docker protein in various signalling pathways are discussed.

Multiple ErbB-2/Neu Phosphorylation Sites Mediate Transformation through Distinct Effector Proteins

Amplification of the type I receptor tyrosine kinase ErbB-2 (HER2/Neu) is observed in 20-30% of human mammary carcinomas, correlating with a poor clinical prognosis. We have previously demonstrated that four (Tyr(1144), Tyr(1201), Tyr(1226/1227), or Tyr(1253)) of the five known Neu/ErbB-2 autophosphorylation sites can independently mediate transforming signals. The transforming potential of at least two of these autophosphorylation sites (Tyr(1144) and Tyr(1226/1227)) has been further correlated with their ability to associate with Grb2 and Shc adapter proteins, respectively. To confirm the specificity of these interactions, we have created a series of second site mutants in these phosphorylation sites. The results showed that Grb2 recruitment to site 1144 is absolutely required for transforming signal from this autophosphorylation site, whereas association of Shc-mediated transformation is dependent on conservation of the NPXY motif spanning Tyr(1227). A stretch of amino acid identity around tyrosines 1201 (ENPEYLTP)and 1253 (ENPEYLDL) exists, and mutation of key residues within this motif reveals distinct requirements for an intact protein tyrosine-binding protein (NPXY). We show that DOK-R, a protein tyrosine-binding site-containing protein implicated in Ras signaling, interacts with Neu/ErbB-2 at Tyr(1253) as do two unidentified proteins, p150 and p34, the latter correlating with transformation. Together these data argue that ErbB-2/Neu is capable of mediating transformation through distinct effector pathways.

Signaling via Shc family adapter proteins

The adapter protein Shc was initially identified as an SH2 containing proto-oncogene involved in growth factor signaling. Since then a number of studies in multiple systems have implicated a role for Shc in signaling via many different types of receptors, such as growth factor receptors, antigen receptors, cytokine receptors, G-protein coupled receptors, hormone receptors and integrins. In addition to the ubiquitous ShcA, two other shc gene products, ShcB and ShcC, which are predominantly expressed in neuronal cells, have also been identified. ShcA knockout mice are embryonic lethal and have clearly suggested an important role for ShcA in vivo. Based on dominant negative studies and mouse embryos deficient in ShcA, a clear role for Shc in leading to mitogen activated protein kinase (MAPK) activation has been established. However MAPK activation may not be the sole function of Shc proteins. Although Shc has also been linked to other signaling events such as c-Myc activation and cell survival, the mechanistic understanding of these signaling events remains poorly characterized. Given the apparently central role that Shc plays signaling via many receptors, delineating the precise mechanism(s) of Shc-mediated signaling may be critical to our understanding of the effects mediated through these receptors.

Further evidence for the involvement of insulin receptor substrates in epidermal growth factor-induced activation of phosphatidylinositol 3-kinase

In accordance with our recent results obtained with cultured rat hepatocytes [Fujioka, T. & Ui, M. (2001) Eur. J. Biochem. 268, 25-34], epidermal growth factor (EGF) gave rise to transient tyrosine phosphorylation of insulin receptor substrates (IRS-1 and IRS-2), thereby activating the bound phosphatidylinositol 3-kinase in human epidermoid carcinoma A431 cells normally abundant in EGF receptors (EGFR) and Chinese hamster ovary (CHO) cells transfected with full-length EGFR. These actions of EGF, although much smaller in magnitude than those of insulin or IGF-I in the same cells, were accompanied by tyrosine phosphorylation of EGFR rather than insulin or IGF-I receptors, never observed in wild-type CHO cells expressing no EGFR, and totally inhibited by an inhibitor of EGFR kinase, AG1478, that was without effect on insulin or IGF-I actions. Recombinant IRS-1 was phosphorylated on tyrosines upon incubation with purified EGFR from A431 cells and 32P-labeled ATP. When CHO cells were transfected with C-terminal truncated EGFR lacking three NPXY motifs responsible for direct binding to phosphotyrosine-binding domains of IRSs, no effect of EGF could be observed. We suggest that tyrosine phosphorylation of IRS-1 or IRS-2 could mediate EGFR-induced activation of phosphatidylinositol 3-kinase in mammalian cells.

Autosomal recessive hypercholesterolemia caused by mutations in a putative LDL receptor adaptor protein

Atherogenic low density lipoproteins are cleared from the circulation by hepatic low density lipoprotein receptors (LDLR). Two inherited forms of hypercholesterolemia result from loss of LDLR activity: autosomal dominant familial hypercholesterolemia (FH), caused by mutations in the LDLR gene, and autosomal recessive hypercholesterolemia (ARH), of unknown etiology. Here we map the ARH locus to an approximately 1-centimorgan interval on chromosome 1p35 and identify six mutations in a gene encoding a putative adaptor protein (ARH). ARH contains a phosphotyrosine binding (PTB) domain, which in other proteins binds NPXY motifs in the cytoplasmic tails of cell-surface receptors, including the LDLR. ARH appears to have a tissue-specific role in LDLR function, as it is required in liver but not in fibroblasts.

Insulin signals to prenyltransferases via the Shc branch of intracellular signaling

We assessed the roles of insulin receptor substrate-1 (IRS-1) and Shc in insulin action on farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) using Chinese hamster ovary (CHO) cells that overexpress wild-type human insulin receptors (CHO-hIR-WT) or mutant insulin receptors lacking the NPEY domain (CHO-DeltaNPEY) or 3T3-L1 fibroblasts transfected with adenoviruses that express the PTB or SAIN domain of IRS-1 and Shc, the pleckstrin homology (PH) domain of IRS-1, or the Src homology 2 (SH2) domain of Shc. Insulin promoted phosphorylation of the alpha-subunit of FTase and GGTase I in CHO-hIR-WT cells, but was without effect in CHO-DeltaNPEY cells. Insulin increased FTase and GGTase I activities and the amounts of prenylated Ras and RhoA proteins in CHO-hIR-WT (but not CHO-DeltaNPEY) cells. Overexpression of the PTB or SAIN domain of IRS-1 (which blocked both IRS-1 and Shc signaling) prevented insulin-stimulated phosphorylation of the FTase and GGTase I alpha-subunit activation of FTase and GGTase I and subsequent increases in prenylated Ras and RhoA proteins. In contrast, overexpression of the IRS-1 PH domain, which impairs IRS-1 (but not Shc) signaling, did not alter insulin action on the prenyltransferases, but completely inhibited the insulin effect on the phosphorylation of IRS-1 and on the activation of phosphatidylinositol 3-kinase and Akt. Finally, overexpression of the Shc SH2 domain completely blocked the insulin effect on FTase and GGTase I activities without interfering with insulin signaling to MAPK. These data suggest that insulin signaling from its receptor to the prenyltransferases FTase and GGTase I is mediated by the Shc pathway, but not the IRS-1/phosphatidylinositol 3-kinase pathway. Shc-mediated insulin signaling to MAPK may be necessary (but not sufficient) for activation of prenyltransferase activity. An additional pathway involving the Shc SH2 domain may be necessary to mediate the insulin effect on FTase and GGTase I.

IRS-1 tyrosine phosphorylation reflects insulin-induced metabolic and mitogenic responses in 3T3-L1 pre-adipocytes

We determined the involvement of Tyr-1158 within the regulatory loop of the insulin receptor (IR) in the generation of insulin-specific responses in situ. For this purpose chimeric receptors with an epidermal growth factor (EGF) receptor extracellular domain and an IR cytoplasmic domain (EIR) were constructed, which allow activation of the cytoplasmic IR domain without activation of endogenous wt-IRs. Tyr-1158 of the chimera EIR was exchanged for Phe, creating a mutant chimeric receptor (EIR-Y1158F). Chimeric receptors were expressed in 3T3-L1 pre-adipocytes, which do not show insulin-specific responses upon EGF stimulation. We found that pre-adipocytes expressing EIR-Y1158F were impaired in their ability to stimulate glycogen synthesis and DNA synthesis upon maximal stimulation with EGF. EIR-Y1158F was impaired in its ability to phosphorylate insulin receptor substrate (IRS)-1 and induce downstream signals of IRS-1 phosphorylation, such as the association of IRS-1 with phosphatidyl-inositol-3'-kinase and the activation of protein kinase B (Akt). In contrast with the phosphorylation of IRS-1, the phosphorylation of IRS-2 and extracellular regulated protein kinase-1/-2 was normal in EIR-Y1158F expressing cells. These observations suggest that the level of IRS-1 phosphorylation rather than the level of IRS-2 phosphorylation mediates insulin-induced glycogen synthesis and DNA synthesis in 3T3-L1 pre-adipocytes.

Tyrosine phosphorylation of the beta 4 integrin cytoplasmic domain mediates Shc signaling to extracellular signal-regulated kinase and antagonizes formation of hemidesmosomes

Ligation of the alpha(6)beta(4) integrin induces tyrosine phosphorylation of the beta(4) cytoplasmic domain, followed by recruitment of the adaptor protein Shc and activation of mitogen-activated protein kinase cascades. We have used Far Western analysis and phosphopeptide competition assays to map the sites in the cytoplasmic domain of beta(4) that are required for interaction with Shc. Our results indicate that, upon phosphorylation, Tyr(1440), or secondarily Tyr(1422), interacts with the SH2 domain of Shc, whereas Tyr(1526), or secondarily Tyr(1642), interacts with its phosphotyrosine binding (PTB) domain. An inactivating mutation in the PTB domain of Shc, but not one in its SH2 domain, suppresses the activation of Shc by alpha(6)beta(4). In addition, mutation of beta(4) Tyr(1526), which binds to the PTB domain of Shc, but not of Tyr(1422) and Tyr(1440), which interact with its SH2 domain, abolishes the activation of ERK by alpha(6)beta(4). Phenylalanine substitution of the beta(4) tyrosines able to interact with the SH2 or PTB domain of Shc does not affect incorporation of alpha(6)beta(4) in the hemidesmosomes of 804G cells. Exposure to the tyrosine phosphatase inhibitor orthovanadate increases tyrosine phosphorylation of beta4 and disrupts the hemidesmosomes of 804G cells expressing recombinant wild type beta(4). This treatment, however, exerts a decreasing degree of inhibition on the hemidesmosomes of cells expressing versions of beta(4) containing phenylalanine substitutions at Tyr(1422) and Tyr(1440), at Tyr(1526) and Tyr(1642), or at all four tyrosine phosphorylation sites. These results suggest that beta(4) Tyr(1526) interacts in a phosphorylation-dependent manner with the PTB domain of Shc. This event is required for subsequent tyrosine phosphorylation of Shc and signaling to ERK but not formation of hemidesmosomes.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Modulation of amyloid precursor protein metabolism by X11alpha /Mint-1. A deletion analysis of protein-protein interaction domains

Modulation of amyloid precursor protein (APP) metabolism plays a pivotal role in the pathogenesis of Alzheimer's disease. The phosphotyrosine-binding/protein interaction (PTB/PI) domain of X11alpha, a neuronal cytosolic adaptor protein, binds to the YENPTY sequence in the cytoplasmic carboxyl terminus of APP. This interaction prolongs the half-life of APP and inhibits Abeta40 and Abeta42 secretion. X11alpha/Mint-1 has multiple protein-protein interaction domains, a Munc-18 interaction domain (MID), a Cask/Lin-2 interaction domain (CID), a PTB/PI domain, and two PDZ domains. These X11alpha protein interaction domains may modulate its effect on APP processing. To test this hypothesis, we performed a deletion analysis of X11alpha effects on metabolism of APP(695) Swedish (K595N/M596L) (APP(sw)) by transient cotransfection of HEK 293 cells with: 1) X11alpha (X11alpha-wt, N-MID-CID-PTB-PDZ-PDZ-C), 2) amino-terminal deletion (X11alpha-DeltaN, PTB-PDZ-PDZ), 3) carboxyl-terminal deletion (X11alpha-DeltaPDZ, MID-CID-PTB), or 4) deletion of both termini (PTB domain only, PTB). The carboxyl terminus of X11alpha was required for stabilization of APP(sw) in cells. In contrast, the amino terminus of X11alpha was required to stimulate APPs secretion. X11alpha, X11alpha-DeltaN, and X11alpha-PTB, but not X11alpha-DeltaPDZ, were effective inhibitors of Abeta40 and Abeta42 secretion. These results suggest that additional protein interaction domains of X11alpha modulate various aspects of APP metabolism.

The Raf signal transduction cascade as a target for chemotherapeutic intervention in growth factor-responsive tumors

This review focuses on the Ras-Raf-mitogen-activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) signal transduction pathway and the consequences of its unregulation in the development of cancer. The roles of some of the cell membrane receptors involved in the activation of this pathway, the G-protein Ras, the Raf, MEK and ERK kinases, the phosphatases that regulate these kinases, as well as the downstream transcription factors that become activated, are discussed. The roles of the Ras-Raf-MEK-ERK pathway in the regulation of apoptosis and cell cycle progression are also analyzed. In addition, potential targets for pharmacological intervention in growth factor-responsive cells are evaluated.

Identification and characterization of an activating TrkA deletion mutation in acute myeloid leukemia

In this study, we utilized retroviral transfer of cDNA libraries in order to identify oncogenes that are expressed in acute myeloid leukemia (AML). From screens using two different cell types as targets for cellular transformation, a single cDNA encoding a variant of the TrkA protooncogene was isolated. The protein product of this protooncogene, TrkA, is a receptor tyrosine kinase for nerve growth factor. The isolated transforming cDNA encoded a TrkA protein that contains a 75-amino-acid deletion in the extracellular domain of the receptor and was named DeltaTrkA. DeltaTrkA readily transformed fibroblast and epithelial cell lines. The deletion resulted in activation of the tyrosine kinase domain leading to constitutive tyrosine phosphorylation of the protein. Expression of DeltaTrkA in cells led to the constitutive activation of intracellular signaling pathways that include Ras, extracellular signal-regulated kinase/mitogen-activated protein kinase, and Akt. Importantly, DeltaTrkA altered the apoptotic and growth properties of 32D myeloid progenitor cells, suggesting DeltaTrkA may have contributed to the development and/or maintenance of the myeloid leukemia from which it was isolated. Unlike Bcr-Abl, expression of DeltaTrkA did not activate Stat5 in these cells. We have detected expression of DeltaTrkA in the original AML sample by reverse transcriptase PCR and by Western blot analysis. While previous TrkA mutations identified from human tumors involved fusion to other proteins, this report is the initial demonstration that deletions within TrkA may play a role in human cancers. Finally, this report is the first to indicate mutations in TrkA may contribute to leukemogenesis.

Structural basis of SNT PTB domain interactions with distinct neurotrophic receptors

SNT adaptor proteins transduce activation of fibroblast growth factor receptors (FGFRs) and neurotrophin receptors (TRKs) to common signaling targets. The SNT-1 phosphotyrosine binding (PTB) domain recognizes activated TRKs at a canonical NPXpY motif and, atypically, binds to nonphosphorylated FGFRs in a region lacking tyrosine or asparagine. Here, using NMR and mutational analyses, we show that the PTB domain utilizes distinct sets of amino acid residues to interact with FGFRs or TRKs in a mutually exclusive manner. The FGFR1 peptide wraps around the beta sandwich structure of the PTB domain, and its binding is possibly regulated by conformational change of a unique C-terminal beta strand in the protein. Our results suggest mechanisms by which SNTs serve as molecular switches to mediate the essential interplay between FGFR and TRK signaling during neuronal differentiation.

The Shc adaptor protein forms interdependent phosphotyrosine-mediated protein complexes in mast cells stimulated with interleukin 3

The Shc adaptor protein possesses 2 distinct phosphotyrosine (pTyr) recognition modules—the pTyr binding (PTB) domain and the Src homology 2 (SH2) domain—and multiple potential sites for tyrosine (Tyr) phosphorylation (Tyr residues 239, 240, and 317). On stimulation of hematopoietic cells with interleukin 3 (IL-3), Shc becomes phosphorylated and may therefore contribute to IL-3 signaling. We investigated the interactions mediated by the Shc modular domains and pTyr sites in IL-3–dependent IC2 premast cells. The Shc PTB domain, rather than the SH2 domain, associated both in vitro and in vivo with the Tyr-phosphorylated β subunit of the IL-3 receptor and with the SH2-containing 5′ inositol phosphatase (SHIP), and it recognized specific NXXpY phosphopeptides from these binding partners. In IL-3–stimulated mast cells, Shc phosphorylation occurred primarily on Tyr239 and 317 and was dependent on a functional PTB domain. Phosphorylated Tyr317, and to a lesser extent, Tyr239, bound the Grb2 adaptor and SHIP. Furthermore, a pTyr317 Shc phosphopeptide selectively recognized Grb2, Sos1, SHIP, and the p85 subunit of phosphatidylinositol 3′ kinase from mast cells, as characterized by mass spectrometry. These results indicate that Shc undergoes an interdependent series of pTyr-mediated interactions in IL-3–stimulated mast cells, resulting in the recruitment of proteins that regulate the Ras pathway and phospholipid metabolism.

The Shc adaptor protein forms interdependent phosphotyrosine-mediated protein complexes in mast cells stimulated with interleukin 3

The Shc adaptor protein possesses 2 distinct phosphotyrosine (pTyr) recognition modules—the pTyr binding (PTB) domain and the Src homology 2 (SH2) domain—and multiple potential sites for tyrosine (Tyr) phosphorylation (Tyr residues 239, 240, and 317). On stimulation of hematopoietic cells with interleukin 3 (IL-3), Shc becomes phosphorylated and may therefore contribute to IL-3 signaling. We investigated the interactions mediated by the Shc modular domains and pTyr sites in IL-3–dependent IC2 premast cells. The Shc PTB domain, rather than the SH2 domain, associated both in vitro and in vivo with the Tyr-phosphorylated β subunit of the IL-3 receptor and with the SH2-containing 5′ inositol phosphatase (SHIP), and it recognized specific NXXpY phosphopeptides from these binding partners. In IL-3–stimulated mast cells, Shc phosphorylation occurred primarily on Tyr239 and 317 and was dependent on a functional PTB domain. Phosphorylated Tyr317, and to a lesser extent, Tyr239, bound the Grb2 adaptor and SHIP. Furthermore, a pTyr317 Shc phosphopeptide selectively recognized Grb2, Sos1, SHIP, and the p85 subunit of phosphatidylinositol 3′ kinase from mast cells, as characterized by mass spectrometry. These results indicate that Shc undergoes an interdependent series of pTyr-mediated interactions in IL-3–stimulated mast cells, resulting in the recruitment of proteins that regulate the Ras pathway and phospholipid metabolism.

Mouse numb is an essential gene involved in cortical neurogenesis

During neurogenesis of the mammalian neocortex, neural progenitor cells divide to generate daughter cells that either become neurons or remain as progenitor cells. The mouse numb (m-numb) gene encodes a membrane-associated protein that is asymmetrically localized to the apical cell membrane of dividing cortical progenitor cells and may be segregated to only the apical daughter cell that has been suggested to remain as a progenitor cell. To examine m-numb function during neural development, we generated a loss-of-function mutant allele of m-numb. Mice homozygous for this mutation exhibit severe defects in cranial neural tube closure and precocious neuron production in the forebrain and die around embryonic day 11.5 (E11. 5). These findings suggest that m-numb is an essential gene that plays a role in promoting progenitor cell fate during cortical neurogenesis.

Proteolytic processing and cell biological functions of the amyloid precursor protein

Recent research has identified some key players involved in the proteolytic processing of amyloid precursor protein (APP) to amyloid beta-peptide, the principal component of the amyloid plaques in Alzheimer patients. Interesting parallels exists with the proteolysis of other proteins involved in cell differentiation, cholesterol homeostasis and stress responses. Since the cytoplasmic domain of APP is anchored to a complex protein network that might function in axonal elongation, dendritic arborisation and neuronal cell migration, the proteolysis of APP might be critically involved in intracellular signalling events.

IL-3 signaling and the role of Src kinases, JAKs and STATs: a covert liaison unveiled

Hematopoiesis is the cumulative result of intricately regulated signal transduction cascades that are mediated by cytokines and their cognate receptors. Proper culmination of these diverse signaling pathways forms the basis for an orderly generation of different cell types and aberrations in these pathways is an underlying cause for diseases such as cancer. Over the past several years, downstream events initiated upon cytokine/growth factor stimulation have been a major focus of biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of novel transcription factors, termed signal transducers and activators of transcription (STATs) appear to orchestrate the downstream events propagated by cytokine/growth factor interactions with their cognate receptors. Until recently, the JAK proteins were considered to be the tyrosine kinases, which dictated the levels of phosphorylation and activation of STAT proteins, forming the basis of the JAK-STAT model. However, over the past few years, increasing evidence has accumulated which indicates that at least some of the STAT protein activation may be mediated by members of the Src gene family following cytokine/growth factor stimulation. Studies have demonstrated that the Src-family of tyrosine kinases can phosphorylate and activate certain STAT proteins, in lieu of JAK kinases. In such a scenario, JAK kinases may be more crucial to phosphorylation of the cytokine/growth factor receptors and in the process create docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Tyrosine phosphorylation and activation of STAT proteins can be achieved either by JAKs or Src-kinases depending on the nature of STAT that is being activated. This forms the basis for the JAK-Src-STAT model proposed in this review. The concerted action of JAK kinases, members of the Src-kinase family and STAT proteins, leads to cell proliferation and cell survival, the end-point of the cytokine/growth factor stimulus. Oncogene (2000).

The pleiotropic effects of fibroblast growth factor receptors in mammalian development

In recent years the study of fibroblast growth factor receptors (FGFRs) in normal development and human genetic disorders has increased our understanding of some complex cellular processes. At least fifteen genetic disorders result from mutations within FGFR genes including skeletal dysplasias such as Apert syndrome and achondroplasia. In vitro experiments and the generation of animal models indicate that these mutations result in activation of the receptors and that FGFRs act as negative regulators of bone growth. FGFRs also play a role in wound healing and cancer. In this article, we review the expression of FGFRs in human development, the phenotypes resulting from FGFR mutations, and recent data identifying pathways downstream of the activated receptors.

Identification and characterization of a dimerization domain in CED-6, an adapter protein involved in engulfment of apoptotic cells

Phagocytosis of apoptotic cells is a key step in the completion of programmed cell death that occurs throughout life in multicellular organisms. The molecular events involved in clearance of apoptotic cells are just beginning to be elucidated. Recently, CED-6, an adapter protein involved in engulfment has been cloned in Caenorhabditis elegans and in humans. CED-6 is composed of a phosphotyrosine-binding (PTB) domain and a proline-rich C-terminal domain with no apparent catalytic domain. Since PTB domains, originally identified in Shc, mediate intracellular signaling downstream of cell surface receptors, CED-6 has also been proposed to mediate intracellular signals leading to engulfment. In this report, we demonstrate that CED-6 dimerizes through a leucine zipper domain that is immediately adjacent to the PTB domain. Several lines of evidence based on co-immunoprecipitation studies, yeast two-hybrid assays, and gel filtration studies suggest that CED-6 exists as a dimer in vivo. Through mutational analyses, we show that the leucine zipper is necessary and sufficient for CED-6 dimerization and that this dimerization is conserved among C. elegans, rodent, and human CED-6 proteins. We propose that dimerization may have unique implications for ligand binding via CED-6 and its function during the phagocytosis of apoptotic cells.

Shc associates with the IL-3 receptor beta subunit, SHIP and Gab2 following IL-3 stimulation. Contribution of Shc PTB and SH2 domains

p46(Shc) and p52(Shc) become heavily tyrosine phosphorylated in response to interleukin 3 (IL-3) treatment. We have investigated the potential of Shc to integrate IL-3 signalling pathways and demonstrate that Shc associates with the beta subunits of the human (betac) and murine (Aic2A) IL-3 receptors, SHIP and Gab2 following IL-3 stimulation. The interaction between Shc and the IL-3 receptor beta chains was direct, mediated by both the SH2 and PTB domains. Interaction with SHIP was via the Shc PTB domain and the Shc SH2 domain mediated the interaction with Gab2. Phosphopeptide competition studies suggest that the SH2 domain interacts primarily with tyrosine 612 of betac (610 of Aic2A), and the PTB domain with tyrosine 577 of betac (575 of Aic2A). PTB binding to IL-3R beta chains was of highest affinity, and appeared to play the primary role in binding. These findings suggest that Shc may play an important role in coordinately integrating IL-3 signalling pathways.

Intracellular adapter molecules

Lymphocyte antigen receptor engagement leads to the initiation of numerous signal transduction pathways that direct ultimate cellular responses. In recent years, it has become apparent that adapter molecules regulate the coupling of receptor-proximal events, such as protein tyrosine kinase activation, with end results such as inducible gene expression and cytoskeletal rearrangements. While adapter molecules possess no intrinsic enzymatic activity, their ability to mediate protein-protein interactions is vital for the integration and propagation of signal transduction cascades in lymphocytes. Recent studies demonstrate that intracellular adapter molecules function as both positive and negative regulators of lymphocyte activation.

Effects of Src homology domain 2 (SH2)-containing inositol phosphatase (SHIP), SH2-containing phosphotyrosine phosphatase (SHP)-1, and SHP-2 SH2 decoy proteins on Fc gamma RIIB1-effector interactions and inhibitory functions

Coaggregation of Fc gamma RIIB1 with B cell Ag receptors (BCR) leads to inhibition of BCR-mediated signaling via recruitment of Src homology domain 2 (SH2)-containing phosphatases. In vitro peptide binding experiments using phosphotyrosine-containing sequences derived from the immunoreceptor tyrosine-based inhibitory motif (ITIM) known to mediate Fc gamma RIIB1 effects suggest that the receptor uses SH2-containing inositol phosphatase (SHIP) and SH2-containing phosphotyrosine phosphatase (SHP)-1, as well as SHP-2 as effectors. In contrast, coimmunoprecipitation studies of receptor-effector associations suggest that the predominant Fc gamma RIIB1 effector protein is SHIP. However, biologically significant interactions may be lost in such studies if reactants' dissociation rates (Kd) are high. Thus, it is unclear to what extent these assays reflect the relative recruitment of SHIP, SHP-1, and SHP-2 to the receptor in vivo. As an alternative approach to this question, we have studied the effects of ectopically expressed SHIP, SHP-1, or SHP-2 SH2-containing decoy proteins on Fc gamma RIIB1 signaling. Results demonstrate the SHIP is the predominant intracellular ligand for the phosphorylated Fc gamma RIIB1 ITIM, although the SHP-2 decoy exhibits some ability to bind Fc gamma RIIB1 and block Fc receptor function. The SHIP SH2, while not affecting Fc gamma RIIB1 tyrosyl phosphorylation, blocks receptor-mediated recruitment of SHIP, SHIP phosphorylation, recruitment of p52 Shc, phosphatidylinositol 3,4,5-trisphosphate hydrolysis, inhibition of mitogen-activated protein kinase activation, and, albeit more modestly, Fc gamma RIIB1 inhibition of Ca2+ mobilization. Taken together, results implicate ITIM interactions with SHIP as a major mechanism of Fc gamma RIIB1-mediated inhibitory signaling.

Analysis of receptor signaling pathways by mass spectrometry: identification of vav-2 as a substrate of the epidermal and platelet-derived growth factor receptors

Oligomerization of receptor protein tyrosine kinases such as the epidermal growth factor receptor (EGFR) by their cognate ligands leads to activation of the receptor. Transphosphorylation of the receptor subunits is followed by the recruitment of signaling molecules containing src homology 2 (SH2) or phosphotyrosine interaction domains (PID). Additionally, several cytoplasmic proteins that may or may not associate with the receptor undergo tyrosine phosphorylation. To identify several components of the EGFR signaling pathway in a single step, we have immunoprecipitated molecules that are tyrosine phosphorylated in response to EGF and analyzed them by one-dimensional gel electrophoresis followed by mass spectrometry. Combining matrix-assisted laser desorption/ionization (MALDI) and nanoelectrospray tandem mass spectrometry (MS/MS) led to the identification of nine signaling molecules, seven of which had previously been implicated in EGFR signaling. Several of these molecules were identified from low femtomole levels of protein loaded onto the gel. We identified Vav-2, a recently discovered guanosine nucleotide exchange factor that is expressed ubiquitously, as a substrate of the EGFR. We demonstrate that Vav-2 is phosphorylated on tyrosine residues in response to EGF and associates with the EGFR in vivo. Binding of Vav-2 to the EGFR is mediated by the SH2 domain of Vav-2. In keeping with its ubiquitous expression, Vav-2 seems to be a general signaling molecule, since it also associates with the platelet-derived growth factor (PDGF) receptor and undergoes tyrosine phosphorylation in fibroblasts upon PDGF stimulation. The strategy suggested here can be used for routine identification of downstream components of cell surface receptors in mammalian cells.

Immunocytochemical localization of Shc and activated EGF receptor in early endosomes after EGF stimulation of HeLa cells

After binding of epidermal growth factor (EGF), the EGF receptor (EGFR) becomes autophosphorylated via tyrosine. The ligand-activated receptor is internalized by endocytosis and subsequently degraded in the lysosomal pathway. To follow EGFR activation after EGF stimulation, we generated antisera to the EGFR phosphotyrosine sites pY992 and pY1173. The SH2 region of Shc binds to both these sites. Both antisera identified EGFR after EGF binding and did not crossreact with the unactivated receptor. The intracellular distribution of phosphorylated EGFR after ligand binding was traced by two-color immunofluorescence confocal microscopy and immunoelectron microscopy. Before EGF stimulation EGFR was primarily located along the cell surface. When internalization of activated EGFR was inhibited by incubation with EGF on ice, Y992- and Y1173-phosphorylated EGFR were located along the plasma membrane. Ten minutes after internalization at 37C, Y992- and Y1173-phosphorylated EGFR were almost exclusively located in early endosomes, as shown by co-localization with EEA1. Immunoelectron microscopy confirmed that phosphorylated EGFR was located in intracellular vesicles resembling early endosomes. After EGF stimulation, the adaptor protein Shc redistributed to EGFR-containing early endosomes. Our results indicate that EGFR activation of Shc via tyrosine-phosphorylated Y992 and Y1173 occurred in early endocytic compartments, and support a role for membrane trafficking in intracellular signaling.

Bifurcation of cell migratory and proliferative signaling by the adaptor protein Shc

Cytokines and extracellular matrix proteins initiate signaling cascades that regulate cell migration and proliferation. Evidence is provided that the adaptor protein Shc can differentially regulate these processes. Specifically, under growth factor-limiting conditions, Shc stimulates haptotactic cell migration without affecting anchorage-dependent proliferation. However, when growth factors are present, Shc no longer influences cell migration; rather, Shc is crucial for DNA synthesis. Mutational analysis of Shc demonstrates that, while tyrosine phosphorylation is required for both DNA synthesis and cell migration, the switch in Shc signaling is associated with differential use of Shc's phosphotyrosine interacting domains; the PTB domain regulates haptotaxis, while the SH2 domain is selectively required for proliferation.

Interaction of c-Jun amino-terminal kinase interacting protein-1 with p190 rhoGEF and its localization in differentiated neurons

c-Jun amino-terminal kinase (JNK) interacting protein-1 (JIP-1) was originally identified as a cytoplasmic inhibitor of JNK. More recently, JIP-1 was proposed to function as a scaffold protein by complexing specific components of the JNK signaling pathway, namely JNK, mitogen-activated protein kinase kinase 7, and mixed lineage kinase 3. We have identified the human homologue of JIP-1 that contains a phosphotyrosine binding (PTB) domain in addition to a JNK binding domain and an Src homology 3 domain. To identify binding targets for the hJIP-1 PTB domain, a mouse embryo cDNA library was screened using the yeast two-hybrid system. One clone encoded a 191-amino acid region of the neuronal protein rhoGEF, an exchange factor for rhoA. Overexpression of rhoGEF promotes cytoskeletal rearrangement and cell rounding in NIE-115 neuronal cells. The interaction of JIP-1 with rhoGEF was confirmed by coimmunoprecipitation of these proteins from lysates of transiently transfected HEK 293 cells. Using glutathione S-transferase rhoGEF fusion proteins containing deletion or point mutations, we identified a putative PTB binding site within rhoGEF. This binding site does not contain tyrosine, indicating that the JIP PTB domain, like that of Xll alpha and Numb, binds independently of phosphotyrosine. Several forms of endogenous JIP-1 protein can be detected in neuronal cell lines. Indirect immunofluorescence analysis localized endogenous JIP-1 to the tip of the neurites in differentiated NIE-115 and PC12 cells. The interaction of JIP-1 with rhoGEF and its subcellular localization suggests that JIP-1 may function to specifically localize a signaling complex in neuronal cells.

Diversity in protein recognition by PTB domains

Phosphotyrosine-binding (PTB) domains were originally identified as modular domains that recognize phosphorylated Asn-Pro-Xxx-p Tyr-containing proteins. Recent binding and structural studies of PTB domain complexes with target peptides have revealed a number of deviations from the previously described mode of interaction, with respect to both the sequences of possible targets and their structures within the complexes. This diversity of recognition by PTB domains extends and strengthens our general understanding of modular binding domain recognition.

Characterization of four mammalian numb protein isoforms. Identification of cytoplasmic and membrane-associated variants of the phosphotyrosine binding domain

Numb is a membrane-associated, phosphotyrosine binding (PTB) domain-containing protein that functions as an intrinsic determinant of cell fate during Drosophila development. We have identified four isoforms of mammalian Numb with predicted molecular masses of 65, 66, 71, and 72 kDa that are generated by alternative splicing of the Numb mRNA. The different isoforms result from the presence of two sequence inserts within the PTB domain and the central region of the protein. The endogenous expression pattern of these isoforms, examined using specific antisera, varied in different tissues and cell lines. In addition, differentiation of P19 cells with retinoic acid leads to the specific loss of expression of the 71- and 72-kDa Numb proteins, suggesting that the expression of certain forms of Numb protein is regulated in a cell type-specific manner. Expression of Numb proteins fused to green fluorescent protein revealed that the form of the PTB domain with the alternatively spliced insert constitutively associated with the plasma membrane in polarized Madin-Darby canine kidney cells. In contrast, the isoform without the insert was cytoplasmic, suggesting that different PTB domain isoforms may regulate the subcellular localization of Numb proteins. The membrane localization may be due, in part, to differential affinity for acidic phospholipids. The distinct expression and localization patterns of the different mammalian Numb isoforms suggest that they have distinct functional properties.

Functional Rac-1 and Nck signaling networks are required for FGF-2-induced DNA synthesis in MCF-7 cells

The effects of Fibroblast Growth Factor-2 (FGF-2) on breast cancer cell DNA synthesis are controversial. To elucidate the mechanisms by which FGF-2 stimulates or inhibits DNA synthesis, we analysed FGF-2 signaling pathways in breast cancer MCF-7 and MCF-7 cells overexpressing Ha-Ras (MCF-7ras). We found that FGF-2-induction of DNA synthesis correlates with Ras transient activation, FRS-2 tyrosine phosphorylation and low level of expression of p66Shc. In addition, Nck-associated proteins are highly tyrosine phosphorylated and JNK reaches a higher level of activation when FGF-2 triggers DNA synthesis. Interestingly upon FGF-2 treatment, JNK activation and DNA synthesis are dependent on Rac-1 activity. These results confirm that in MCF-7 cells, induction of DNA synthesis by FGF-2 requires a transient activation of the Ras/MAPK cascade and demonstrates for the first time that intact Rac-1 and Nck signaling networks are required.

The function of PTB domain proteins

Phosphotyrosine binding (PTB) domains have been identified in a large number of proteins. In proteins like Shc and IRS-1, the PTB domain binds in a phosphotyrosine-dependent fashion to peptides that form a b turn. In these proteins, PTB domains play an important role in signal transduction by growth factor receptors. However, in several other proteins, the PTB domains have been found to participate in phosphotyrosine-independent interactions. The X11 family of proteins contains a PTB domain that binds peptides in a phosphotyrosine-independent fashion. The homologue of X11 in C. elegans is the lin-10 gene, a gene crucial for receptor targeting to the basolateral surface of body wall epithelia. The X11/Lin-10 proteins are found in a complex with two other proteins, Lin-2 and Lin-7, which have also been implicated in basolateral targeting in worm epithelia. This protein complex is also likely to be important in the targeting of cell surface proteins in mammalian neurons and epithelia. The ability of the PTB domain to bind peptides in a phosphotyrosine-dependent and -independent fashion allows this domain to be involved in diverse cellular functions.

Lipoprotein receptors: new roles for ancient proteins

Lipoprotein receptors used to be viewed simply as the means by which cells were supplied with lipids for energy production and membrane synthesis. This perception has now changed dramatically. Megalin, a member of the low density lipoprotein receptor gene family, turns out to mediate the endocytic uptake of retinoids and steroids, thus helping to regulate their biological function. Other members of this receptor family interact with cytosolic signalling proteins, giving this evolutionarily ancient family of receptors an entirely unexpected new role as transducers of extracellular signals.

The PTB Domain: The Name Doesn't Say It All

The phosphotyrosine-binding domain is a recently described protein-protein interaction domain which, despite its name, is involved in both phosphotyrosine-dependent and -independent interactions. Proteins with this domain are involved in diverse cellular functions, ranging from receptor signaling to protein targeting.

hFE65L influences amyloid precursor protein maturation and secretion

The amyloid precursor protein (APP) is processed in the secretory and endocytic pathways, where both the neuroprotective alpha-secretase-derived secreted APP (APPs alpha) and the Alzheimer's disease-associated beta-amyloid peptide are generated. All three members of the FE65 protein family bind the cytoplasmic domain of APP, which contains two sorting signals, YTS and YENPTY. We show here that binding of APP to the C-terminal phosphotyrosine interaction domain of hFE65L requires an intact YENPTY clathrin-coated pit internalization sequence. To study the effects of the hFE65L/APP interaction on APP trafficking and processing, we performed pulse/chase experiments and examined APP maturation and secretion in an H4 neuroglioma cell line inducible for expression of the hFE65L protein. Pulse/chase analysis of endogenous APP in these cells showed that the ratio of mature to total cellular APP increased after the induction of hFE65L. We also observed a three-fold increase in the amount of APPs alpha recovered from conditioned media of cells overexpressing hFE65L compared with uninduced controls. The effect of hFE65L on the levels of APPs alpha secreted is due neither to a simple increase in the steady-state levels of APP nor to activation of the protein kinase C-regulated APP secretion pathway. We conclude that the effect of hFE65L on APP processing is due to altered trafficking of APP as it transits through the secretory pathway.

The disabled 1 phosphotyrosine-binding domain binds to the internalization signals of transmembrane glycoproteins and to phospholipids

Disabled gene products are important for nervous system development in drosophila and mammals. In mice, the Dab1 protein is thought to function downstream of the extracellular protein Reln during neuronal positioning. The structures of Dab proteins suggest that they mediate protein-protein or protein-membrane docking functions. Here we show that the amino-terminal phosphotyrosine-binding (PTB) domain of Dab1 binds to the transmembrane glycoproteins of the amyloid precursor protein (APP) and low-density lipoprotein receptor families and the cytoplasmic signaling protein Ship. Dab1 associates with the APP cytoplasmic domain in transfected cells and is coexpressed with APP in hippocampal neurons. Screening of a set of altered peptide sequences showed that the sequence GYXNPXY present in APP family members is an optimal binding sequence, with approximately 0.5 microM affinity. Unlike other PTB domains, the Dab1 PTB does not bind to tyrosine-phosphorylated peptide ligands. The PTB domain also binds specifically to phospholipid bilayers containing phosphatidylinositol 4P (PtdIns4P) or PtdIns4,5P2 in a manner that does not interfere with protein binding. We propose that the PTB domain permits Dab1 to bind specifically to transmembrane proteins containing an NPXY internalization signal.

Integration of T cell receptor-dependent signaling pathways by adapter proteins

The initiation of biochemical signal transduction following ligation of surface receptors with intrinsic cytoplasmic tyrosine kinase activity is common for many cell types. T lymphocytes also require activation of tyrosine kinases following T cell receptor (TCR) ligation for maximal stimulation. However, the TCR has no intrinsic tyrosine kinase activity. Instead, the TCR must rely on cytoplasmic tyrosine kinases that localize to the TCR complex and initiate TCR-mediated signaling events. Although much has been learned regarding how these cytosolic tyrosine kinases are activated and recruited to the TCR complex, relatively little is understood about how these initial events are translated into transcriptional activation of genes that regulate cytokine production, cell proliferation, and cell death. Recently, it has become clear that the class of intracellular molecules known collectively as adapter proteins, molecules with modular domains capable of recruiting additional proteins but that exhibit no intrinsic enzymatic activity, serve to couple proximal biochemical events initiated by TCR ligation with more distal signaling pathways.

Re-engineering the target specificity of the insulin receptor by modification of a PTB domain binding site

Shc and IRS-1 (and their relatives) are cytoplasmic docking proteins that possess phosphotyrosine-binding (PTB) domains, through which they bind specific activated receptor tyrosine kinases (RTK). The subsequent phosphorylation of Shc or IRS-1 creates binding sites for the SH2 domains of multiple signaling proteins, leading to the activation of intracellular biochemical pathways. The PTB domains of Shc and IRS-1 both recognize autophosphorylation sites in RTKs with the consensus sequence NPXpY, but show distinct abilities to bind stably to RTKs such as the TrkA nerve growth factor receptor and the insulin receptor. In vitro analysis has suggested that residues N-terminal to the NPXpY motif may determine the affinity with which phosphopeptide ligands are recognized by the Shc and IRS-1 PTB domains. Unlike IRS-1, the Shc PTB domain binds poorly to the insulin-receptor (IR) beta subunit in vitro, owing to its low affinity for the NPXpY autophosphorylation site at Tyr 960 of the IR. As a consequence, Shc does not bind stably to the activated IR in cells. We show that substitution of Ser 955, five residues N-terminal to the Tyr 960 autophosphorylation site (the -5 position), with Ile alters the target specificity of the IR such that it stably associates with Shc in insulin-stimulated cells. A triple substitution of the -5, -8 and -9 residues relative to Tyr 960 of the IR to the corresponding amino acids found in the Shc PTB domain binding site of TrkA results in even stronger binding of the IR to Shc in vivo. The variant IRs with enhanced ability to bind Shc showed an increased ability to activate the MAPK pathway in response to insulin stimulation. These results demonstrate that subtle differences in residues N-terminal to NPXpY autophosphorylation sites determine the ability of RTKs to bind specific PTB domain proteins in vivo, and thus modify the signaling properties of activated receptors.

The insulin receptor tyrosine kinase domain in a chimaeric epidermal growth factor-insulin receptor generates Ca2+ signals through the PLC-gamma1 pathway

The receptors for insulin (IR) and epidermal growth factor (EGFR) are members of the tyrosine kinase receptor (TKR) family. Despite homology of their cytosolic TK domains, both receptors induce different cellular responses. Tyrosine phosphorylation of insulin receptor substrate (IRS) molecules is a specific IR post-receptor response. The EGFR specifically activates phospholipase C-gamma1 (PLC-gamma1). Recruitment of substrate molecules with Src homology 2 (SH2) domains or phosphotyrosine binding (PTB) domains to phosphotyrosines in the receptor is one of the factors creating substrate specificity. In addition, it has been shown that the TK domains of the IR and EGFR show preferences to phosphorylate distinct peptides in vitro, suggesting additional mechanisms of substrate recognition. We have examined to what extent the substrate preference of the TK domain contributes to the specificity of the receptor in vivo. For this purpose we determined whether the IR TK domain, in situ, is able to tyrosine-phosphorylate substrates normally used by the EGFR. A chimaeric receptor, consisting of an EGFR in which the juxtamembrane and tyrosine kinase domains were exchanged by their IR counterparts, was expressed in CHO-09 cells lacking endogenous EGFR. This receptor was found to activate PLC-gamma1, indicating that the IR TK domain, in situ, is able to tyrosine phosphorylate substrates normally used by the EGFR. These findings suggest that the IR TK domain, in situ, has a low specificity for selection and phosphorylation of non-cognate substrates.