Interactions between the protein-tyrosine kinase ZAP-70, the proto-oncoprotein Vav, and tubulin in Jurkat T cells

End-binding protein 1 controls signal propagation from the T cell receptor

The role of microtubules (MTs) in the control and dynamics of the immune synapse (IS) remains unresolved. Here, we show that T cell activation requires the growth of MTs mediated by the plus-end specific protein end-binding 1 (EB1). A direct interaction of the T cell receptor (TCR) complex with EB1 provides the molecular basis for EB1 activity promoting TCR encounter with signalling vesicles at the IS. EB1 knockdown alters TCR dynamics at the IS and prevents propagation of the TCR activation signal to LAT, thus inhibiting activation of PLCγ1 and its localization to the IS. These results identify a role for EB1 interaction with the TCR in controlling TCR sorting and its connection with the LAT/PLCγ1 signalosome.

The Rac activator Tiam1 is required for polarized protrusional outgrowth of primary astrocytes by affecting the organization of the microtubule network

Polarized cell migration is a crucial process in the development and repair of tissues, as well as in pathological conditions, including cancer. Recent studies have elucidated important roles for Rho GTPases in the establishment and maintenance of polarity prior to and during cell migration. Here, we show that Tiam1, a specific activator of the small GTPase Rac, is required for the polarized outgrowth of protrusions in primary astrocytes during the initial phase of cell polarization after scratch-wounding monolayers of cells. Tiam1 deficiency delays closure of wounds in confluent monolayers. Lack of Tiam1 impairs adoption of an asymmetrical cell shape as well as microtubule organization within protrusions. Positioning of the centrosome and Golgi apparatus, however, are independent of Tiam1-Rac signaling. We speculate that the function of Tiam1 in polarized outgrowth of astrocyte protrusions involves regulation of microtubule organization, possibly by stabilizing the microtubule cytoskeleton. Our results add Tiam1 as a player to the growing list of proteins involved in polarized outgrowth of protrusions and further elucidate the signaling pathways leading to cell polarization.

Vav1 and Vav2 play different roles in macrophage migration and cytoskeletal organization

Vav family proteins act as guanine nucleotide exchange factors for Rho family proteins, which are known to orchestrate cytoskeletal changes and cell migration in response to extracellular stimuli. Using mice deficient for Vav1, Vav2 and/or Vav3, overlapping and isoform-specific functions of the three Vav proteins have been described in various hematopoietic cell types, but their roles in regulating cell morphology and migration have not been studied in detail. To investigate whether Vav isoforms have redundant or unique functions in regulating adhesion and migration, we investigated the properties of Vav1-deficient and Vav2-deficient macrophages. Both Vav1-deficient and Vav2-deficient cells have a smaller adhesive area; yet, only Vav1-deficient cells have a reduced migration speed, which coincides with a lower level of microtubules. Vav2-deficient macrophages display a high level of constitutive membrane ruffling, but neither Vav1 nor Vav2 is required for colony stimulating factor-1-induced membrane ruffling and cell spreading. Our results suggest that the migration speed of macrophages is regulated independently of spread area or membrane ruffling and that Vav1 is selectively required to maintain a normal migration speed.

The Immunological Synapse and Rho GTPases

Rho GTPases are molecular switches controlling a broad range of cellular processes including lymphocyte activation. Not surprisingly, Rho GTPases are now recognized as pivotal regulators of antigen-specific T cell activation by APCs and immunological synapse formation. This review summarizes recent advances in our understanding of how Rho GTPase-dependent pathways control T lymphocyte motility, polarization and activation.

HDAC6 deacetylase activity links the tubulin cytoskeleton with immune synapse organization

We investigated the role of acetylated microtubules in the antigen-specific interaction of T helper and antigen-presenting cells (APCs). In T cells, acetylated microtubules concentrated at contact site with APCs, surrounding clusters of CD3 and LFA-1. TcR engagement induced a transient deacetylation of microtubules at early times and an enhanced acetylation at late times. Confocal videomicroscopy studies revealed that the HDAC6 tubulin deacetylase was translocated and concentrated at the contact site of T cells with APCs. Overexpression of HDAC6 but not a dead deacetylase mutant in T cells disorganized CD3 and LFA-1 at the immune synapse. This effect was reverted by treatment with the deacetylase inhibitor trichostatin A. The antigen-specific translocation of the microtubule organizing center (MTOC) and IL-2 production were also severely impaired by overexpression of HDAC6. Our results underscore the key role for HDAC6 in the organization of the immune synapse and the antigen-specific reorientation of the MTOC.

Detection and identification of Vav1 protein in primary cultured murine cerebellar neurons and in neuroblastoma cells (SH-SY5Y and Neuro-2a)

Vav1 was detected in neuronal cells during a screening for 1-methylthiodihydroceramide (1-MSDH-Cer) binding proteins. 1-MSDH-Cer is a metabolically stable analogue of dihydroceramide that was reported to strongly interfere with the formation of ceramide and hence the biosynthesis of all sphingolipids in neuronal cells. To identify target proteins that function as putative mediators of this molecule, a 1-MSDH-Cer affinity chromatography was utilised. When the cytosolic fraction of human neuroblastoma SH-SY5Y cells was subjected to 1-MSDH-Cer affinity chromatography, the sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the eluted protein fraction revealed an about 2-fold enrichment of the 98 kD protein band. Tryptic digestion of the excised band in combination with MALDI mass spectrometry strongly suggested that this band contained Vav1 protein. This was surprising since Vav1 in contrast to the other two isoforms Vav2 and Vav3 is believed to be exclusively expressed in hematopoietic cells and has not been detected in neuronal cells until now. The expression of Vav1 was confirmed in human SH-SY5Y neuroblastoma cells and additionally in murine Neuro-2A neuroblastoma cells as well as in primary cultured murine cerebellar neurons by Western blot analysis and reverse transcription polymerase chain reaction.

The Polycomb-group protein ENX-2 interacts with ZAP-70

Human ENX-2 is a homologue of Drosophila Enhancer of zeste, which is a member of Polycomb-group proteins regulating the expression of homeotic genes as chromatin-associated proteins. In this study, we demonstrate that ENX-2 plays an important role as a signaling molecule involved in T cell receptor-mediated signaling pathway. In immunoprecipitation experiments, ENX-2 and zeta associated protein-70 (ZAP-70) were co-precipitated from T cell lysate. When probed with an anti-phospho-tyrosine antibody, ENX-2 was found to be phosphorylated on tyrosine. On the other hand, ENX-2 was not phosphorylated on tyrosine in the mutant Jurkat cell, J.Cam1.6 lacking the activity of lymphocyte protein tyrosine kinase p56(lck). The interaction between ENX-2 and ZAP-70 was abolished in the mutant cell. Furthermore, in-vitro kinase assay using purified p56(lck) demonstrated that ENX-2 became tyrosine phosphorylated by this kinase. These findings show that the phosphorylation of ENX-2 is responsible for the interaction between ENX-2 and ZAP-70.

All roads lead to actin: the intimate relationship between TCR signaling and the cytoskeleton

Regardless of cell type, the regulation of the actin cytoskeleton is tightly linked to vital biological properties such as polarity, motility, cell-cell contact, exocytosis and proliferation. In the immune system, where rapid and efficient response to antigen-provoked stimuli is crucial, an overwhelming amount of data implicate the actin cytoskeleton and its regulators as central to immune function. Increasingly, the cytoskeleton is considered an essential amplification step in T cell receptor (TCR)-, costimulatory-, and integrin-mediated signaling. Advances in genetic manipulation and confocal imaging have led to a keener appreciation of the importance of TCR signal integration by the actin cytoskeleton. This review outlines recent advances in elucidating the regulation of T cell function through the actin cytoskeleton. We also examine intriguing parallels between the immune system and other models of cytoskeletal regulation.

Visualization of signaling pathways and cortical cytoskeleton in cytolytic and noncytolytic natural killer cell immune synapses

Recent applications of imaging approaches and other methods of cell biology have provided high-resolution visualization of the location of fluorescent proteins in living and fixed cells during cell-cell interactions between lymphocytes, antigen presenting cells and target cells. We review the composition and dynamics of molecular and cytoskeletal events occurring during natural killer cell interactions with susceptible and nonsusceptible target cells. The natural killer cell immune synapse and the concomitant changes in cytoskeletal components and cytoplasmic organelles are described. The findings are compared with the observations made in T helper cells and cytotoxic T cells. It is concluded that the cytolytic immune synapses display spatial-temporal dynamics that are accelerated as compared with T helper cells. In addition, the cytolytic conjugates have unique characteristics relating to their effector function. Furthermore, the natural killer cell immune synapses in cytolytic and noncytolytic interactions are distinctly different and display patterns consistent with characteristic signaling pathways identified in biochemical studies of disrupted cells. The precise relationship between different stages of the natural killer cell immune synapse formation and progression in signal transduction pathways is yet to be established.

Cell-cell adhesion and signalling

Signalling pathways activated by Rho small GTPases have recently been identified that coordinate junction assembly, stability and function, as well as interactions of adhesive complexes with the underlying cortical cytoskeleton. Particularly exciting is the interplay between adherens junctions, activation of Rho proteins and the dynamics of microtubule, actin and intermediate filaments. This interplay has important implications for functional regulation of cell-cell adhesion, and points to a more integrated view of signalling processes.

CD38 is associated with lipid rafts and upon receptor stimulation leads to Akt/protein kinase B and Erk activation in the absence of the CD3-zeta immune receptor tyrosine-based activation motifs

T lymphocytes can be activated via the T cell receptor (TCR) or by triggering through a number of other cell surface structures, including the CD38 co-receptor molecule. Here, we show that in TCR+ T cells that express a CD3-zeta lacking the cytoplasmic domain, cross-linking with CD38- or CD3-specific monoclonal antibodies induces tyrosine phosphorylation of CD3-epsilon, zeta-associated protein-70, linker for activation of T cells, and Shc. Moreover, in these cells, anti-CD38 or anti-CD3 stimulation leads to protein kinase B/Akt and Erk activation, suggesting that the CD3-zeta-immunoreceptor tyrosine-based activation motifs are not required for CD38 signaling in T cells. Interestingly, in unstimulated T cells, lipid rafts are highly enriched in CD38, including the T cells lacking the cytoplasmic tail of CD3-zeta. Moreover, CD38 clustering by extensive cross-linking with an anti-CD38 monoclonal antibody and a secondary antibody leads to an increased resistance of CD38 to detergent solubilization, suggesting that CD38 is constitutively associated with membrane rafts. Consistent with this, cholesterol depletion with methyl-beta-cyclodextrin substantially reduces CD38-mediated Akt activation while enhancing CD38-mediated Erk activation. CD38/raft association may improve the signaling capabilities of CD38 via formation of protein/lipid domains to which signaling-competent molecules, such as immunoreceptor tyrosine-based activation motif-bearing CD3 molecules and protein-tyrosine kinases, are recruited.

Bacterial peptidoglycan binds to tubulin

A search for cellular binding proteins for peptidoglycan (PGN), a CD14- and TLR2-dependent macrophage activator from Gram-positive bacteria, using PGN-affinity chromatography and N-terminal micro-sequencing, revealed that tubulin was a major PGN-binding protein in mouse macrophages. Tubulin also co-eluted with PGN from anti-PGN vancomycin affinity column and bound to PGN coupled to agarose. Tubulin-PGN binding was preferential under the conditions that promote tubulin polymerization, required macromolecular PGN, was competitively inhibited by soluble PGN and tubulin, did not require microtubule-associated proteins, and had an affinity of 100-150 nM. By contrast, binding of tubulin to lipopolysaccharide (LPS) had 2-3 times lower affinity, faster kinetics of binding, and showed positive cooperativity. PGN enhanced tubulin polymerization in the presence of 4 M glycerol, but in the absence of glycerol, both PGN and LPS decreased microtubule polymerization. These results indicate that tubulin is a major PGN-binding protein and that PGN modulates tubulin polymerization.

Microtubule-dependent formation of podosomal adhesion structures in primary human macrophages

Podosomes are unique actin-rich adhesion structures of monocyte-derived cells such as macrophages and osteoclasts. They clearly differ from other substratum-contacting organelles like focal adhesions in morphological and functional regards. Formation of podosomes has been shown to be dependent on the small GTPase CDC42Hs and its effector Wiskott-Aldrich syndrome protein (WASp). In this study, we investigated the functional relation between podosomes and the microtubule system in primary human macrophages. We demonstrate that, in contrast to focal adhesions, assembly of podosomes in macrophages and their monocytic precursors is dependent on an intact microtubule system. In contrast, experiments using Wiskott-Aldrich syndrome (WAS) macrophages indicate that the microtubule system is not reciprocally dependent on podosomes. A potential linker between podosomes and microtubules may be WASp itself, considering that microinjection of the WASp polyproline domain prevents podosome reassembly. This polyproline domain is thought to link WASp to microtubules via CDC42 interacting protein 4 (CIP4). Consistently, macrophages microinjected with CIP4 constructs deficient in either the microtubule- or the WASp-binding domain also fail to reassemble podosomes. In sum, our findings show that microtubules are essential for podosome formation in primary human macrophages and that WASp and CIP4 may be involved in this phenomenon.

Regulation of beta 1 integrin-mediated adhesion by T cell receptor signaling involves ZAP-70 but differs from signaling events that regulate transcriptional activity

Stimulation of the CD3/TCR results within minutes in an increase in T cell adhesion mediated by beta(1) integrins. The biochemical pathways that control CD3-mediated increases in beta(1) integrin-mediated adhesion remain poorly characterized. In this study, the role of the tyrosine kinase ZAP-70 in the regulation of beta(1) integrin activity by the CD3/TCR was investigated. CD3 stimulation did not increase beta(1) integrin-mediated adhesion of the ZAP-70-deficient Jurkat T cell line, P116, to the beta(1) integrin ligand fibronectin. Reintroduction of wild-type ZAP-70, but not a kinase-inactive variant, K369R, corrected the adhesive defect observed in P116 T cells. In addition, the kinase-inactive ZAP-70 mutant inhibited CD3-induced adhesion of primary human T cell blasts. Interestingly, a ZAP-70 mutant with a tyrosine to phenylalanine substitution at position 319 (Y319F) restored the adhesive defect in P116 T cells, even though Y319F ZAP-70 failed to fully reconstitute CD3-initiated NF-AT-dependent transcription and tyrosine phosphorylation of the LAT adapter protein. Finally, expression of mutants of LAT and the SLP-76 adapter protein that modulate CD3-mediated activation of an NF-AT reporter gene failed to block CD3-induced increases in beta(1) integrin-mediated adhesion. These observations support a model in which the tyrosine kinase activity of ZAP-70 kinase is critical for regulation of beta(1) integrin activity by CD3/TCR. However, the signaling events downstream of ZAP-70 that regulate CD3/TCR-mediated activation of beta(1) integrin function exhibit key differences when compared with the signaling pathways that regulate transcriptional events initiated by CD3/TCR stimulation.

A comprehensive characterization of the T-cell antigen receptor complex composition by microcapillary liquid chromatography-tandem mass spectrometry

It has become apparent that many intracellular signaling processes involve the dynamic reorganization of cellular proteins into complex signaling assemblies that have a specific subunit composition, function, and subcellular location. Since the elements of such assemblies interact physically, multiprotein signaling complexes can be isolated and analyzed. Recent technical advances in highly sensitive protein identification by electrospray-tandem mass spectrometry have dramatically increased the sensitivity with which such analyses can be performed. The T-cell antigen receptor (TCR) is an oligomeric transmembrane protein complex that is essential to T-cell recognition and function. The extracellular protein domains are responsible for ligand binding while intracellular domains generate and transduce signals in response to specific receptor-ligand interactions. We used microbore capillary chromatography-tandem mass spectrometry to investigate the composition of the TCR protein complex isolated from resting and activated cells of the murine T-cell line CD11.3. We identified all the previously known subunits of the TCR/CD3 complex as well as proteins previously not known to associate with the TCR. The catalytic activities of some of these proteins could potentially be used to interfere pharmacologically with TCR signaling.

Searching for significance in TCR-cytoskeleton interactions

Two T-cell receptor (TCR) populations are expressed on T cells; one is linked to the cytoskeleton via its zeta chain. These cytoskeleton-linked receptors (30-40% of the total number of TCRs) might be important in TCR-mediated signaling and/or concurrent events. Here, differences between the two populations are summarized, and new data are examined to speculate on the functional significance of cytoskeleton-linked TCRs.

T lymphocytes and neutrophil granulocytes differ in regulatory signaling and migratory dynamics with regard to spontaneous locomotion and chemotaxis

Chemotactic migration of T lymphocytes and neutrophil granulocytes within a three-dimensional collagen matrix is distinct from spontaneous, matrix-induced migration concerning dynamic parameters and regulatory intracellular signaling. Both spontaneous T lymphocyte locomotion and stromal-cell-derived factor-1 (SDF-1)-induced chemotaxis-involved protein tyrosine kinase (PTK) activity, whereas only SDF-1-induced migration was protein kinase C (PKC) dependent. Spontaneous locomotion of neutrophil granulocytes was independent of PKC and PTK activity, but formyl-methionyl-leucyl-phenylalanine-induced migration involved PKC activity. In addition, the microtubule cytoskeleton was not changed after induction of chemotaxis in both cell types. T lymphocytes had a well-developed microtubule cytoskeleton with the microtubule organizing center located in the uropod, whereas neutrophil granulocytes revealed a clustered tubulin distribution at the leading edge of the migrating cell. Therefore, differences of the microtubule cytoskeleton might contribute to differences in locomotion between T lymphocytes and neutrophil granulocytes but not to differences between spontaneous locomotion and chemotaxis.

Src-Regulated Extracellular Signal-Related Kinase and Syk-Regulated c-Jun N-Terminal Kinase Pathways Act in Conjunction to Induce IL-1 Synthesis in Response to Microtubule Disruption in HL60 Cells

A microtubule reorganization is often observed during cellular contacts that are associated to IL-1 production. Here, we show that in HL60 cells, vincristine, a microtubule-disrupting agent that induces a strong production of IL-1, triggers the activation of both extracellular signal-related kinase (ERK) and c-Jun N-terminal kinase (JNK-1). While ERK activation is rapid and transient, peaking at 10 min, the JNK1 activation is delayed and more sustained reaching a maximum at 2 h. ERK activation was blocked by CP 118556, indicating it is regulated by a Src-like kinase, while JNK1 was inhibited by piceatannol, revealing an upstream regulation by Syk. Each kind of the nonreceptor tyrosine kinase blockers efficiently inhibits the vincristine-induced IL-1 production and diminishes the level of IL-1 transcripts, indicating that the ERK and JNK pathways act coordinately to elicit the transcription of the IL-1 gene. Furthermore, we found that pertussis toxin, a blocker of Go/Gi proteins, abrogated the vincristine-induced activation of both Src and Syk. Our data support a model where the status of microtubule polymerization influences the activity of Go or Gi proteins that control, in turn, two independent Src/ERK and Syk/JNK1 cascades that are both necessary to sustain IL-1 synthesis.

Protein tyrosine kinase p53/p56(lyn) forms complexes with gamma-tubulin in rat basophilic leukemia cells

The aggregation of receptors with high affinity for IgE (FcepsilonRI) on the surface of mast cells and basophils initiates a chain of biochemical events culminating in the release of allergy mediators. Although microtubules have been implicated in the activation process, the molecular mechanism of their interactions with signal transduction molecules is poorly understood. Here we show that in rat basophilic leukemia cells large amounts of alphabeta-tubulin dimers ( approximately 70%) and gamma-tubulin ( approximately 85%) are found in a soluble pool which was released from the cells after permeabilization with saponin, or extraction with non-ionic detergents. Soluble tubulins were found in large complexes with other molecules. Complexes of soluble gamma-tubulin released from activated cells contained tyrosine-phosphorylated proteins of relative mol. wt approximately 25, 50, 53, 56, 60, 75, 80, 97, 115 and 200 kDa. Increased tyrosine phosphorylation of proteins associated with the cytoskeleton, i.e. around centrosomes, was detected by immunofluorescence microscopy. In vitro kinase assays revealed increased tyrosine phosphorylation of proteins in gamma-tubulin complexes isolated from activated cells. Two of the tyrosine phosphorylated proteins in these complexes were identified as the p53/56(lyn) kinase. Furthermore, gamma-tubulin bound to the N-terminal fragment of recombinant Lyn kinase and its binding was slightly enhanced in activated cells. Pretreatment of the cells with Src family-selective tyrosine kinase inhibitor, PP1, decreased the amount of tyrosine phosphorylated proteins in gamma-tubulin complexes, as well as the amount of gamma-tubulin in Lyn kinase immunocomplexes. The combined data suggest that gamma-tubulin is involved in early stages of mast cell activation.

Aggregation of lipid rafts accompanies signaling via the T cell antigen receptor

The role of lipid rafts in T cell antigen receptor (TCR) signaling was investigated using fluorescence microscopy. Lipid rafts labeled with cholera toxin B subunit (CT-B) and cross-linked into patches displayed characteristics of rafts isolated biochemically, including detergent resistance and colocalization with raft-associated proteins. LCK, LAT, and the TCR all colocalized with lipid patches, although TCR association was sensitive to nonionic detergent. Aggregation of the TCR by anti-CD3 mAb cross-linking also caused coaggregation of raft-associated proteins. However, the protein tyrosine phosphatase CD45 did not colocalize to either CT-B or CD3 patches. Cross-linking of either CD3 or CT-B strongly induced tyrosine phosphorylation and recruitment of a ZAP-70(SH2)(2)-green fluorescent protein (GFP) fusion protein to the lipid patches. Also, CT-B patching induced signaling events analagous to TCR stimulation, with the same dependence on expression of key TCR signaling molecules. Targeting of LCK to rafts was necessary for these events, as a nonraft- associated transmembrane LCK chimera, which did not colocalize with TCR patches, could not reconstitute CT-B-induced signaling. Thus, our results indicate a mechanism whereby TCR engagement promotes aggregation of lipid rafts, which facilitates colocalization of LCK, LAT, and the TCR whilst excluding CD45, thereby triggering protein tyrosine phosphorylation.

Microtubule cytoskeleton involvement in muscarinic suppression of voltage-gated calcium channel current in guinea-pig ileal smooth muscle

1. Effects of agents, which affect microtubule polymerization-depolymerization cycle, on Ba2+ current (IBa) flowing through voltage-gated Ca2+ channels and carbachol (CCh)-induced sustained suppression of IBa were examined in whole-cell voltage-clamped smooth muscle cells of guinea-pig ileum. 2. offchicine (100 microM) and vinblastine (100 microM), microtubule depolymerizers, increased the ampitude of IBa. Lumicolchicine (100 microM), an inactive analogue of colchicine, had no effect on IBa. 3. Taxol (1 - 100 microM), a microtubule polymerizer, decreased IBa in a concentration-dependent manner and accelerated the rate of inactivation of IBa. Baccatin III (100 microM), an inactive analogue of taxol, had no effect on IBa. 4. Colchicine (100 microM) and vinblastine (100 microM), but not lumicolchicine (100 microM), decreased or abolished the sustained component of CCh (10 microM)-induced IBa suppression. 5. Pretreatment with taxol (10 - 100 microM) resulted in a concentration-dependent decrease in IBa and the action of CCh on IBa. The inhibitory effects of taxol and CCh on IBa were not additive. 6. Colchicine (100 microM) or taxol (100 microM) had no effect on voltage-gated K+ channel current or CCh-induced non-selective cationic channel current. 7. These results suggest that polymerization of microtubules leads to suppression of Ca2+ channel activity, and that muscarinic sustained suppression of Ca2+ channel current is mediated by a signal transduction element which involves microtubule cytoskeleton.

The CD3-gamma delta epsilon transducing module mediates CD38-induced protein-tyrosine kinase and mitogen-activated protein kinase activation in Jurkat T cells

We have examined the ability of the CD3-gamma delta epsilon and CD3-zeta signaling modules of the T cell receptor (TCR) to couple CD38 to intracellular signaling pathways. The results demonstrated that in TCR+ T cells that express the whole set of CD3 subunits CD38 ligation led to complete tyrosine phosphorylation of both CD3-zeta and CD3-epsilon polypeptide chains. In contrast, in TCR+ cells with a defective CD3-zeta association CD38 engagement caused tyrosine phosphorylation of CD3-epsilon but not of CD3-zeta. Despite these differences, in both cell types CD38 ligation resulted in protein-tyrosine kinase and mitogen-activated protein kinase activation. However, in cells expressing chimerical CD25-zeta or CD25-epsilon receptors or in a TCR-beta- Jurkat T cell line, CD38 ligation did not result in tyrosine phosphorylation of the chimeric receptors, or CD3 subunits, or protein-tyrosine kinase or mitogen-activated protein kinase activation. In summary, these results support a model in which CD38 transduces activating signals inside the cell by means of CD3-epsilon and CD3-zeta tyrosine phosphorylation. Moreover, these data identify the CD3-gamma delta epsilon signaling module as a necessary and sufficient component of the TCR/CD3 complex involved in T cell activation through CD38.

Cytoskeletal rearrangement during migration and activation of T lymphocytes

T lymphocytes have an inherent ability to migrate along a chemotactic gradient, which enables them to exit the bloodstream and reach different tissues. Motile T cells display a polarized morphology with two distinct cell compartments: the leading edge and the uropod. During cell polarization, chemoattractant receptors, cell-adhesion molecules and cytoskeletal proteins are redistributed within these cellular compartments. The polarity of T lymphocytes changes during the establishment of antigen-specific cell-cell interactions, and this involves rearrangement of cytoskeletal proteins. This article discusses the regulation of these cytoskeletal rearrangements, and their role in the activation, migration and effector function of T cells.

Integrin-dependent tyrosine phosphorylation and growth regulation by Vav

The proto-oncogene product p95Vav (Vav) undergoes rapid phosphorylation on tyrosine following stimulation of the T or B cell antigen receptor, and in response to a variety of other cell surface stimuli. Vav contains, among other, a guanine nucleotide exchange factor domain with homology to the Rho/Rac/CDC42 exchange protein Db1. It has been recently shown that Vav is functionally linked to small GTPases of the Rho family, suggesting that it is an activator of Rho GTPases and may participate in regulation of cytoskeletal organization. The present study shows that cell adhesion to fibronectin triggers rapid phosphorylation of Vav on tyrosine in Vav-transfected CHO cells and in Jurkat T cells. Vav phosphorylation is strongly dependent on adhesion and is mediated by beta 1 integrins. Furthermore, Vav overexpression enhances the adhesion-dependent increase in the rate and extent of phosphorylation on focal adhesion kinase and paxillin, and the formation of stress fibers and lamellipodia. In addition, there is a marked increase in the amount of Vav localized to the triton-insoluble fraction following 1 h of incubation on FN. Finally, Vav increases the growth rate of the cells in an adhesion-dependent manner. Our results strongly implicate Vav as a mediator of integrin signal transduction.

Tyrosine 319, a newly identified phosphorylation site of ZAP-70, plays a critical role in T cell antigen receptor signaling

Following T cell antigen receptor (TCR) engagement, the protein tyrosine kinase (PTK) ZAP-70 is rapidly phosphorylated on several tyrosine residues, presumably by two mechanisms: an autophosphorylation and a trans-phosphorylation by the Src-family PTK Lck. These events have been implicated in both positive and negative regulation of ZAP-70 activity and in coupling this PTK to downstream signaling pathways in T cells. We show here that Tyr315 and Tyr319 in the interdomain B of ZAP-70 are autophosphorylated in vitro and become phosphorylated in vivo upon TCR triggering. Moreover, by mutational analysis, we demonstrate that phosphorylation of Tyr319 is required for the positive regulation of ZAP-70 function. Indeed, overexpression in Jurkat cells and in a murine T cell hybridoma of a ZAP-70 mutant in which Tyr319 was replaced by phenylalanine (ZAP-70-Y319F) dramatically impaired anti-TCR-induced activation of the nuclear factor of activated T cells and interleukin-2 production, respectively. Surprisingly, an analogous mutation of Tyr315 had little or no effect. The inhibitory effect of ZAP-70-Y319F correlated with a substantial loss of its activation-induced tyrosine phosphorylation and up-regulation of catalytic activity, as well as with a decreased in vivo capacity to phosphorylate known ZAP-70 substrates, such as SLP-76 and LAT. Collectively, our data reveal the pivotal role of Tyr319 phosphorylation in the positive regulation of ZAP-70 and in TCR-mediated signaling.

Activation of Vav and Ras through the nerve growth factor and B cell receptors by different kinases

Engagement of the B-cell antigen receptor (BCR) or the nerve growth factor receptor (NGFR/TrkA) induces activation of multiple tyrosine kinases, resulting in phosphorylation of numerous intracellular substrates. We show that addition of NGF or anti-IgM antibody leads to the early tyrosine phosphorylation of p95(vav), which is expressed exclusively in hematopoietic cells; NGF, similar to crosslinking the BCR, also results in the rapid activation of Ras. The phosphorylation of Vav and activation of Ras triggered by NGF is mediated through Trk tyrosine kinase, whereas signaling through the BCR uses a different tyrosine kinase. We also show that NGF induces tyrosine phosphorylation of Shc and its association with Grb2. Vav and Ras with the adaptor proteins Shc and Grb2 appear to serve as a link between different receptor-mediated signaling pathways and, in human B cells, may play an important regulatory role in neuroimmune interactions.

Microtubules and signal transduction

Although molecular components of signal transduction pathways are rapidly being identified, how elements of these pathways are positioned spatially and how signals traverse the intracellular environment from the cell surface to the nucleus or to other cytoplasmic targets are not well understood. The discovery of signaling molecules that interact with microtubules (MTs), as well as the multiple effects on signaling pathways of drugs that destabilize or hyperstabilize MTs, indicate that MTs are likely to be critical to the spatial organization of signal transduction. MTs themselves are also affected by signaling pathways and this may contribute to the transmission of signals to downstream targets.

Phosphorylation- and activation-independent association of the tyrosine kinase Syk and the tyrosine kinase substrates Cbl and Vav with tubulin in B-cells

Aggregation of the B-cell antigen receptor leads to the activation of the 72-kDa Syk protein-tyrosine kinase and the phosphorylation of tubulin on tyrosine. To explore the requirement of Syk catalytic activity for tubulin phosphorylation, tubulin was isolated from cytosolic fractions from anti-IgM-activated B-cells (DT40) that lacked endogenous Syk and immunoblotted with anti-phosphotyrosine antibodies. Tubulin was not tyrosine-phosphorylated in Syk- B-cells. Phosphorylation could be restored by the expression of wild-type, but not catalytically inactive, Syk. However, both catalytically inactive and wild-type Syk were capable of constitutive association with tubulin, indicating that tubulin phosphorylation is not required for this interaction. Anti-phosphotyrosine antibody immunoblotting of proteins adsorbed to colchicine-agarose revealed the presence of three major tubulin-associated phosphoproteins of 110, 90, and 74 kDa, the phosphorylation of which was dependent on Syk expression. The proteins of 110 and 90 kDa were identified as Cbl and Vav, two proto-oncogene products known to become prominently phosphorylated following receptor engagement. Both proteins were shown to be constitutively associated with tubulin.

Fyn and ZAP-70 are required for Vav phosphorylation in T cells stimulated by antigen-presenting cells

In T cells, triggering of the T cell antigen receptor or of the co-stimulatory receptor CD28 can direct tyrosine phosphorylation of the signaling protein Vav. We investigated the role played by the protein tyrosine kinases Fyn, Lck, and ZAP-70 in these processes in a T cell hybridoma after physiological stimulation of the T cell receptor (TCR) and CD28. A dominant-negative mutant approach based on overexpression of catalytically inactive alleles of these kinases showed that CD28-induced Vav phosphorylation preferentially requires Fyn, whereas ZAP-70 had no role. Consistently, Vav was strongly phosphorylated in Lck-deficient JCAM-1 cells after CD28 ligation. In contrast, ZAP-70 appeared to control TCR-directed Vav phosphorylation. However, overexpression of ZAP-70 carrying a mutated Tyr315, contained within a motif previously suggested to be a Vav Src homology 2 domain binding site, had little or no effect. Immunoprecipitation assays showed that phosphorylated Vav associated with Fyn after CD28 triggering and that this interaction, likely to involve binding of Fyn Src homology 2 domain to Vav, was more strongly detectable after concomitant CD28 and TCR stimulation. These data suggest that Fyn plays a major role in controlling Vav phosphorylation upon T cell activation and that the mechanism implicating ZAP-70 in this process may be more complex than previously anticipated.

The Vav-Rac1 pathway in cytotoxic lymphocytes regulates the generation of cell-mediated killing

The Rac1 guanine nucleotide exchange factor, Vav, is activated in hematopoietic cells in response to a large variety of stimuli. The downstream signaling events derived from Vav have been primarily characterized as leading to transcription or transformation. However, we report here that Vav and Rac1 in natural killer (NK) cells regulate the development of cell-mediated killing. There is a rapid increase in Vav tyrosine phosphorylation during the development of antibody-dependent cellular cytotoxicity and natural killing. In addition, overexpression of Vav, but not of a mutant lacking exchange factor activity, enhances both forms of killing by NK cells. Furthermore, dominant-negative Rac1 inhibits the development of NK cell-mediated cytotoxicity by two mechanisms: (a) conjugate formation between NK cells and target cells is decreased; and (b) those NK cells that do form conjugates have decreased ability to polarize their granules toward the target cell. Therefore, our results suggest that in addition to participating in the regulation of transcription, Vav and Rac1 are pivotal regulators of adhesion, granule exocytosis, and cellular cytotoxicity.

Altered peptide ligands induce quantitatively but not qualitatively different intracellular signals in primary thymocytes

Interaction of the T cell receptor (TCR) with peptide/major histocompatibility complexes (MHC) in the thymus is of critical importance for developing thymocytes. In a previous study, we described an antagonist peptide that inhibited negative selection of transgenic thymocytes induced by an agonist peptide. In this study we show that this antagonist peptide can induce positive selection of CD8(+) thymocytes more efficiently than the agonist or the weak agonist peptides, whereas the opposite is true for their ability to cause negative selection. The intracellular signals induced in thymocytes by such peptides after TCR ligation was examined in CD4(+)8(+) double-positive thymocytes from F5/beta2mo/Rag-1(o) transgenic mice. TCR ligation with either the agonist, weak agonist, or antagonist peptide variants resulted in hyperphosphorylation of CD3zeta, CD3epsilon, ZAP-70, Syk, Vav, SLP-76, and pp36-38. The extent of phosphorylation of these intracellular proteins correlated with the efficiency with which the peptide analogs induced apoptosis of immature thymocytes. Unexpectedly, there was no correlation between the upstream TCR signaling pathways analyzed and the capacity of the different peptides to induce positive selection.

Effect of microtubule disruption on cell adhesion and spreading

Microtubules have been involved in a variety of cellular processes. In this study, we examined the role of the microtubular system in the adhesion and spreading of the adenocarcinoma cell line HT29-D4. Disruption of microtubules by nocodazole or navelbine resulted in an increase in cell adhesion to purified ECM proteins. This enhanced cell adhesion is mediated by integrins, but is not attributable to quantitative changes in the number of integrin receptors at the cell surface, as determined by flow cytometric analysis. In contrast to attachment, spreading of HT29-D4 cells was reduced by nocodazole treatment in a dose-dependent manner. Thus, microtubule depolymerization appears to increase initial attachment of cells to extracellular matrix, while impeding subsequent cell spreading.

Nocodazole inhibits signal transduction by the T cell antigen receptor

The potential role of the cytoskeleton in signaling via the T cell antigen receptor (TCR) was investigated using pharmacological agents. In Jurkat T cells, disruption of the actin-based cytoskeleton with cytochalasin D or disruption of the microtubules with colchicine did not affect TCR induction of proximal signaling events triggered by CD3 mAb. Polymerized actin and tubulin, therefore, were not required for TCR-mediated signal transduction. Nocodazole, however, was found to inhibit dramatically TCR signaling, independently of its ability to depolymerize microtubules. This effect was TCR-specific, because signaling via the human muscarinic acetylcholine receptor 1 in the same cells was unaffected. A mechanism for the inhibition of TCR signaling by nocodazole was suggested by in vitro assays, which revealed that the drug inhibited the kinase activity of LCK and, to a lesser extent, FYN. The kinase activity of ZAP-70 in vitro, however, was unaffected. These results, therefore, suggested that nocodazole prevented initial phosphorylation of the TCR by LCK after stimulation, and as a result, it blocked activation of downstream signaling pathways. Immunofluorescence analyses also revealed that nocodazole and the specific SRC-family kinase inhibitor PP1 delocalized ZAP-70 from its constitutive site at the cell cortex. These effects did not require the SH2 domains of ZAP-70. The localization of ZAP-70 to the cell cortex is, therefore, regulated by the activity of SRC-family kinases, independently of their ability to phosphorylate immunoreceptor tyrosine-based activation motifs of the TCR.

Two-dimensional electrophoretic analysis of mixed lineage kinase 2 N-terminal domain binding proteins

The mixed lineage kinase 2 (MLK2) protein contains several structurally distinct domains including an src homology (SH) 3 domain, a kinase catalytic domain, two leucine zippers, a basic motif and a cdc42/rac interactive binding motif. These domains have been recognized mainly for their involvement in protein-protein interactions in signal transduction networks. The SH3 domain in particular has been implicated in control of signaling events. To identify proteins that interact with MLK2, the N-terminal 100 amino acids, including the SH3 domain, were expressed as a glutathione S-transferase (GST) fusion protein. This fusion protein (MLK2N) was used as an affinity ligand to isolate binding proteins from lysates of 35S-radiolabeled MDA-MB231 breast carcinoma cells. When the radiolabeled binding proteins were subjected to 2-DE, proteins of Mr 55,000, 31,500 and 34,000 bound consistently to the MLK2N domain fusion protein, but not to the GST control. Two of the binding proteins were isolated from whole cell lysates by preparative 2-DE and subjected to in-gel digestion and capillary or microbore reverse-phase high performance liquid chromatography (RP-HPLC). Resultant peptides were analyzed by peptide mass fingerprinting, N-terminal Edman degradation or tandem mass spectrometry. The 55,000 protein was identified as the cytoskeletal protein, beta-tubulin, and this was verified by immunoblotting of proteins in the MLK2N binding fraction with anti-tubulin antibodies. The 31,500 protein has been identified as prohibitin, a protein that has been implicated in both signal transduction and cell cycle arrest.

T cell receptor (TCR) engagement in apoptosis-defective, but interleukin 2 (IL-2)-producing, T cells results in impaired ZAP70/CD3-zeta association

We have previously shown that a tyrosine to leucine replacement in the transmembrane region of T cell receptor (TCR)-beta results in a deficient induction of CD95-L and apoptosis upon TCR triggering in a transfected T cell line. By contrast, interleukin (IL)-2 production and the expression of CD25 and CD69 were normally induced. Since the mutation in TCR-beta also resulted in impaired association of CD3-zeta, it was proposed that this chain is specifically required for the induction of apoptosis. We now show that the deficient induction of CD95-L and apoptosis does not derive from a general lower production of second messengers, since intracellular Ca2+ fluxes and tyrosine phosphorylation of total proteins were elicited at wild-type levels. Unlike in T cell clones stimulated with partial agonists, both p21 and p18 forms of tyrosine-phosphorylated CD3-zeta were detected, although the overall level of tyrosine-phosphorylated CD3-zeta was low. More strikingly, inducible association of ZAP70 to CD3-zeta was strongly inhibited, despite a normal induction of ZAP70 tyrosine phosphorylation. Finally, ZAP70 was not concentrated near the plasma membrane in the apoptosis-deficient cells. These results suggest that CD3-zeta is necessary for engagement of a specific signaling pathway leading to CD95-L expression that also needs the recruitment of ZAP70.

Anti-peptide antibodies detect conformational changes of the inter-SH2 domain of ZAP-70 due to binding to the zeta chain and to intramolecular interactions

T cell receptor (TCR) triggering induces association of the protein tyrosine kinase ZAP-70, via its two src-homology 2 (SH2) domains, to di-phosphorylated Immunoreceptor Tyrosine-based Activation Motifs (2pY-ITAMs) present in the intracellular tail of the TCR-zeta chain. The crystal structure of the SH2 domains complexed with a 2pY-ITAM peptide suggests that the 60-amino acid-long inter-SH2 spacer helps the SH2 domains to interact with each other to create the binding site for the 2pY-ITAM. To investigate whether the inter-SH2 spacer has additional roles in the whole ZAP-70, we raised antibodies against two peptides of this region and probed ZAP-70 structure under various conditions. We show that the reactivity of antibodies directed at both sequences was dramatically augmented toward the tandem SH2 domains alone compared with that of the entire ZAP-70. This indicates that the conformation of the inter-SH2 spacer is not maintained autonomously but is controlled by sequences C-terminal to the SH2 domains, namely, the linker region and/or the kinase domain. Moreover, antibody binding to the same two determinants was also inhibited when ZAP-70 or the SH2 domains bound to the zeta chain or to a 2pY-ITAM. Together, these two observations suggest a model in which intramolecular contacts keep ZAP-70 in a closed configuration with the two SH2 domains near to each other.

Cytoskeletal polarization of T cells is regulated by an immunoreceptor tyrosine-based activation motif-dependent mechanism

Binding of a T cell to an appropriate antigen-presenting cell (APC) induces the rapid reorientation of the T cell cytoskeleton and secretory apparatus towards the cell-cell contact site in a T cell antigen receptor (TCR) and peptide/major histocompatibility complex-dependent process. Such T cell polarization directs the delivery of cytokines and cytotoxic mediators towards the APC and contributes to the highly selective and specific action of effector T cells. To study the signaling pathways that regulate cytoskeletal rearrangements in T lymphocytes, we set up a conjugate formation assay using Jurkat T cells as effectors and cell-sized latex beads coated with various antibodies as artificial APCs. Here, we report that beads coated with antibodies specific for the TCR-CD3 complex were sufficient to induce T cell polarization towards the bead attachment site, as judged by reorientation of the microtubule-organizing center (MTOC) and localized actin polymerization. Thus, these cytoskeletal changes did not depend on activation of additional coreceptors. Moreover, single subunits of the TCR complex, namely TCR-zeta and CD3epsilon, were equally effective in inducing cytoskeletal polarization. However, mutagenesis of the immunoreceptor tyrosine-based activation motifs (ITAMs), present three times in TCR-zeta and once in CD3epsilon, revealed that the induction of cytoskeletal rearrangements required the presence of at least one intact ITAM. In agreement with this result, lack of functional Lck, the protein tyrosine kinase responsible for ITAM phosphorylation, abolished both MTOC reorientation and polarized actin polymerization. Both inhibitor and transient overexpression studies demonstrated that MTOC reorientation could occur in the absence of Ras activation. Our results suggest that APC-induced T cell polarization is a TCR-mediated event that is coupled to the TCR by the same signaling motif as TCR-induced gene activation, but diverges in its distal signaling requirements.

ZAP-70 protein tyrosine kinase is constitutively targeted to the T cell cortex independently of its SH2 domains

ZAP-70 is a nonreceptor protein tyrosine kinase that is essential for signaling via the T cell antigen receptor (TCR). ZAP-70 becomes phosphorylated and activated by LCK protein tyrosine kinase after interaction of its two NH2-terminal SH2 domains with tyrosine-phosphorylated subunits of the activated TCR. In this study, the localization of ZAP-70 was investigated by immunofluorescence and confocal microscopy. ZAP-70 was found to be localized to the cell cortex in a diffuse band under the plasma membrane in unstimulated T cells, and this localization was not detectably altered by TCR stimulation. Analysis of mutants indicated that ZAP-70 targeting was independent of its SH2 domains but required its active kinase domain. The specific compartmentalization of ZAP-70 suggests that it may interact with an anchoring protein in the cell cortex via its hinge or kinase domains. It is likely that the maintenance of high concentrations of ZAP-70 at the cell cortex, that only has to move a short distance to interact with phophorylated TCR subunits, facilitates rapid initiation of signaling by the TCR. In addition, as the major increase in tyrosine phosphorylation induced by the TCR also occurs at the cell cortex (Ley, S.C., M. Marsh, C.R. Bebbington, K. Proudfoot, and P. Jordan. 1994. J. Cell. Biol. 125:639-649), ZAP-70 may be localized close to its downstream targets.

Regulation of microtubule dynamics by Ca2+/calmodulin-dependent kinase IV/Gr-dependent phosphorylation of oncoprotein 18

Oncoprotein 18 (Op18; also termed p19, 19K, p18, prosolin, and stathmin) is a regulator of microtubule (MT) dynamics and is phosphorylated by multiple kinase systems on four Ser residues. In addition to cell cycle-regulated phosphorylation, external signals induce phosphorylation of Op18 on Ser-25 by the mitogen-activated protein kinase and on Ser-16 by the Ca2+/calmodulin-dependent kinase IV/Gr (CaMK IV/Gr). Here we show that induced expression of a constitutively active mutant of CaMK IV/Gr results in phosphorylation of Op18 on Ser-16. In parallel, we also observed partial degradation of Op18 and a rapid increase of total cellular MTs. These results suggest a link between CaMK IV/Gr, Op18, and MT dynamics. To explore such a putative link, we optimized a genetic system that allowed conditional coexpression of a series of CaMK IV/Gr and Op18 derivatives. The result shows that CaMK IV/Gr can suppress the MT-regulating activity of Op18 by phosphorylation on Ser-16. In line with these results, by employing a chemical cross-linking protocol, it was shown that phosphorylation of Ser-16 is involved in weakening of the interactions between Op18 and tubulin. Taken together, these data suggest that the mechanism of CaMK IV/Gr-mediated suppression of Op18 activity involves both partial degradation of Op18 and direct modulation of the MT-destabilizing activity of this protein. These results show that Op18 phosphorylation by CaMK IV/Gr may couple alterations of MT dynamics in response to external signals that involve Ca2+.

Tubulin post-translational modifications--enzymes and their mechanisms of action

This review describes the enzymes responsible for the post-translational modifications of tubulin, including detyrosination/tyrosination, acetylation/deacetylation, phosphorylation, polyglutamylation, polyglycylation and the generation of non-tyrosinatable alpha-tubulin. Tubulin tyrosine-ligase, which reattaches tyrosine to detyrosinated tubulin, has been extensively characterized and its gene sequenced. Enzymes such as tubulin-specific carboxypeptidase and alpha-tubulin acetyltransferase, required, respectively, for detyrosination and acetylation of tubulin, have yet to be purified to homogeneity and examined in defined systems. This has produced some conflicting results, especially for the carboxypeptidase. The phosphorylation of tubulin by several different types of kinases has been studied in detail but drawing conclusions is difficult because many of these enzymes modify proteins other than their actual substrates, an especially pertinent consideration for in vitro experiments. Tubulin phosphorylation in cultured neuronal cells has proven to be the best model for evaluation of kinase effects on tubulin/microtubule function. There is little information on the enzymes required for polyglutamylation, polyglycylation, and production of non-tyrosinatable tubulin, but the available data permit interesting speculation of a mechanistic nature. Clearly, to achieve a full appreciation of tubulin post-translational changes the responsible enzymes must be characterized. Knowing when the enzymes are active in cells, if soluble or polymerized tubulin is the preferred substrate and the amino acid residues modified by each enzyme are all important. Moreover, acquisition of purified enzymes will lead to cloning and sequencing of their genes. With this information, one can manipulate cell genomes in order to either modify key enzymes or change their relative amounts, and perhaps reveal the physiological significance of tubulin post-translational modifications.

Structure and function of vav

The proto-oncogene vav is expressed solely in cells of hematopoietic origin regardless of their differentiation lineage. However, recently an homologue of vav, which is widely expressed (vav2) has been identified. Vav is a complicated and interesting molecule that contains a number of structural features found in proteins involved in cell signaling. Vav has a leucine-rich region, a leucine-zipper, a calponin homology domain, an acidic domain, a Dbl-homology domain, a pleckstrin homology domain, a cysteine-rich domain, two Src homology 3 domains, with a proline-rich region in the amino-SH3 domain, and finally an Src homology 2 domain. These domains have been implicated in protein protein interactions and strongly suggest that vav is involved in signaling events. vav is also rapidly and transiently tyrosine phosphorylated through the activation of multiple receptors on hematopoietic cells. Furthermore, vav is tyrosine phosphorylated upon the activation of several cytokines and growths factors. Recently, the generation of nice vav-/- showed that vav has an essential role in proliferation/activation of T and B cells. The purpose of this review is to summarize the current knowledge on vav and to evaluate the roles of vav in cellular functions.

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.