Protein-tyrosine phosphatase activity of CD45 is activated by sequential phosphorylation by two kinases

Regulation of the catalytic activity of PTP1B: Roles for cell adhesion, tyrosine residue 66, and proline residues 309 and 310

The reversible phosphorylation of proteins on tyrosine residues is fundamental to a variety of intracellular signaling pathways and is controlled by the actions of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). While much progress has been made in understanding the regulation of PTKs, there is still relatively little known concerning the regulation of PTPs. Using immune complex phosphatase assays, we demonstrated that the enzymatic activity of the nonreceptor type PTP, PTP1B, is regulated by cell adhesion. Placing primary human foreskin fibroblasts (HFFs) in suspension leads to a distinct increase in PTP1B activity, whereas the readhesion of suspended HFFs onto fibronectin or collagen I inhibited activity. To gain insight into the mechanisms involved, we analyzed recombinant forms of PTP1B mutated at potential regulatory sites. Our results indicated that tyrosine residue 66 is essential for maintaining activity at 37 degrees C. We also found that the C-terminal region of PTP1B and localization to the endoplasmic reticulum are not required for the inhibition of activity by cell adhesion. However, analysis of PA-PTP1B, in which alanines are substituted for prolines 309 and 310, revealed an important role for these residues as the catalytic activity of this mutant did not decrease following readhesion onto collagen I. Since the binding of p130cas and Src to PTP1B is dependent upon these proline residues, we assayed the regulation of PTP1B in mouse embryo fibroblasts deficient in these proteins. We found that neither p130cas nor Src is required for the inhibition of PTP1B activity by adhesion to extracellular matrix proteins. Additionally, pretreatment with cytochalasin D did not prevent the reduction of PTP1B activity when cells adhered to collagen I, indicating that cell spreading is not required for this regulation. The control of the catalytic activity of PTP1B by cell adhesion demonstrated in this study is likely to have important implications for growth factor and insulin signaling.

Subdomain X of the kinase domain of Lck binds CD45 and facilitates dephosphorylation

CD45 is a transmembrane, two-domain protein-tyrosine phosphatase expressed exclusively in nucleated hematopoietic cells. The Src family kinase, Lck, is a major CD45 substrate in T cells and CD45 dephosphorylation of Lck is important for both T cell development and activation. However, how the substrate specificity of phosphatases such as CD45 is achieved is not well understood. Analysis of the interaction between the cytoplasmic domain of CD45 and its substrate, Lck, revealed that the active, membrane-proximal phosphatase domain of CD45 (CD45-D1) bound to the phosphorylated Lck kinase domain, the SH2 domain, and the unique N-terminal region of Lck. The second, inactive phosphatase domain (CD45-D2) bound only to the kinase domain of Lck. CD45-D2 was unable to bind phosphotyrosine, and its interaction with the kinase domain of Lck was independent of tyrosine phosphorylation. The binding of CD45-D2 was localized to subdomain X (SD10) of Lck. CD45-D2 bound similarly to Src family kinases but bound Csk to a lesser extent and did not bind significantly to the less related kinase, Erk1. CD45 dephosphorylated Lck and Src at similar rates but dephosphorylated Csk and Erk1 at lower rates. Replacement of Erk1 SD10 with that of Lck resulted in the binding of CD45-D2 and the conversion of Erk1 to a more efficient CD45 substrate. This demonstrates a role for CD45-D2 in binding substrate and identifies the SD10 region in Lck as a novel site involved in substrate recognition.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

An extracellular ligand increases the specific activity of the receptor-like protein tyrosine phosphatase DEP-1

Cellular growth, differentiation and migration is regulated by protein tyrosine phosphorylation. Receptor-like protein tyrosine phosphatases are thus likely to be key regulators of vital cellular processes. The regulation of these enzymes is in general poorly understood. Ligands have been identified only for a small subset of the receptor-like protein tyrosine phosphatases and in no case has upregulation of the specific activity by extracellular ligands been demonstrated. Prompted by earlier findings of ligands for receptor-like protein tyrosine phosphatases in extracellular matrix we investigated if Matrigel, a preparation of extracellular matrix proteins, contained modulators of the specific activity of the receptor-like protein tyrosine phosphatase DEP-1. Matrigel stimulation of cells increased the specific activity of immunoprecipitated DEP-1. Also, incubation of immunoprecipitated DEP-1 with Matrigel led to an increase in DEP-1 activity, which was blocked by soluble DEP-1 extracellular domain. Finally, immunoprecipitated DeltaECD-DEP-1, a mutant form of DEP-1 lacking most of the extracellular domain, failed to respond to Matrigel stimulation. These experiments identify Matrigel as a source of DEP-1 agonist(s) and provide the first evidence for upregulation of the specific activity of receptor-like protein tyrosine phosphatases by extracellular ligands.

CD45 function is regulated by an acidic 19-amino acid insert in domain II that serves as a binding and phosphoacceptor site for casein kinase 2

In this study experiments were conducted to elucidate the physical/functional relationship between CD45 and casein kinase 2 (CK2). Immunoprecipitation experiments demonstrated that CK2 associates with CD45 and that this interaction is inducible upon Ag receptor cross-linking in B and T cell lines as well as murine thymocytes and splenic B cells. However, yeast two-hybrid analysis failed to demonstrate a physical interaction between the individual CK2 alpha, alpha', or beta subunits and CD45. In contrast, a yeast three-hybrid assay in which either CK2 alpha and beta or alpha' and beta subunits were coexpressed with the cytoplasmic domain of CD45, demonstrated that both CK2 subunits are necessary for the interaction with CD45. Experiments using the yeast three-hybrid assay also revealed that a 19-aa acidic insert in domain II of CD45 mediates the physical interaction between CK2 and CD45. Structure/function experiments in which wild-type or mutant CD45RA and CD45RO isoforms were expressed in CD45-deficient Jurkat cells revealed that the 19-aa insert is important for optimal CD45 function. The ability of both CD45RA and CD45RO to reconstitute CD3-mediated signaling based on measurement of calcium mobilization and mitogen-activated protein kinase activation was significantly decreased by deletion of the 19-aa insert. Mutation of four serine residues within the 19-aa insert to alanine affected CD45 function to a similar extent compared with that of the deletion mutants. These findings support the hypothesis that a physical interaction between the CD45 cytoplasmic domain and CK2 is important for post-translational modification of CD45, which, in turn, regulates its catalytic function.

CD45: new jobs for an old acquaintance

Identified as the first and prototypic transmembrane protein tyrosine phosphatase (PTPase), CD45 has been extensively studied for over two decades and is thought to be important for positively regulating antigen-receptor signaling via the dephosphorylation of Src kinases. However, new evidence indicates that CD45 can function as a Janus kinase PTPase that negatively controls cytokine-receptor signaling. A point mutation in CD45, which appears to affect CD45 dimerization, and a genetic polymorphism that affects alternative CD45 splicing are implicated in autoimmunity in mice and multiple sclerosis in humans. CD45 is expressed in multiple isoforms and the modulation of specific CD45 splice variants with antibodies can prevent transplant rejections. In addition, loss of CD45 can affect microglia activation in a mouse model for Alzheimer's disease. Thus, CD45 is moving rapidly back into the spotlight as a drug target and central regulator involved in differentiation of multiple hematopoietic cell lineages, autoimmunity and antiviral immunity.

Switch in the protein tyrosine phosphatase associated with human CD100 semaphorin at terminal B-cell differentiation stage

Human CD100, the first semaphorin identified in the immune system, is a transmembrane protein involved in T-cell activation. In the present study, we showed that activation of peripheral blood or tonsillar B lymphocytes induced the expression of CD100 in CD38+CD138− cell populations, including in CD148+ subpopulations, thus expressing a memory B-cell–like phenotype. Using an in vitro enzymatic assay, we found that protein tyrosine phosphatase (PTP) activities were immunoprecipitated with CD100 in these cell populations, which were isolated by cell sorting, as well as in most B-cell lines representing various stages of B-cell differentiation. Immunodepletion and Western blotting experiments demonstrated that CD45 was the PTP associated with CD100 in cell lines displaying pre-B, activated B, and pre-plasma cell phenotypes. CD45 also accounted for PTP activity immunoprecipitated with CD100 in CD38+CD138− cells sorted after activation of peripheral blood or tonsillar B lymphocytes. In contrast, no CD100-CD45 association was observed in plasma cell lines corresponding to the terminal B-cell differentiation stage. CD148, the other transmembrane PTP known to be implicated in lymphocyte signaling pathways, was either only partly involved in the CD100-associated PTP activity or not expressed in plasma cell lines, indicating the association of CD100 with another main PTP. Our data show that CD100 is differentially expressed and can functionally associate with distinct PTPs in B cells depending on their activation and maturation state. They also provide evidence for a switch in the CD100-associated PTP at terminal stage of B-cell differentiation.

Modulation of human neutrophil responses to CD32 cross-linking by serine/threonine phosphatase inhibitors: cross-talk between serine/threonine and tyrosine phosphorylation

The interplay between serine/threonine and tyrosine phosphorylation was studied in human neutrophils. The direct effects of calyculin and okadaic acid, potent inhibitors of PP1 and PP2A serine/threonine phosphatases, on the patterns of neutrophil phosphorylation, and their effects on the responses of neutrophils to CD32 cross-linking were monitored. After a 2-min incubation with 10-6 M calyculin, a transient tyrosine phosphorylation of a subset of proteins, among which Cbl and Syk, was observed. After a longer incubation (>5 min) with calyculin, concomitant with an accumulation of serine and threonine phosphorylation, neutrophil responses to CD32 cross-linking were selectively altered. Tyrosine phosphorylation of Cbl in response to CD32 cross-linking was inhibited by calyculin, and this inhibition was linked with a slower electrophoretic mobility of Cbl as a consequence of its phosphorylation on serine/threonine residues. However, tyrosine phosphorylation of Syk and of the receptor itself were not affected. Furthermore, the mobilization of intracellular calcium stimulated by CD32 cross-linking was totally abrogated by calyculin. Finally, the stimulation of superoxide production observed in response to CD32 cross-linking was enhanced in calyculin-treated cells. These results suggest that serine/threonine phosphorylation events regulate the signaling pathways activated by CD32 cross-linking in neutrophils and identify a novel mechanism of modulation of the functional responsiveness of human neutrophils to CD32 cross-linking.

Phosphorylation of CD45 by casein kinase 2. Modulation of activity and mutational analysis

CD45 is a receptor-type protein-tyrosine phosphatase (PTP) that is required for antigen-specific stimulation and proliferation in lymphocytes. This study was designed to determine the nature of specific kinases in lymphocytes that phosphorylate CD45 and to determine the effect of phosphorylation on CD45 PTP activity. A major cytoplasmic lymphocyte kinase that phosphorylated CD45 was identified as casein kinase 2 (CK2) by use of an in-gel kinase assay in combination with immunoprecipitation, immunodepletion, and specific inhibition. Mutational analysis of CK2 consensus sites showed that the target for CK2 was in an acidic insert of 19 amino acids in the D2 domain, and Ser to Ala mutations at amino acids 965, 968, 969, and 973 abrogated CK2 phosphorylation of CD45. CK2 phosphorylation increased CD45 activity 3-fold toward phosphorylated myelin basic protein, and this increase was reversible by PP2A treatment. Mutation of Ser to Glu at the CK2 sites had the same effect as phosphorylation and also tripled the Vmax of CD45. CD45 isolated in vivo was highly phosphorylated and could not be phosphorylated by CK2 without prior dephosphorylation with phosphatase PP2A. We conclude that CK2 is a major lymphocyte kinase that is responsible for in vivo phosphorylation of CD45, and phosphorylation at specific CK2 sites regulates CD45 PTP activity.

Human CD5 Signaling and Constitutive Phosphorylation of C-Terminal Serine Residues by Casein Kinase II

CD5 is a lymphocyte surface glycoprotein with a long cytoplasmic domain suitable for phosphorylation and signal transduction, which is involved in the modulation of Ag-specific receptor-mediated activation and differentiation signals. In this study, we use Jurkat T cell transfectants of CD5 cytoplasmic tail mutants to reveal phosphorylation sites relevant to signal transduction. Our results show that casein kinase II (CKII) is responsible for the constitutive phosphorylation of CD5 molecules at a cluster of three serine residues located at the extreme C terminus (S458, S459, and S461). Furthermore, the yeast two-hybrid system demonstrates the specific association between the C-terminal regions of the CD5 cytoplasmic tail and the regulatory β subunit of CKII. We demonstrate that CKII associates with and phosphorylates the C-terminal region of CD5, a conserved domain known to be relevant for the generation of second lipid messengers, and thereby enables at least one component of its signaling funcion.

Characterization of recombinant CD45 cytoplasmic domain proteins. Evidence for intramolecular and intermolecular interactions

CD45 is a transmembrane two-domain tyrosine phosphatase required for efficient signal transduction initiated by lymphocyte antigen receptors. As with most transmembrane two-domain phosphatases, the role of the second phosphatase domain is unclear. In this study, recombinant CD45 cytoplasmic domain proteins purified from bacteria were used to evaluate the function of the individual phosphatase domains. A recombinant protein expressing the membrane-proximal region, first phosphatase domain, and spacer region of CD45 (rD1) was catalytically active and found to exist primarily as a dimer. In contrast to this, a recombinant protein expressing the spacer region, the second phosphatase domain and the carboxy tail of CD45 (rD2) existed as a monomer and had no catalytic activity against any of the substrates tested. Comparison of rD1 with the recombinant protein expressing the entire cytoplasmic domain of CD45 (rD1/D2) indicated that rD1/D2 was 2-3-fold more catalytically active, was more thermostable, and existed primarily as a monomer. Limited trypsin digestion of rD1/D2 provided evidence for a noncovalent association between an N-terminal 27-kDa fragment and a C-terminal 53-kDa fragment, suggesting an intramolecular interaction. Furthermore, rD1 was found to specifically associate with rD2 in an in vitro binding assay. Taken together, these data provide evidence for an intramolecular interaction occurring in the cytoplasmic domain of CD45. In the absence of the C-terminal region containing the second phosphatase domain, intermolecular interactions occur, resulting in dimer formation.

Protein kinase CK2 ("casein kinase-2") and its implication in cell division and proliferation

Protein kinase CK2 (also termed casein kinase-2 or -II) is a ubiquitous Ser/Thr-specific protein kinase required for viability and for cell cycle progression. CK2 is especially elevated in proliferating tissues, either normal or transformed, and the expression of its catalytic subunit in transgenic mice is causative of lymphomas. CK2 is highly pleiotropic: more than 160 proteins phosphorylated by it at sites specified by multiple acidic residues are known. Despite its heterotetrameric structure generally composed by two catalytic (alpha and/or alpha') and two non catalytic beta-subunits, the regulation of CK2 is still enigmatic. A number of functional features of the beta-subunit which could cooperate to the modulation of CK2 targeting/activity will be discussed.

The human semaphorin-like leukocyte cell surface molecule CD100 associates with a serine kinase activity

CD100 is a 150-kDa homodimeric glycoprotein broadly expressed on the surface of human hematopoietic cells. CD100 has been recently identified as the first lymphoid gene that belongs to the semaphorin gene family. Semaphorins function as chemorepellent molecules in the nervous system, but the function of CD100 remains poorly understood. In lymphoid cells, it has been suggested to play a role in homotypic cell adhesion and in T cell activation. We demonstrate that in T cells and natural killer cells a serine kinase activity is immunoprecipitated with CD100. Distinct epitopes of CD100 have been defined with specific monoclonal antibodies, mediating opposite effects at the functional level, especially in T cells. The kinase activity is retained only with an antibody against a particular epitope of CD100. Additionally, a fusion protein containing the cytoplasmic domain of the molecule retains the kinase activity in cellular lysates, and CD100 itself is presumably a favorite substrate of the kinase. These findings suggest that a serine kinase pathway may participate in the different functional effects triggered through the distinct epitopes of CD100 and is likely involved in the biological effects of this semaphorin-like leukocyte cell surface molecule.

The MBP fusion protein restores the activity of the first phosphatase domain of CD45

CD45 is a receptor-like protein tyrosine phosphatase critically involved in the regulation of initial effector functions in B- and T-cells. The protein comprises two phosphatase (PTP) domains in its cytoplasmic region. However, whether each PTP domain has enzyme activity by itself or whether both domains are required to build up a functional enzyme is unclear. We have studied different constructions of human CD45 comprising the two PTP domains, both separately and as a single protein, fused to maltose-binding protein (MBP). In apparent contrast with previous studies, we show that the first PTP domain of CD45 (when fused to MBP) may be a viable phosphatase in the absence of the second domain. Phosphatase activity resides in the monomeric form of the protein and is lost after proteolytic cleavage of the fusion partner, indicating that MBP specifically activates the first PTP domain. Furthermore, changes in the optimal pH for activity with respect to wild-type CD45 suggest that protein-protein interactions involving residues in the neighbourhood of the catalytic site mediate enzyme activation.

The receptor-like protein-tyrosine phosphatase DEP-1 is constitutively associated with a 64-kDa protein serine/threonine kinase

Protein-tyrosine phosphatases (PTPs) are involved in the regulation of diverse cellular processes and may function as positive effectors as well as negative regulators of intracellular signaling. Recent data demonstrate that malignant transformation of cells is frequently associated with changes in PTP expression or activity. Our analysis of PTP expression in mammary carcinoma cell lines resulted in the molecular cloning of a receptor-like PTP, also known as DEP-1. DEP-1 was found to be expressed at varying levels in mammary carcinoma cell lines and A431 cells. In all tumor cell lines analyzed, DEP-1 was constitutively phosphorylated on tyrosine residues. Phosphorylation of DEP-1 increased significantly after treatment of cells with the PTP inhibitor pervanadate. In A431 cells, tyrosine phosphorylation of DEP-1 was also observed after stimulation with epidermal growth factor, however, only after prolonged exposure of the cells to the ligand, suggesting an indirect mechanism of phosphorylation. In addition, DEP-1 coprecipitated with several tyrosine-phosphorylated proteins from pervanadate-treated cells. In vitro binding experiments using a glutathione S-transferase fusion protein containing the catalytically inactive PTP domain of DEP-1 (Gst-DEP-1-C/S) identify these proteins as potential substrates of DEP-1. In addition, we found a 64-kDa serine/threonine kinase to be constitutively associated with DEP-1 in all tumor cell lines tested. The 64-kDa kinase forms a stable complex with DEP-1 and phosphorylates DEP-1 and DEP-1-interacting proteins in vitro. These data suggest a possible mechanism of DEP-1 regulation in tumor cell lines involving serine/threonine and/or tyrosine phosphorylation.

Phosphatidic acid-mediated regulation of neutrophil plasma membrane CD45-phosphotyrosine phosphatase

CD45-phosphotyrosine phosphatase (PTPase) constitutes the major portion of thr PTPase activity within plasma membranes of neutrophilic leukocytes, where it regulates signals leading to functional activation. We have previously demonstrated that the catalytic component of neutrophil plasma membrane CD45-PTPASE is regulated by a cytosolic inactivator which itself is attenuated upon cellular stimulation, allowing enzyme translocated from granule stores to express full activity. The present study investigated mechanisms of cytosolic inactivator attenuation. Preincubation of plasma membranes of stimulated neutrophils with cytosol from resting cells resulted in a rapid loss of membrane-associated PTPase activity. Phosphatidic acid had no direct effect on plasma membrane PTPase activity but blunted in a dose dependent manner the effects of the PTPase inactivator. Inactivator attenuation was not observed with equivalent concentrations of either diacylglycerol or lysophosphatidic acid. Optimal attenuation of inactivator activity was obtained with long chain, soluble ligands, such as dicapryl phosphatidic acid. Inhibitors of neutrophil plasma membrane ecto-phosphatidic acid phosphohydrolase did not block inactivator attenuation, suggesting that phosphatidic acid and not one of its metabolites was the entity responsible. In conclusion, neutrophil plasma membrane PTPase is dynamically regulated by a cytosolic inactivator, the inhibition of which may potentiate the effects of PTPase translocated during cellular stimulation. Phosphatidic acid generated as a consequence of cellular stimulation may mediate this inhibition and thereby regulate the effects of tyrosine kinases activated during the initial phases of cell stimulation.

Identification of in vivo phosphorylation sites of CD45 protein-tyrosine phosphatase in 70Z/3.12 cells

Phosphorylation of CD45, a transmembrane protein-tyrosine phosphatase (PTPase), has been proposed to mediate docking of signaling proteins and to modulate PTPase activity. To study the role of phosphorylation in CD45, in vivo phosphorylation sites of CD45 from 70Z/3.12 cells were identified using 32P labeling, trypsin digestion, two-dimensional peptide mapping, high performance liquid chromatography, phosphoamino acid analysis, matrix-assisted laser desorption/ionization mass spectrometry, and specific enzymatic degradation. Eight phosphopeptides, a through h, were isolated and four phosphorylation sites were identified. All four phosphorylation sites were in the membrane-distal PTPase domain (D2) and the C-terminal tail and none were in the membrane-proximal PTPase domain (D1). One site, Ser(P)939 peptide h, was in the D2 domain and, by comparison to the three-dimensional structure of PTP1B, is predicted to lie at the apex of the substrate binding loop. Ser939 was the only in vitro phosphorylation site for protein kinase C among the phosphorylation sites identified. Four of the C-terminal peptides identified (d, e, f, and g) spanned the same sequence and were derived from the same phosphorylation site in the C-terminal tail, Ser1204. Peptide a was derived from the intact C terminus and comprised a mixture of monophosphorylated peptides containing either Ser(P)1248 or Thr(P)1246. Knowledge of the precise phosphorylation sites of CD45 will lead to the design of experiments to define the role of phosphorylation in PTPase activity and in signaling.

Differential signaling by lymphocyte antigen receptors

Studies performed during the past several years make plain that ligand occupancy of antigen receptors need not necessarily provoke identical responses in all instances. For example, ligation of antigen receptors may stimulate a proliferative response, induce a state of unresponsiveness to subsequent stimulation (anergy), or induce apoptosis. How does a single type of transmembrane receptor induce these very heterogeneous cellular responses? In the following pages, we outline evidence supporting the view that the nature of the ligand/receptor interaction directs the physical recruitment of signaling pathways differentially inside the lymphocyte and hence defines the nature of the subsequent immune response. We begin by providing a functional categorization of antigen receptor components, considering the ways in which these components interact with the known set of signal transduction pathways, and then review the evidence suggesting that differential signaling through the TCR is achieved by qualitative differences in the effector pathways recruited by TCR, perhaps reflecting the time required to bring complicated signal transduction elements into proximity within the cell. The time-constant of the interaction between antigen and receptor in this way determines, at least in part, the nature of the resulting response. Finally, although our review focuses substantially on T cell receptor signaling, we have included a less detailed description of B cell receptor signaling as well, simply to emphasize the parallels that exist in these two closely related systems.

Association of the protein tyrosine phosphatase PTP1C with the protein tyrosine kinase c-Src in human platelets

Protein tyrosine phosphatase 1C (PTP1C), highly expressed in hematopoietic cells, is a soluble protein tyrosine phosphatase containing two Src homology 2 (SH2) domains at the N-terminus and two putative sites of tyrosine phosphorylation at the C-terminus. This paper reports that PTP1C and c-Src could be coimmunoprecipitated during thrombin-induced platelet activation. Moreover, association between the two signalling proteins occurred only after PTP1C had been tyrosine phosphorylated. In in vitro experiments, PTP1C bound to the SH2 domain of c-Src, suggesting that association between tyrosine phosphorylated PTP1C and c-Src was mediated by the SH2 domain of c-Src. Finally, in resting platelets, PTP1C was mainly found in the Nonidet P-40 soluble fraction whereas following thrombin-induced activation, around 17% of PTP1C was associated with the insoluble fraction.

Protein tyrosine phosphatases take off

Protein tyrosine phosphatases (PTPs) are a family of signal transduction enzymes that dephosphorylate phosphotyrosine containing proteins. Structural and kinetic studies provide a molecular understanding of how these enzymes regulate a wide range of intracellular processes.

The receptor-like protein-tyrosine phosphatase, RPTP alpha, is phosphorylated by protein kinase C on two serines close to the inner face of the plasma membrane

To determine whether the receptor-like protein-tyrosine phosphatase, RPTP alpha, which is widely expressed in both the developing and adult mouse, is regulated by phosphorylation, we raised antiserum against a C-terminal peptide. This antiserum precipitated a 140-kDa protein from metabolically 35S-labeled NIH3T3 cells. Using this antiserum, we showed that endogenous RPTP alpha is constitutively phosphorylated in NIH3T3 cells, predominantly on two serines, which we identified as Ser-180 and Ser-204, lying in the juxtamembrane domain. 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulation of quiescent NIH3T3 cells rapidly increased phosphorylation of Ser-180 and Ser-204. Purified protein kinase C (PKC) phosphorylated bacterially expressed RPTP alpha at Ser-180 and Ser-204. When wild type and S180A/S204A double mutant RPTP alpha S were transiently expressed in 293 human embryonic kidney cells, TPA stimulated phosphorylation of wild type but not of double mutant RPTP alpha. PKC down-regulation following prolonged exposure to TPA diminished TPA-stimulated RPTP alpha phosphorylation. Taken together, these results indicate that RPTP alpha is a direct substrate for (PKC). Examination of 293 cells expressing exogenous RPTP alpha using immunofluorescence confocal microscopy showed that RPTP alpha exists predominantly in two subcellular compartments: in dense intracellular granules or dispersed within the plasma membrane. TPA treatment caused redistribution of some intracellular RPTP alpha to the cell surface, but this did not require direct phosphorylation of RPTP alpha at Ser-180/Ser-204. Our results suggest that activation of PKC by cytokines modulates RPTP alpha function in several different ways.

The coordinated action of protein tyrosine phosphatases and kinases in cell signaling

The levels of tyrosine phosphorylation required for cell growth and differentiation are achieved through the coordinated action of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Depending upon the cellular context, these two types of enzymes may either antagonize or cooperate with each other during the signal transmission process. An imbalance between these enzymes may impair normal cell growth, leading to cellular transformation. Both PTKs and PTPs have evolved to a level of structural diversity that allows them to regulate many cellular processes. This review will focus on several specific examples that highlight the interplay between PTPs and PTKs in cell signaling.