Interconversion of the kinetic identities of the tandem catalytic domains of receptor-like protein-tyrosine phosphatase PTPalpha by two point mutations is synergistic and substrate-dependent

Protein Tyrosine Phosphatases: A new paradigm in an old signaling system?

Protein Tyrosine Phosphatases reverse cellular signals initiated by growth factors receptors and other tyrosine kinases by dephosphorylating phosphotyrosine on target proteins. The activity of these enzymes is crucial for maintaining cell homeostasis, yet these enzymes have been often dismissed as humble house-keeping proteins. Understandably, mutations and changes in expression patterns of Protein Tyrosine Phosphatases are implicated in tumorigenesis and various carcinomas. The conserved nature of their catalytic domains makes drug discovery a challenging pursuit. In this review, we focus on describing the various classes of Protein Tyrosine Phosphatases and their catalytic domains. We also summarize their role in cancer and neurodegenerative diseases using specific members as the model system. Finally, we explain the dichotomy in the biological role of catalytically active vs the pseudoenzyme forms of Protein Tyrosine Phosphatases in the context of their membrane bound receptor forms. This chapter aims to provide a current understanding of these proteins, in the background of their foundational past research.

Vascular Endothelial Protein Tyrosine Phosphatase Regulates Endothelial Function

Vascular endothelial protein tyrosine phosphatase (VE-PTP) is a receptor-type PTP (RPTP), predominantly expressed in vascular endothelial cells. It regulates embryonic and tumor angiogenesis and controls vascular permeability and homeostasis in inflammation. Major substrates are the tyrosine kinase receptor Tie-2 and the adhesion molecule VE-cadherin. This review describes how VE-PTP controls vascular functions by its various substrates and the therapeutic potential of VE-PTP in various pathophysiological settings.

Receptor-type protein tyrosine phosphatase alpha (PTPα) mediates MMP14 localization and facilitates triple-negative breast cancer cell invasion

The ability of cancer cells to invade surrounding tissues requires degradation of the extracellular matrix (ECM). Invasive structures, such as invadopodia, form on the plasma membranes of cancer cells and secrete ECM-degrading proteases that play crucial roles in cancer cell invasion. We have previously shown that the protein tyrosine phosphatase alpha (PTPα) regulates focal adhesion formation and migration of normal cells. Here we report a novel role for PTPα in promoting triple-negative breast cancer cell invasion in vitro and in vivo. We show that PTPα knockdown reduces ECM degradation and cellular invasion of MDA-MB-231 cells through Matrigel. PTPα is not a component of TKS5-positive structures resembling invadopodia; rather, PTPα localizes with endosomal structures positive for MMP14, caveolin-1, and early endosome antigen 1. Furthermore, PTPα regulates MMP14 localization to plasma membrane protrusions, suggesting a role for PTPα in intracellular trafficking of MMP14. Importantly, we show that orthotopic MDA-MB-231 tumors depleted in PTPα exhibit reduced invasion into the surrounding mammary fat pad. These findings suggest a novel role for PTPα in regulating the invasion of triple-negative breast cancer cells.

Identifying pseudoenzymes using functional annotation: pitfalls of common practice

Pseudoenzymes are proteins that are evolutionary related to enzymes but lack relevant catalytic activity. They are usually evolved from enzymatic ancestors that have lost their catalytic activities. The loss of catalytic function is one extreme amongst the other evolutionary changes that can occur to enzymes, like the changing of substrate specificity or the reaction catalysed. However, the loss of catalytic function events remain poorly characterised, except for some notable examples, like the pseudokinases. In this review, we aim to analyse current knowledge related to pseudoenzymes across a large number of enzymes families. This aims to be a review of the data available in biological databases, rather than a more traditional literature review. In particular, we use UniProtKB as the source for functional annotation and M-CSA (Mechanism and Catalytic Site Atlas) for information on the catalytic residues of enzymes. We show that explicit annotation of lack of activity is not exhaustive in UniProtKB and that a protocol using lack of catalytic annotation as an indication for lack of function can be an adequate alternative, after some corrections. After identifying pseudoenzymes related to enzymes in M-CSA, we were able to comment on their prevalence across enzyme families, and on the correlation between lack of catalytic function and the mutation of catalytic residues. These analyses challenge two common ideas in the emerging literature: that pseudoenzymes are ubiquitous across enzyme families and that mutations in the catalytic residues of enzyme homologues are always a good indication of lack of activity.

High-resolution crystal structures of the D1 and D2 domains of protein tyrosine phosphatase epsilon for structure-based drug design

Here, new crystal structures are presented of the isolated membrane-proximal D1 and distal D2 domains of protein tyrosine phosphatase epsilon (PTPℇ), a protein tyrosine phosphatase that has been shown to play a positive role in the survival of human breast cancer cells. A triple mutant of the PTPℇ D2 domain (A455N/V457Y/E597D) was also constructed to reconstitute the residues of the PTPℇ D1 catalytic domain that are important for phosphatase activity, resulting in only a slight increase in the phosphatase activity compared with the native D2 protein. The structures reported here are of sufficient resolution for structure-based drug design, and a microarray-based assay for high-throughput screening to identify small-molecule inhibitors of the PTPℇ D1 domain is also described.

Regulation of RPTPα-c-Src signalling pathway by miR-218

Receptor protein tyrosine phosphatase alpha (RPTPα), an activator of Src family kinases, is found significantly overexpressed in human cancer tissues. However, little is known about the regulation of RPTPα expression. miRNAs target multiple genes and play important roles in many cancer processes. Here, we identified a miRNA, miR-218 that binds directly to the 3'-UTR of RPTPα. Ectopic overexpression of miR-218 decreased RPTPα protein leading to decreased dephosphorylation of c-Src and decreased tumour growth in vitro and in vivo. A feedback loop between c-Src and miR-218 was revealed where c-Src inhibits transcription of SLIT2, which intronically hosts miR-218. These results show a novel regulatory pathway for RPTPα-c-Src signalling.

Unix interfaces, Kleisli, bucandin structure, etc. -- the heroic beginning of bioinformatics in Singapore

Remarkably, Singapore as one of today's hotspots for bioinformatics and computational biology research appeared de novo out of pioneering efforts of engaged local individuals in the early 90-s that, supported with increasing public funds from 1996 on, morphed into the present vibrant research community. This article brings to mind the pioneers, their first successes and early institutional developments.

Bi-domain protein tyrosine phosphatases reveal an evolutionary adaptation to optimize signal transduction

SIGNIFICANCE

The bi-domain protein tyrosine phosphatases (PTPs) exemplify functional evolution in signaling proteins for optimal spatiotemporal signal transduction. Bi-domain PTPs are products of gene duplication. The catalytic activity, however, is often localized to one PTP domain. The inactive PTP domain adopts multiple functional roles. These include modulation of catalytic activity, substrate specificity, and stability of the bi-domain enzyme. In some cases, the inactive PTP domain is a receptor for redox stimuli. Since multiple bi-domain PTPs are concurrently active in related cellular pathways, a stringent regulatory mechanism and selective cross-talk is essential to ensure fidelity in signal transduction.

RECENT ADVANCES

The inactive PTP domain is an activator for the catalytic PTP domain in some cases, whereas it reduces catalytic activity in other bi-domain PTPs. The relative orientation of the two domains provides a conformational rationale for this regulatory mechanism. Recent structural and biochemical data reveal that these PTP domains participate in substrate recruitment. The inactive PTP domain has also been demonstrated to undergo substantial conformational rearrangement and oligomerization under oxidative stress.

CRITICAL ISSUES AND FUTURE DIRECTIONS

The role of the inactive PTP domain in coupling environmental stimuli with catalytic activity needs to be further examined. Another aspect that merits attention is the role of this domain in substrate recruitment. These aspects have been poorly characterized in vivo. These lacunae currently restrict our understanding of neo-functionalization of the inactive PTP domain in the bi-domain enzyme. It appears likely that more data from these research themes could form the basis for understanding the fidelity in intracellular signal transduction.

Day of the dead: pseudokinases and pseudophosphatases in physiology and disease

Pseudophosphatases and pseudokinases are increasingly viewed as integral elements of signaling pathways, and there is mounting evidence that they have frequently retained the ability to interact with cellular 'substrates', and can exert important roles in different diseases. However, these pseudoenzymes have traditionally received scant attention compared to classical kinases and phosphatases. In this review we explore new findings in the emerging pseudokinase and pseudophosphatase fields, and discuss their different modes of action which include exciting new roles as scaffolds, anchors, spatial modulators, traps, and ligand-driven regulators of canonical kinases and phosphatases. Thus, it is now apparent that pseudokinases and pseudophosphatases both support and drive a panoply of signaling networks. Finally, we highlight recent evidence on their involvement in human pathologies, marking them as potential novel drug targets.

Receptor type protein tyrosine phosphatases (RPTPs) - roles in signal transduction and human disease

Protein tyrosine phosphorylation is a fundamental regulatory mechanism controlling cell proliferation, differentiation, communication, and adhesion. Disruption of this key regulatory mechanism contributes to a variety of human diseases including cancer, diabetes, and auto-immune diseases. Net protein tyrosine phosphorylation is determined by the dynamic balance of the activity of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Mammals express many distinct PTKs and PTPs. Both of these families can be sub-divided into non-receptor and receptor subtypes. Receptor protein tyrosine kinases (RPTKs) comprise a large family of cell surface proteins that initiate intracellular tyrosine phosphorylation-dependent signal transduction in response to binding of extracellular ligands, such as growth factors and cytokines. Receptor-type protein tyrosine phosphatases (RPTPs) are enzymatic and functional counterparts of RPTKs. RPTPs are a family of integral cell surface proteins that possess intracellular PTP activity, and extracellular domains that have sequence homology to cell adhesion molecules. In comparison to extensively studied RPTKs, much less is known about RPTPs, especially regarding their substrate specificities, regulatory mechanisms, biological functions, and their roles in human diseases. Based on the structure of their extracellular domains, the RPTP family can be grouped into eight sub-families. This article will review one representative member from each RPTP sub-family.

Catalytically active membrane-distal phosphatase domain of receptor protein-tyrosine phosphatase alpha is required for Src activation

Receptor protein-tyrosine phosphatase alpha (RPTPalpha) is a transmembrane protein with tandem cytoplasmic phosphatase domains. Most of the catalytic activity is contained by the membrane-proximal catalytic domain (D1). We found a spontaneous Arg554 to His mutation in the pTyr recognition loop of the membrane-distal phosphatase domain (D2) of a human patient. This mutation was not linked to the disease. Here, we report that the R554H mutation abolished RPTPalpha-D2 catalytic activity. The R554H mutation impaired Src binding to RPTPalpha. RPTPalpha, with a catalytic site cysteine to serine mutation in D2, also displayed diminished binding to Src. Concomitant with decreased Src binding of the R554H and C723S mutants compared with wild-type RPTPalpha, enhanced phosphorylation of the inhibitory Src Tyr527 site was observed, as well as reduced Src activation. To confirm that catalytic activity of RPTPalpha-D2 was required for these effects, we analyzed a third mutant, RPTPalpha-R729K, which had an inactive D2. Again, Src binding was reduced and Tyr527 phosphorylation was enhanced. Our results suggest that a catalytically active D2 is required for RPTPalpha to bind and dephosphorylate its well-characterized substrate, Src.

Differential function of PTPalpha and PTPalpha Y789F in T cells and regulation of PTPalpha phosphorylation at Tyr-789 by CD45

CD45 is a major membrane protein tyrosine phosphatase (PTP) expressed in T cells where it regulates the activity of Lck, a Src family kinase important for T cell receptor-mediated activation. PTPalpha is a more widely expressed transmembrane PTP that has been shown to regulate the Src family kinases, Src and Fyn, and is also present in T cells. Here, PTPalpha was phosphorylated at Tyr-789 in CD45(-) T cells but not in CD45(+) T cells suggesting that CD45 could regulate the phosphorylation of PTPalpha at this site. Furthermore, CD45 could directly dephosphorylate PTPalpha in vitro. Expression of PTPalpha and PTPalpha-Y789F in T cells revealed that the mutant had a reduced ability to decrease Fyn and Cbp phosphorylation, to regulate the kinase activity of Fyn, and to restore T cell receptor-induced signaling events when compared with PTPalpha. Conversely, this mutant had an increased ability to prevent Pyk2 phosphorylation and CD44-mediated cell spreading when compared with PTPalpha. These data demonstrate distinct activities of PTPalpha and PTPalpha-Y789F in T cells and identify CD45 as a regulator of PTPalpha phosphorylation at tyrosine 789 in T cells.

Protein tyrosine phosphatases: from genes, to function, to disease

The protein tyrosine phosphatase (PTP) superfamily of enzymes functions in a coordinated manner with protein tyrosine kinases to control signalling pathways that underlie a broad spectrum of fundamental physiological processes. In this review, I describe recent breakthroughs in our understanding of the role of the PTPs in the regulation of signal transduction and the aetiology of human disease.

Integrin-induced Tyrosine Phosphorylation of Protein-tyrosine Phosphatase-α Is Required for Cytoskeletal Reorganization and Cell Migration

Protein-tyrosine phosphatase-alpha (PTPalpha) activates Src family kinases (SFKs) to promote the integrin-stimulated early autophosphorylation of focal adhesion kinase (FAK). We report here that integrin stimulation induces tyrosine phosphorylation of PTPalpha. PTPalpha was dephosphorylated upon fibroblast detachment from the substratum and rephosphorylated when cells were plated on the integrin ligand fibronectin. alpha PTP phosphorylation occurred at Tyr789 and required SFKs (Src or Fyn/Yes), FAK, and an intact cytoskeleton. It also required active PTPalpha or constitutively active Src. These observations indicate that PTPalpha activates SFKs and that the subsequently activated SFK.FAK tyrosine kinase complex in turn phosphorylates PTPalpha. Reintroduction of wild-type PTPalpha or unphosphorylatable PTPalpha(Y789F) (but not inactive PTPalpha) into PTPalpha-null fibroblasts restored defective integrin-induced SFK activation, FAK phosphorylation, and paxillin phosphorylation. PTPalpha(Y789F) and inactive PTPalpha could not rescue delayed actin stress fiber assembly and focal adhesion formation or defective cell migration. This study distinguishes two roles of PTPalpha in integrin signaling: an early role as an activator of SFKs and FAK with no requirement for PTPalpha phosphorylation and a later downstream role in cytoskeleton-associated events for which PTPalpha phosphorylation at Tyr789 is essential.

Protein Tyrosine Phosphatase α Regulates Fyn Activity and Cbp/PAG Phosphorylation in Thymocyte Lipid Rafts

A role for the receptor protein tyrosine phosphatase alpha (PTPalpha) in immune cell function and regulation of Src family kinases was investigated using thymocytes from PTPalpha-deficient mice. PTPalpha-null thymocytes develop normally, but unstimulated PTPalpha-/- cells exhibit increased tyrosine phosphorylation of specific proteins, increased Fyn activity, and hyperphosphorylation of Cbp/PAG that promotes its association with C-terminal Src kinase. Elevated Fyn activity in the absence of PTPalpha is due to enhanced phosphorylation of Fyn tyrosines 528 and 417. Some PTPalpha is localized in lipid rafts of thymocytes, and raft-associated Fyn is specifically activated in PTPalpha-/- cells. PTPalpha is not a Cbp/PAG phosphatase, because it is not required for Cbp/PAG dephosphorylation in unstimulated or anti-CD3-stimulated thymocytes. Together, our results indicate that PTPalpha, likely located in lipid rafts, regulates the activity of raft Fyn. In the absence of PTPalpha this population of Fyn is activated and phosphorylates Cbp/PAG to enhance association with C-terminal Src kinase. Although TCR-mediated tyrosine phosphorylation was apparently unaffected by the absence of PTPalpha, the long-term proliferative response of PTPalpha-/- thymocytes was reduced. These findings indicate that PTPalpha is a component of the complex Src family tyrosine kinase regulatory network in thymocytes and is required to suppress Fyn activity in unstimulated cells in a manner that is not compensated for by the major T cell PTP and SFK regulator, CD45.

Differential Oxidation of Protein-tyrosine Phosphatases

Oxidation is emerging as an important regulatory mechanism of protein-tyrosine phosphatases (PTPs). Here we report that PTPs are differentially oxidized, and we provide evidence for the underlying mechanism. The membrane-proximal RPTPalpha-D1 was catalytically active but not readily oxidized as assessed by immunoprobing with an antibody that recognized oxidized catalytic site cysteines in PTPs (oxPTPs). In contrast, the membrane-distal RPTPalpha-D2, a poor PTP, was readily oxidized. Oxidized catalytic site cysteines in PTP immunoprobing and mass spectrometry demonstrated that mutation of two residues in the Tyr(P) loop and the WPD loop that reverse catalytic activity of RPTPalpha-D1 and RPTPalpha-D2 also reversed oxidizability, suggesting that oxidizability and catalytic activity are coupled. However, catalytically active PTP1B and LAR-D1 were readily oxidized. Oxidizability was strongly dependent on pH, indicating that the microenvironment of the catalytic cysteine has an important role. Crystal structures of PTP domains demonstrated that the orientation of the absolutely conserved PTP loop arginine correlates with oxidizability of PTPs, and consistently, RPTPmu-D1, with a similar conformation as RPTPalpha-D1, was not readily oxidized. In conclusion, PTPs are differentially oxidized at physiological pH and H(2)O(2) concentrations, and the PTP loop arginine is an important determinant for susceptibility to oxidation.

Structural basis for the function and regulation of the receptor protein tyrosine phosphatase CD45

CD45 is the prototypic member of transmembrane receptor-like protein tyrosine phosphatases (RPTPs) and has essential roles in immune functions. The cytoplasmic region of CD45, like many other RPTPs, contains two homologous protein tyrosine phosphatase domains, active domain 1 (D1) and catalytically impaired domain 2 (D2). Here, we report crystal structure of the cytoplasmic D1D2 segment of human CD45 in native and phosphotyrosyl peptide-bound forms. The tertiary structures of D1 and D2 are very similar, but doubly phosphorylated CD3ζ immunoreceptor tyrosine-based activation motif peptide binds only the D1 active site. The D2 “active site” deviates from the other active sites significantly to the extent that excludes any possibility of catalytic activity. The relative orientation of D1 and D2 is very similar to that observed in leukocyte common antigen–related protein with both active sites in an open conformation and is restrained through an extensive network of hydrophobic interactions, hydrogen bonds, and salt bridges. This crystal structure is incompatible with the wedge model previously suggested for CD45 regulation.

Evolution of the multifunctional protein tyrosine phosphatase family

The protein tyrosine phosphatase (PTP) family plays a central role in signal transduction pathways by controlling the phosphorylation state of serine, threonine, and tyrosine residues. PTPs can be divided into dual specificity phosphatases and the classical PTPs, which can comprise of one or two phosphatase domains. We studied amino acid substitutions at functional sites in the phosphatase domain and identified putative noncatalytic phosphatase domains in all subclasses of the PTP family. The presence of inactive phosphatase domains in all subclasses indicates that they were invented multiple times in evolution. Depending on the domain composition, loss of catalytic activity can result in different consequences for the function of the protein. Inactive single-domain phosphatases can still specifically bind substrate and protect it from dephosphorylation by other phosphatases. The inactive domains of tandem phosphatases can be further subdivided. The first class is more conserved, still able to bind phosphorylated tyrosine residues and might recruit multiphosphorylated substrates for the adjacent active domain. The second has accumulated several variable amino acid substitutions in the catalytic center, indicating a complete loss of tyrosine-binding capabilities. To study the impact of substitutions in the catalytic center to the evolution of the whole domain, we examined the evolutionary rates for each individual site and compared them between the classes. This analysis revealed a release of evolutionary constraint for multiple sites surrounding the catalytic center only in the second class, emphasizing its difference in function compared with the first class. Furthermore, we found a region of higher conservation common to both domain classes, suggesting a new regulatory center. We discuss the influence of evolutionary forces on the development of the phosphatase domain, which has led to additional functions, such as the specific protection of phosphorylated tyrosine residues, substrate recruitment, and regulation of the catalytic activity of adjacent domains.

Dimerization in vivo and inhibition of the nonreceptor form of protein tyrosine phosphatase epsilon

cyt-PTP epsilon is a naturally occurring nonreceptor form of the receptor-type protein tyrosine phosphatase (PTP) epsilon. As such, cyt-PTP epsilon enables analysis of phosphatase regulation in the absence of extracellular domains, which participate in dimerization and inactivation of the receptor-type phosphatases receptor-type protein tyrosine phosphatase alpha (RPTPalpha) and CD45. Using immunoprecipitation and gel filtration, we show that cyt-PTP epsilon forms dimers and higher-order associations in vivo, the first such demonstration among nonreceptor phosphatases. Although cyt-PTP epsilon readily dimerizes in the absence of exogenous stabilization, dimerization is increased by oxidative stress. Epidermal growth factor receptor stimulation can affect cyt-PTP epsilon dimerization and tyrosine phosphorylation in either direction, suggesting that cell surface receptors can relay extracellular signals to cyt-PTP epsilon, which lacks extracellular domains of its own. The inactive, membrane-distal (D2) phosphatase domain of cyt-PTP epsilon is a major contributor to intermolecular binding and strongly interacts in a homotypic manner; the presence of D2 and the interactions that it mediates inhibit cyt-PTP epsilon activity. Intermolecular binding is inhibited by the extreme C and N termini of D2. cyt-PTP epsilon lacking these regions constitutively dimerizes, and its activities in vitro towards para-nitrophenylphosphate and in vivo towards the Kv2.1 potassium channel are markedly reduced. We conclude that physiological signals can regulate dimerization and phosphorylation of cyt-PTP epsilon in the absence of direct interaction between the PTP and extracellular molecules. Furthermore, dimerization can be mediated by the D2 domain and does not strictly require the presence of PTP extracellular domains.

PTP alpha regulates integrin-stimulated FAK autophosphorylation and cytoskeletal rearrangement in cell spreading and migration

We investigated the molecular and cellular actions of receptor protein tyrosine phosphatase (PTP) alpha in integrin signaling using immortalized fibroblasts derived from wild-type and PTP alpha-deficient mouse embryos. Defects in PTP alpha-/- migration in a wound healing assay were associated with altered cell shape and focal adhesion kinase (FAK) phosphorylation. The reduced haptotaxis to fibronectin (FN) of PTP alpha-/- cells was increased by expression of active (but not inactive) PTP alpha. Integrin-mediated formation of src-FAK and fyn-FAK complexes was reduced or abolished in PTP alpha-/- cells on FN, concomitant with markedly reduced phosphorylation of FAK at Tyr397. Reintroduction of active (but not inactive) PTP alpha restored FAK Tyr-397 phosphorylation. FN-induced cytoskeletal rearrangement was retarded in PTP alpha-/- cells, with delayed filamentous actin stress fiber assembly and focal adhesion formation. This mimicked the effects of treating wild-type fibroblasts with the src family protein tyrosine kinase (Src-PTK) inhibitor PP2. These results, together with the reduced src/fyn tyrosine kinase activity in PTP alpha-/- fibroblasts (Ponniah et al., 1999; Su et al., 1999), suggest that PTP alpha functions in integrin signaling and cell migration as an Src-PTK activator. Our paper establishes that PTP alpha is required for early integrin-proximal events, acting upstream of FAK to affect the timely and efficient phosphorylation of FAK Tyr-397.

Multimerization of the protein-tyrosine phosphatase (PTP)-like insulin-dependent diabetes mellitus autoantigens IA-2 and IA-2beta with receptor PTPs (RPTPs). Inhibition of RPTPalpha enzymatic activity

Most receptor-type protein-tyrosine phosphatases (RPTPs) contain two tandem PTP domains. For some RPTPs the enzymatically inactive membrane-distal phosphatase domains (D2) were found to bind enzymatically active membrane proximal PTP (D1) domains, and oligomerization has been proposed as a general regulatory mechanism. The RPTP-like proteins IA-2 and IA-2beta, major autoantigens in insulin-dependent diabetes mellitus, contain just a single enzymatically inactive PTP-like domain. Their physiological role is as yet enigmatic. To investigate whether the catalytically inactive cytoplasmic domains of IA-2 and IA-2beta are involved in oligomerization, we exploited interaction trap assay in yeast and glutathione S-transferase pull-down and co-immunoprecipitation strategies on lysates of transfected COS-1 cells. The results show that IA-2 and IA-2beta are capable of homo- and heterodimerization to which both the juxtamembrane region and the phosphatase-like segment can contribute. Furthermore, they can form heterodimers with some other RPTP members, most notably RPTPalpha and RPTPepsilon, and down-regulate RPTPalpha enzymatic activity. Thus, in addition to homo-dimerization, the enzymatic activity of receptor-type PTPs can be regulated through heterodimerization with other RPTPs, including the catalytically inactive IA-2 and IA-2beta.

Intra- and intermolecular interactions between intracellular domains of receptor protein-tyrosine phosphatases

The presence of two protein-tyrosine phosphatase (PTP) domains is a striking feature in most transmembrane receptor PTPs (RPTPs). The generally inactive membrane-distal PTP domains (RPTP-D2s) bind and are proposed to regulate the membrane-proximal PTP domains (RPTP-D1s). We set out to characterize the interactions between RPTP-D1s and RPTP-D2s in vivo by co-immunoprecipitation of hemagglutinin-tagged fusion proteins encoding the transmembrane domain and RPTP-D1 and myc-tagged RPTP-D2. Seven RPTPs from four different subfamilies were used: RPTPalpha, RPTPepsilon, LAR, RPTPvarsigma, RPTPdelta, CD45, and RPTP(mu). We found that RPTP-D2s bound to RPTPs with different affinities. The presence of intrinsic RPTP-D2 altered the binding specificity toward other RPTP-D2s positively or negatively, depending on the identity of the RPTPs. Furthermore, the C terminus of RPTP-D2s and the "wedge" in RPTP-D1s played a central role in binding specificity. Finally, full-length RPTPalpha and LAR heterodimerized in an oxidative stress-dependent manner. Like RPTPalpha-D2, the LAR-D2 conformation was affected by oxidative stress, suggesting a common regulatory mechanism for RPTP complex formation. Taken together, interactions between RPTP-D1s and RPTP-D2s are a common but specific mechanism that is likely to be regulated. The RPTP-D2s and the wedge structures are crucial determinants of binding specificity, thus regulating cross-talk between RPTPs.

Gain control of N-methyl-D-aspartate receptor activity by receptor-like protein tyrosine phosphatase alpha

Src kinase regulation of N-methyl-D-aspartate (NMDA) subtype glutamate receptors in the central nervous system (CNS) has been found to play an important role in processes related to learning and memory, ethanol sensitivity and epilepsy. However, little is known regarding the mechanisms underlying the regulation of Src family kinase activity in the control of NMDA receptors. Here we report that the distal phosphatase domain (D2) of protein tyrosine phosphatase alpha (PTPalpha) binds to the PDZ2 domain of post-synaptic density 95 (PSD95). Thus, Src kinase, its activator (PTPalpha) and substrate (NMDA receptors) are linked by the same scaffold protein, PSD95. Removal of PTPalpha does not affect the association of Src with NMDA receptors, but turns off the constitutive regulation of NMDA receptors by the kinase. Further more, we found that application of the PTPalpha catalytic domains (D1 + D2) into neurones enhances NMDA receptor-mediated synaptic responses. Conversely, the blockade of endogenous PTPalpha inhibits NMDA receptor activity and the induction of long-term potentiation in hippocampal neurones. Thus, PTPalpha is a novel up-regulator of synaptic strength in the CNS.

Regulation of receptor protein-tyrosine phosphatase alpha by oxidative stress

The presence of two protein-tyrosine phosphatase (PTP) domains is a striking feature in most transmembrane receptor PTPs (RPTPs). The function of the generally inactive membrane-distal PTP domain (RPTP-D2) is unknown. Here we report that an intramolecular interaction between the spacer region (Sp) and the C-terminus in RPTPalpha prohibited intermolecular interactions. Interestingly, stress factors such as H(2)O(2), UV and heat shock induced reversible, free radical-dependent, intermolecular interactions between RPTPalpha and RPTPalpha-SpD2, suggesting an inducible switch in conformation and binding. The catalytic site cysteine of RPTPalpha-SpD2, Cys723, was required for the H(2)O(2) effect on RPTPalpha. H(2)O(2) induced a rapid, reversible, Cys723-dependent conformational change in vivo, as detected by fluorescence resonance energy transfer, with cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) flanking RPTPalpha-SpD2 in a single chimeric protein. Importantly, H(2)O(2) treatment stabilized RPTPalpha dimers, resulting in inactivation. We propose a model in which oxidative stress induces a conformational change in RPTPalpha-D2, leading to stabilization of RPTPalpha dimers, and thus to inhibition of RPTPalpha activity.

Intramolecular interactions in protein tyrosine phosphatase RPTPmu: kinetic evidence

The receptor-like protein tyrosine phosphatase RPTPmu contains three intracellular domains: the juxtamembrane (JM) and two phosphatase domains (D1 and D2). D1 is catalytically active in vitro. The functional roles of JM and D2 are still unclear. To find out whether and how they modulate the phosphatase activity of D1, we compared the enzymatic characteristics of two constructs, containing a truncated JM and either D1 or both phosphatase domains. p-Nitrophenyl phosphate and two peptide substrates were efficiently dephosphorylated by both constructs. The specificity constant of D1 alone was up to 50% higher. D2 induces (a) decreased K(m) values for peptide substrates, (b) decreased catalytic efficiency for these substrates, (c) shifting of the optimal pH to slightly lower values, and (d) looser binding of competitive inhibitors. These data suggest that the phosphatase activity of D1 is negatively modulated and its ligand binding capacity is sensibly modified by domain D2, having possible functional significance.

Calmodulin binds to and inhibits the activity of the membrane distal catalytic domain of receptor protein-tyrosine phosphatase alpha

cDNA expression library screening revealed binding between the membrane distal catalytic domain (D2) of protein-tyrosine phosphatase alpha (PTPalpha) and calmodulin. Characterization using surface plasmon resonance showed that calmodulin bound to PTPalpha-D2 in a Ca(2+)-dependent manner but did not bind to the membrane proximal catalytic domain (D1) of PTPalpha, to the two tandem catalytic domains (D1D2) of PTPalpha, nor to the closely related D2 domain of PTPepsilon. Calmodulin bound to PTPalpha-D2 with high affinity, exhibiting a K(D) approximately 3 nm. The calmodulin-binding site was localized to amino acids 520-538 in the N-terminal region of D2. Site-directed mutagenesis showed that Lys-521 and Asn-534 were required for optimum calmodulin binding and that restoration of these amino acids to the counterpart PTPepsilon sequence could confer calmodulin binding. The overlap of the binding site with the predicted lip of the catalytic cleft of PTPalpha-D2, in conjunction with the observation that calmodulin acts as a competitive inhibitor of D2-catalyzed dephosphorylation (K(i) approximately 340 nm), suggests that binding of calmodulin physically blocks or distorts the catalytic cleft of PTPalpha-D2 to prevent interaction with substrate. When expressed in cells, full-length PTPalpha and PTPalpha lacking only D1, but not full-length PTPepsilon, bound to calmodulin beads in the presence of Ca(2+). Also, PTPalpha was found in association with calmodulin immunoprecipitated from cell lysates. Thus calmodulin does associate with PTPalpha in vivo but not with PTPalpha-D1D2 in vitro, highlighting a potential conformational difference between these forms of the tandem catalytic domains. The above findings suggest that calmodulin is a possible specific modulator of PTPalpha-D2 and, via D2, of PTPalpha.

Receptor-like protein tyrosine phosphatase alpha homodimerizes on the cell surface

We reported previously that the N-terminal D1 catalytic domain of receptor protein-tyrosine phosphatase alpha (RPTPalpha) forms a symmetrical, inhibited dimer in a crystal structure, in which a helix-turn-helix wedge element from one monomer is inserted into the catalytic cleft of the other monomer. Previous functional studies also suggested that dimerization inhibits the biological activity of a CD45 chimeric RPTP and the catalytic activity of an isolated RPTPsigma D1 catalytic domain. Most recently, we have also shown that enforced dimerization inhibits the biological activity of full-length RPTPalpha in a wedge-dependent manner. The physiological significance of such inhibition is unknown, due to a lack of understanding of how RPTPalpha dimerization is regulated in vivo. In this study, we show that transiently expressed cell surface RPTPalpha exists predominantly as homodimers, suggesting that dimerization-mediated inhibition of RPTPalpha biological activity is likely to be physiologically relevant. Consistent with our published and unpublished crystallographic data, we show that mutations in the wedge region of D1 catalytic domain and deletion of the entire D2 catalytic domain independently reduced but did not abolish RPTPalpha homodimerization, suggesting that both domains are critically involved but that neither is essential for homodimerization. Finally, we also provide evidence that both the RPTPalpha extracellular domain and the transmembrane domain were independently able to homodimerize. These results lead us to propose a zipper model in which inactive RPTPalpha dimers are stabilized by multiple, relatively weak dimerization interfaces. Dimerization in this manner would provide a potential mechanism for negative regulation of RPTPalpha. Such RPTPalpha dimers could be activated by extracellular ligands or intracellular binding proteins that induce monomerization or by intracellular signaling events that induce an open conformation of the dimer.

Identification of p130cas as an in vivo substrate of receptor protein-tyrosine phosphatase alpha

We have employed a substrate trapping strategy to identify physiological substrates of the receptor protein-tyrosine phosphatase alpha (RPTPalpha). Here we report that a substrate-trapping mutant of the RPTPalpha membrane proximal catalytic domain (D1), RPTPalpha-D1-C433S, specifically bound to tyrosine-phosphorylated proteins from pervanadate-treated cells. The membrane distal catalytic domain of RPTPalpha (D2) and mutants thereof did not bind to tyrosine-phosphorylated proteins. The pattern of tyrosine-phosphorylated proteins that bound to RPTPalpha-D1-C433S varied between cell lines, but a protein of approximately 130 kDa was pulled down from every cell line. This protein was identified as p130(cas). Tyrosine-phosphorylated p130(cas) from fibronectin-stimulated NIH3T3 cells bound to RPTPalpha-D1-C433S as well, suggesting that p130(cas) is a physiological substrate of RPTPalpha. RPTPalpha dephosphorylated p130(cas) in vitro, and RPTPalpha co-localized with a subpopulation of p130(cas) to the plasma membrane. Co-transfection experiments with activated SrcY529F, p130(cas), and RPTPalpha or inactive, mutant RPTPalpha indicated that RPTPalpha dephosphorylated p130(cas) in vivo. Tyrosine-phosphorylated epidermal growth factor receptor was not dephosphorylated by RPTPalpha under these conditions, suggesting that p130(cas) is a specific substrate of RPTPalpha in living cells. In conclusion, our results provide evidence that p130(cas) is a physiological substrate of RPTPalpha in vivo.

Multiple interactions between receptor protein-tyrosine phosphatase (RPTP) alpha and membrane-distal protein-tyrosine phosphatase domains of various RPTPs

Receptor protein-tyrosine phosphatase (RPTP) alpha belongs to the large family of receptor protein-tyrosine phosphatases containing two tandem phosphatase domains. Most of the catalytic activity is retained in the first, membrane-proximal domain (RPTPalpha-D1), and little is known about the function of the second, membrane-distal domain (RPTPalpha-D2). We investigated whether proteins bound to RPTPalpha using the two-hybrid system and found that the second domain of RPTPsigma interacted with the juxtamembrane domain of RPTPalpha. We confirmed this interaction by co-immunoprecipitation experiments. Furthermore, RPTPalpha not only interacted with RPTPsigma-D2 but also with RPTPalpha-D2, LAR-D2, RPTPdelta-D2, and RPTPmu-D2, members of various RPTP subfamilies, although with different affinities. In the yeast two-hybrid system and in glutathione S-transferase pull-down assays, we show that the RPTP-D2s interacted directly with the wedge structure of RPTPalpha-D1 that has been demonstrated to be involved in inactivation of the RPTPalpha-D1/RPTPalpha-D1 homodimer. The interaction was specific because the equivalent wedge structure in LAR was unable to interact with RPTPalpha-D2 or LAR-D2. In vivo, we show that other interaction sites exist as well, including the C terminus of RPTPalpha-D2. The observation that RPTPalpha, but not LAR, bound to multiple RPTP-D2s with varying affinities suggests a specific mechanism of cross-talk between RPTPs that may regulate their biological function.

Receptor tyrosine phosphatases regulate axon guidance across the midline of the Drosophila embryo

Neural receptor-linked protein tyrosine phosphatases (RPTPs) are required for guidance of motoneuron and photoreceptor growth cones in Drosophila. These phosphatases have not been implicated in growth cone responses to specific guidance cues, however, so it is unknown which aspects of axonal pathfinding are controlled by their activities. Three RPTPs, known as DLAR, DPTP69D, and DPTP99A, have been genetically characterized thus far. Here we report the isolation of mutations in the fourth neural RPTP, DPTP10D. The analysis of double mutant phenotypes shows that DPTP10D and DPTP69D are necessary for repulsion of growth cones from the midline of the embryonic central nervous system. Repulsion is thought to be triggered by binding of the secreted protein Slit, which is expressed by midline glia, to Roundabout (Robo) receptors on growth cones. Robo repulsion is downregulated by the Commissureless (Comm) protein, allowing axons to cross the midline. Here we show that the Rptp mutations genetically interact with robo, slit and comm. The nature of these interactions suggests that DPTP10D and DPTP69D are positive regulators of Slit/Roundabout repulsive signaling. We also show that elimination of all four neural RPTPs converts most noncrossing longitudinal pathways into commissures that cross the midline, indicating that tyrosine phosphorylation controls the manner in which growth cones respond to midline signals.

Involvement of the membrane distal catalytic domain in pervanadate-induced tyrosine phosphorylation of receptor protein-tyrosine phosphatase alpha

Receptor protein-tyrosine phosphatase alpha, RPTPalpha, is a typical transmembrane protein-tyrosine phosphatase (PTP) with two cytoplasmic catalytic domains. RPTPalpha became strongly phosphorylated on tyrosine upon treatment of cells with the PTP inhibitor pervanadate. Surprisingly, mutation of the catalytic site Cys in the membrane distal PTP domain (D2), but not of the membrane proximal PTP domain (D1) that harbors the majority of the PTP activity, almost completely abolished pervanadate-induced tyrosine phosphorylation. Pervanadate-induced RPTPalpha tyrosine phosphorylation was not restricted to Tyr789, a known phosphorylation site. Cotransfection of wild-type RPTPalpha did not potentiate tyrosine phosphorylation of inactive RPTPalpha-C433SC723S, suggesting that RPTPalpha-mediated activation of kinase(s) does not underlie the observed effects. Mapping experiments indicated that pervanadate-induced tyrosine phosphorylation sites localized predominantly, but not exclusively, to the C-terminus. Our results demonstrate that RPTPalpha-D2 played a role in pervanadate-induced tyrosine phosphorylation of RPTPalpha, which may suggest that RPTPalpha-D2 is involved in protein-protein interactions.

Catalytic activation of the membrane distal domain of protein tyrosine phosphatase epsilon, but not CD45, by two point mutations

Most, if not all, of the catalytic activity of the tandem catalytic domain-containing receptor-like protein tyrosine phosphatases (PTPs) resides in the membrane proximal domains (D1), with little to no activity associated with the membrane distal domains (D2). Two point mutations in the D2 domain of PTPalpha, which restore invariant amino acids found in the KNRY motif and WPD loop of all active D1 domains, synergistically confer D1-equivalent kinetic properties towards the phosphotyrosine analogue pNPP, and activate PTPalpha-D2 catalysed phosphopeptide hydrolysis (Lim et al., J. Biol. Chem. 273 (1998) 28986-28993; Buist et al., Biochemistry 38 (1999) 914-922). As all D2 domains lack these two D1-invariant amino acids, we have investigated whether other D2 domains are activated by such point mutations. Mutant PTPepsilon-D2, closely related to PTPalpha-D2 and belonging to a subgroup of D2 domains with minimal and conservative substitutions of D1-invariant amino acids, exhibits synergistic activation towards pNPP but not towards a phosphopeptide substrate. CD45-D2, belonging to another subgroup of D2 domains with considerable substitutions in D1-invariant amino acids, is not activated by these mutations, even in the context of a third mutation which restores the minimal essential active site sequence C(X(5))R, indicating that additional defects are sufficient to preclude catalysis. The ability of the KNRY and WPD replacements to activate PTPepsilon-D2 and PTPalpha-D2, but not CD45-D2, in conjunction with the extent and nature of their wild-type amino acid substitutions, suggests that these D2 domains are representative of two functionally distinct groups of D2 domain.

Protein-tyrosine phosphatases in development

One of the most important mechanisms of eukaryotic signalling is protein phosphorylation on tyrosine residues, which plays a pivotal role in development by regulating cell proliferation, differentiation and migration. Cellular phosphotyrosine (P.Tyr) levels are regulated by the antagonistic activities of the protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs). We have good insight into the function of PTKs at the molecular level and into the role of PTK-mediated signalling in development. Intuitively, PTPs and PTKs are equally important in development. Over the past decade, much emphasis has been placed on elucidation of the function of PTPs, which has led to good insights into the mechanism of PTP-mediated dephosphorylation. Although still relatively little is known about the role of PTPs in cell signalling and development, evidence is now emerging that several PTPs are crucial for proper development. Here I will introduce PTP-mediated signalling and discuss recent findings regarding the function of PTPs in development.

Crystal structure of the tandem phosphatase domains of RPTP LAR

Most receptor-like protein tyrosine phosphatases (RPTPs) contain two conserved phosphatase domains (D1 and D2) in their intracellular region. The carboxy-terminal D2 domain has little or no catalytic activity. The crystal structure of the tandem D1 and D2 domains of the human RPTP LAR revealed that the tertiary structures of the LAR D1 and D2 domains are very similar to each other, with the exception of conformational differences at two amino acid positions in the D2 domain. Site-directed mutational changes at these positions (Leu-1644-to-Tyr and Glu-1779-to-Asp) conferred a robust PTPase activity to the D2 domain. The catalytic sites of both domains are accessible, in contrast to the dimeric blocked orientation model previously suggested. The relative orientation of the LAR D1 and D2 domains, constrained by a short linker, is stabilized by extensive interdomain interactions, suggesting that this orientation might be favored in solution.