Proteolytic processing of the receptor-type protein tyrosine phosphatase PTPBR7

The single-copy mouse gene Ptprr gives rise to different protein tyrosine phosphatase (PTP) isoforms in neuronal cells through the use of distinct promoters, alternative splicing, and multiple translation initiation sites. Here, we examined the array of post-translational modifications imposed on the PTPRR protein isoforms PTPBR7, PTP-SL, PTPPBSgamma42 and PTPPBSgamma37, which have distinct N-terminal segments and localize to different parts of the cell. All isoforms were found to be short-lived, constitutively phosphorylated proteins. In addition, the transmembrane isoform, PTPBR7, was subject to N-terminal proteolytic processing, in between amino acid position 136 and 137, resulting in an additional, 65-kDa transmembrane PTPRR isoform. Unlike for some other receptor-type PTPs, the proteolytically produced N-terminal ectodomain does not remain associated with this PTPRR-65. Shedding of PTPBR7-derived polypeptides at the cell surface further adds to the molecular complexity of PTPRR biology.

Altered MAP kinase phosphorylation and impaired motor coordination in PTPRR deficient mice

The neuronal protein tyrosine phosphatases encoded by mouse gene Ptprr (PTPBR7, PTP-SL, PTPPBSgamma-42 and PTPPBSgamma-37) have been implicated in mitogen-activated protein (MAP) kinase deactivation on the basis of transfection experiments. To determine their physiological role in vivo, we generated mice that lack all PTPRR isoforms. Ptprr-/- mice were viable and fertile, and not different from wildtype littermates regarding general physiology or explorative behaviour. Highest PTPRR protein levels are in cerebellum Purkinje cells, but no overt effects of PTPRR deficiency on brain morphology, Purkinje cell number or dendritic branching were detected. However, MAP kinase phosphorylation levels were significantly altered in the PTPRR-deficient cerebellum and cerebrum homogenates. Most notably, increased phospho-ERK1/2 immunostaining density was observed in the basal portion and axon hillock of Ptprr-/- Purkinje cells. Concomitantly, Ptprr-/- mice displayed ataxia characterized by defects in fine motor coordination and balance skills. Collectively, these results establish the PTPRR proteins as physiological regulators of MAP kinase signalling cascades in neuronal tissue and demonstrate their involvement in cerebellum motor function.

Crystal structures and inhibitor identification for PTPN5, PTPRR and PTPN7: a family of human MAPK-specific protein tyrosine phosphatases

Protein tyrosine phosphatases PTPN5, PTPRR and PTPN7 comprise a family of phosphatases that specifically inactivate MAPKs (mitogen-activated protein kinases). We have determined high-resolution structures of all of the human family members, screened them against a library of 24000 compounds and identified two classes of inhibitors, cyclopenta[c]quinolinecarboxylic acids and 2,5-dimethylpyrrolyl benzoic acids. Comparative structural analysis revealed significant differences within this conserved family that could be explored for the design of selective inhibitors. PTPN5 crystallized, in two distinct crystal forms, with a sulphate ion in close proximity to the active site and the WPD (Trp-Pro-Asp) loop in a unique conformation, not seen in other PTPs, ending in a 3(10)-helix. In the PTPN7 structure, the WPD loop was in the closed conformation and part of the KIM (kinase-interaction motif) was visible, which forms an N-terminal aliphatic helix with the phosphorylation site Thr66 in an accessible position. The WPD loop of PTPRR was open; however, in contrast with the structure of its mouse homologue, PTPSL, a salt bridge between the conserved lysine and aspartate residues, which has been postulated to confer a more rigid loop structure, thereby modulating activity in PTPSL, does not form in PTPRR. One of the identified inhibitor scaffolds, cyclopenta[c]quinoline, was docked successfully into PTPRR, suggesting several possibilities for hit expansion. The determined structures together with the established SAR (structure-activity relationship) propose new avenues for the development of selective inhibitors that may have therapeutic potential for treating neurodegenerative diseases in the case of PTPRR or acute myeloblastic leukaemia targeting PTPN7.

ERK2 shows a restrictive and locally selective mechanism of recognition by its tyrosine phosphatase inactivators not shared by its activator MEK1

The two regulatory residues that control the enzymatic activity of the mitogen-activated protein (MAP) kinase ERK2 are phosphorylated by the unique MAP kinase kinases MEK1/2 and dephosphorylated by several tyrosine-specific and dual specificity protein phosphatases. Selective docking interactions facilitate these phosphorylation and dephosphorylation events, controlling the specificity and duration of the MAP kinase activation-inactivation cycles. We have analyzed the contribution of specific residues of ERK2 in the physical and functional interaction with the ERK2 phosphatase inactivators PTP-SL and MKP-3 and with its activator MEK1. Single mutations in ERK2 that abrogated the dephosphorylation by endogenous tyrosine phosphatases from HEK293 cells still allowed efficient phosphorylation by endogenous MEK1/2. Discrete ERK2 mutations at the ERK2 docking groove differentially affected binding and inactivation by PTP-SL and MKP-3. Remarkably, the cytosolic retention of ERK2 by its activator MEK1 was not affected by any of the analyzed ERK2 single amino acid substitutions. A chimeric MEK1 protein, containing the kinase interaction motif of PTP-SL, bound tightly to ERK2 through its docking groove and behaved as a gain-of-function MAP kinase kinase that hyperactivated ERK2. Our results provide evidence that the ERK2 docking groove is more restrictive and selective for its tyrosine phosphatase inactivators than for MEK1/2 and indicate that distinct ERK2 residues modulate the docking interactions with activating and inactivating effectors.

Cloning and characterization of the novel chimeric gene TEL/PTPRR in acute myelogenous leukemia with inv(12)(p13q13)

We have cloned a novel TEL/protein tyrosine phosphatase receptor-type R (PTPRR) chimeric gene generated by inv(12)(p13q13). PTPRR is the first protein tyrosine phosphatase identified as a fusion partner of TEL. The chimeric gene fused exon 4 of the TEL gene with exon 7 of the PTPRR gene, and produced 10 isoforms through alternative splicing. Two isoforms that were expressed at the highest level in the leukemic cells could have been translated into COOH-terminally truncated TEL protein possessing the helix-loop-helix domain (tTEL) and TEL/PTPRR chimeric protein linking the helix-loop-helix domain of TEL to the catalytic domain of PTPRR. These two mutant proteins exerted a dominant-negative effect over transcriptional repression mediated by wild-type TEL, although they themselves did not show any transcriptional activity. Heterodimerization with wild-type TEL might be an underlying mechanism in this effect. TEL/PTPRR did not exhibit any tyrosine phosphatase activity. Importantly, overexpression of TEL/PTPRR in granulocyte macrophage colony-stimulating factor-dependent UT7/GM cells resulted in their factor-independent proliferation, whereas overexpression of tTEL did not. After cytokine depletion, phosphorylated signal transducers and activators of transcription 3 (STAT3) significantly declined in mock cells, but remained in both tTEL- and TEL/PTPRR-overexpressing cells. Loss of tumor suppressive function of wild-type TEL and maintenance of STAT3-mediated signal could at least partly contribute to the leukemogenesis caused by inv(12)(p13q13).

Characterization of multiple transcripts and isoforms derived from the mouse protein tyrosine phosphatase gene Ptprr

The use of alternative splice sites, promoters and translation start sites considerably adds to the complexity of organisms. Four mouse cDNAs (PTPBR7, PTP-SL, PTPPBSgamma+ and PTPPBSgamma-) have been cloned that contain different 5' parts but encode identical protein tyrosine phosphatase PTPRR catalytic domains. We investigated the genomic origin and coding potential of these transcripts to elucidate their interrelationship. Mouse gene Ptprr exons were identified within a 260 kbp segment on chromosome 10, revealing PTP-SL- and PTPPBSgamma-specific transcription start sites within introns two and four, respectively, relative to the 14 PTPBR7 exons. Northern and RT-PCR analyses demonstrated differential expression patterns for these promoters. Furthermore, transfection studies and AUG codon mutagenesis demonstrated that in PTP-SL and PTPPBSgamma messengers multiple translation initiation sites are being used. Resulting 72, 60, 42 and 37 kDa PTPRR protein isoforms differ not only in the length of their N-terminal part but also in their subcellular localization, covering all major PTP subtypes; receptor-like, membrane associated and cytosolic. In summary, mouse gene Ptprr gives rise to multiple isoforms through the use of distinct promoters, alternative splicing and differential translation starts. These results set the stage for further investigations on the physiological roles of PTPRR proteins.

Colocalisation of the protein tyrosine phosphatases PTP-SL and PTPBR7 with beta4-adaptin in neuronal cells

The mouse gene Ptprr encodes the neuronal protein tyrosine phosphatases PTP-SL and PTPBR7. These proteins differ in their N-terminal domains, with PTP-SL being a cytosolic, membrane-associated phosphatase and PTPBR7 a type I transmembrane protein. In this study, we further explored the nature of the PTP-SL-associated vesicles in neuronal cells using a panel of organelle markers and noted a comparable subcellular distribution for PTP-SL and the beta4-adaptin subunit of the AP4 complex. PTP-SL, PTPBR7 and beta4-adaptin are localised at the Golgi apparatus and at vesicles throughout the cytoplasm. Immunohistochemical analysis demonstrated that PTP-SL, PTPBR7 and beta4-adaptin are all endogenously expressed in brain. Interestingly, coexpression of PTP-SL and beta4-adaptin leads to an altered subcellular localisation for PTP-SL. Instead of the Golgi and vesicle-type staining pattern, still observable for beta4-adaptin, PTP-SL is now distributed throughout the cytoplasm. Although beta4-adaptin was found to interact with the phosphatase domain of PTP-SL and PTPBR7 in the yeast two-hybrid system, it failed to do so in transfected neuronal cells. Our data suggest that the tyrosine phosphatases PTP-SL and PTPBR7 may be involved in the formation and transport of AP4-coated vesicles or in the dephosphorylation of their transmembrane cargo molecules at or near the Golgi apparatus.

Phosphotyrosine-specific phosphatase PTP-SL regulates the ERK5 signaling pathway

The duration and the magnitude of mitogen-activated protein kinase (MAPK) activation specifies signal identity and thus allows the regulation of diverse cellular functions by the same kinase cascade. A tight and finely tuned regulation of MAPK activity is therefore critical for the definition of a specific cellular response. We investigated the role of tyrosine-specific phosphatases (PTPs) in the regulation of ERK5. Although unique in its structure, ERK5 is activated in analogy to other MAPKs by dual phosphorylation of threonine and tyrosine residues in its activation motif. In this study we concentrated on whether and how PTP-SL, a kinase-interacting motif-containing PTP, might be involved in the down-regulation of the ERK5 signal. We found that both proteins interact directly with each other in vitro and in intact cells, resulting in mutual modulation of their enzymatic activities. PTP-SL is a substrate of ERK5 and independent of phosphorylation binding to the kinase enhances its catalytic phosphatase activity. On the other hand, interaction with PTP-SL not only down-regulates endogenous ERK5 activity but also effectively impedes the translocation of ERK5 to the nucleus. These findings indicate a direct regulatory influence of PTP-SL on the ERK5 pathway and corresponding downstream responses of the cell.

Two clusters of residues at the docking groove of mitogen-activated protein kinases differentially mediate their functional interaction with the tyrosine phosphatases PTP-SL and STEP

Regulated function of mitogen-activated protein (MAP) kinases involves their selective association through docking sites with both activating MAP kinase kinases and inactivating phosphatases, including dual specificity and protein-tyrosine phosphatases (PTP). Site-directed mutagenesis on the mammalian MAP kinases ERK2 and p38alpha identified within their C-terminal docking grooves two clusters of residues important for association with their regulatory PTPs, PTP-SL and STEP. ERK2 and p38alpha mutations that resembled the sevenmaker gain-of-function mutation in the Rolled D. melanogaster ERK2 homologue failed to associate with PTP-SL, were not retained in the cytosol, and were poorly inactivated by this PTP. Additional ERK2 mutations at the docking groove showed deficient association and dephosphorylation by PTP-SL, although their cytosolic retention was unaffected. Other ERK2 mutations, resembling gain-of-function mutations in the FUS3 yeast ERK2 homologue, associated to PTP-SL and were inactivated normally by this PTP. Our results demonstrate that mutations at distinct regions of the docking groove of ERK2 and p38alpha differentially affect their association and regulation by the PTP-SL and STEP PTPs.

Crystal structure of PTP-SL/PTPBR7 catalytic domain: implications for MAP kinase regulation

Protein tyrosine phosphatases PTP-SL and PTPBR7 are isoforms belonging to cytosolic membrane-associated and to receptor-like PTPs (RPTPs), respectively. They represent a new family of PTPs with a major role in activation and translocation of MAP kinases. Specifically, the complex formation between PTP-SL and ERK2 involves an unusual interaction leading to the phosphorylation of PTP-SL by ERK2 at Thr253 and the inactivating dephosphorylation of ERK2 by PTP-SL. This interaction is strictly dependent upon a kinase interaction motif (KIM) (residues 224-239) situated at the N terminus of the PTP-SL catalytic domain. We report the first crystal structure of the catalytic domain for a member of this family (PTP-SL, residues 254-549, identical with residues 361-656 of PTPBR7), providing an example of an RPTP with single cytoplasmic domain, which is monomeric, having an unhindered catalytic site. In addition to the characteristic PTP-core structure, PTP-SL has an N-terminal helix, possibly orienting the KIM motif upon interaction with the target ERK2. An unusual residue in the catalytically important WPD loop promotes formation of a hydrophobically and electrostatically stabilised clamp. This could induce increased rigidity to the WPD loop and therefore reduced catalytic activity, in agreement with our kinetic measurements. A docking model based on the PTP-SL structure suggests that, in the complex with ERK2, the phosphorylation of PTP-SL should be accomplished first. The subsequent dephosphorylation of ERK2 seems to be possible only if a conformational rearrangement of the two interacting partners takes place.

Type 2 diabetes locus on 12q15. Further mapping and mutation screening of two candidate genes

We recently reported evidence of a novel type 2 diabetes locus placed on chromosome 12q15 between markers D12S375 and D12S1684 (Diabetes 48:2246-2251, 1999). Four multigenerational families having logarithm of odds (LOD) scores >1.0 in the original analysis were genotyped for 11 additional markers in this interval to refine this mapping; this allowed us to narrow the linked region to the interval between markers D12S1693 and D12S326. In a multipoint parametric analysis using the VITESSE software, the LOD score for linkage at this location reached 3.1 in one of these families. This interval contains the gene for protein tyrosine phosphatase receptor type R (PTPRR)--a protein that may be involved in both insulin secretion and action. After determining PTPRR exon-intron structure, we identified several polymorphisms in this gene but no mutation segregating with diabetes. The search for mutations was also negative for carboxypeptidase M (CPM)--another candidate gene mapped to this region. In summary, our data provide further evidence for the existence of a type 2 diabetes locus on chromosome 12q15. This locus, however, does not appear to correspond to the PTPRR or CPM, although a contribution of mutations in regulatory regions cannot be excluded at this time.

Protein tyrosine phosphatase (PC12, Br7,S1) family: expression characterization in the adult human and mouse

Protein tyrosine phosphatases (PTPs) play important roles in modulating signals transduced by tyrosine kinases. Certain phosphatases have been implicated as having important roles in embryonic development as well as in adult physiology. Although both kinases and phosphatases are equally important in regulating signal transduction, phosphatases as a group have not been well characterized. Thus, characterization of sequence, expression, and biological function for additional phosphatases is informative. PTPBr7/PC12 and PTPSl are mouse receptor PTPs sharing similar amino acid sequences. Northern blot analysis demonstrated expression of these genes in adult rodent brain and revealed previously uncharacterized transcripts in the brain and other tissues. Our results demonstrate that PTPBr7/PC12 and PTPSl are members of a larger family of PTPs. We have identified two novel family members as well as several novel transcriptional splice variants from both human and mouse colon cDNA libraries. Expression analysis demonstrated that the various mRNA transcripts are differentially expressed, with the highest levels found in the brain, intestinal tract, uterus, and placenta. In situ hybridization analysis of mouse brain and intestinal tissues established that each isoform has a unique expression pattern in specific cell populations as well as in tissue regions. Furthermore, these restricted patterns suggest that the encoded family of phosphatases may play roles in modulating signal transduction pathways important for specific cell types and biological processes.

A novel regulatory mechanism of MAP kinases activation and nuclear translocation mediated by PKA and the PTP-SL tyrosine phosphatase

Protein tyrosine phosphatase PTP-SL retains mitogen-activated protein (MAP) kinases in the cytoplasm in an inactive form by association through a kinase interaction motif (KIM) and tyrosine dephosphorylation. The related tyrosine phosphatases PTP-SL and STEP were phosphorylated by the cAMP-dependent protein kinase A (PKA). The PKA phosphorylation site on PTP-SL was identified as the Ser(231) residue, located within the KIM. Upon phosphorylation of Ser(231), PTP-SL binding and tyrosine dephosphorylation of the MAP kinases extracellular signal-regulated kinase (ERK)1/2 and p38alpha were impaired. Furthermore, treatment of COS-7 cells with PKA activators, or overexpression of the Calpha catalytic subunit of PKA, inhibited the cytoplasmic retention of ERK2 and p38alpha by wild-type PTP-SL, but not by a PTP-SL S231A mutant. These findings support the existence of a novel mechanism by which PKA may regulate the activation and translocation to the nucleus of MAP kinases.

The mouse Ptprr gene encodes two protein tyrosine phosphatases, PTP-SL and PTPBR7, that display distinct patterns of expression during neural development

The protein tyrosine phosphatases PTP-SL and PTPBR7 differ only in the length of their N-terminal domain. We show here that PTP-SL and PTPBR7 are isoforms derived from a single gene (Ptprr) through developmentally regulated use of alternative promoters. Isoform-specific reverse transcriptase-polymer chain reaction (RT-PCR) and RNA in situ hybridization experiments reveal that PTPBR7 is expressed during early embryogenesis in spinal ganglia cells as well as in developing Purkinje cells. Post-natally, PTPBR7 is expressed in various regions of the adult mouse brain, but expression in Purkinje cells has ceased and is replaced by the PTP-SL-specific transcript. In transient transfection experiments it is confirmed that PTPBR7 is a type I transmembrane protein tyrosine phosphatase (PTPase). PTP-SL, however, appears to be a cytosolic membrane-associated PTPase that is located at perinuclear vesicular structures that partly belong to the endosomal compartment. Thus, during maturation of Purkinje cells, a gene-promoter switch results in the replacement of a receptor-type PTPase by a cytosolic vesicle-associated isoform.

Inactivation of mitogen-activated protein kinases by a mammalian tyrosine-specific phosphatase, PTPBR7

Mitogen-activated protein kinase (MAPK) is inactivated through dephosphorylation of tyrosyl and threonyl regulatory sites. In yeast, both dual-specificity and tyrosine-specific phosphatases are involved in dephosphorylation. In mammals, however, no tyrosine-specific phosphatase has been identified molecularly to dephosphorylate MAPK in vivo. Recently, we and others have cloned a murine tyrosine-specific phosphatase, PTPBR7/PTP-SL, which is expressed predominantly in the brain. Here we report inactivation of the extracellular signal-regulated kinase (ERK) family MAPK by PTPBR7. PTPBR7 made complexes with ERK1/ERK2 in vivo and dephosphorylated ERK1 in vitro. When overexpressed in mammalian cells, wild-type PTPBR7 suppressed the phosphorylation and activation of ERK by epidermal growth factor (EGF), nerve growth factor (NGF), and constitutively active MEK1, a mutant MAPK kinase. In contrast, catalytically inactive and ERK-binding-deficient mutants revealed little inhibition on the ERK cascade. These results indicate that PTPBR7 suppresses MAPK directly in vivo.

PTP-SL and STEP protein tyrosine phosphatases regulate the activation of the extracellular signal-regulated kinases ERK1 and ERK2 by association through a kinase interaction motif

Protein kinases and phosphatases regulate the activity of extracellular signal-regulated kinases 1 and 2 (ERK1/2) by controlling the phosphorylation of specific residues. We report the physical and functional association of ERK1/2 with the PTP-SL and STEP protein tyrosine phosphatases (PTPs). Upon binding, the N-terminal domains of PTP-SL and STEP were phosphorylated by ERK1/2, whereas these PTPs dephosphorylated the regulatory phosphotyrosine residues of ERK1/2 and inactivated them. A sequence of 16 amino acids in PTP-SL was identified as being critical for ERK1/2 binding and termed kinase interaction motif (KIM) (residues 224-239); it was shown to be required for phosphorylation of PTP-SL by ERK1/2 at Thr253. Co-expression of ERK2 with catalytically active PTP-SL in COS-7 cells impaired the EGF-induced activation of ERK2, whereas a PTP-SL mutant, lacking PTP activity, increased the ERK2 response to EGF. This effect was dependent on the presence of the KIM on PTP-SL. Furthermore, ERK1/2 activity was downregulated in 3T3 cells stably expressing PTP-SL. Our findings demonstrate the existence of a conserved ERK1/2 interaction motif within the cytosolic non-catalytic domains of PTP-SL and STEP, which is required for the regulation of ERK1/2 activity and for phosphorylation of the PTPs by these kinases. Our findings suggest that PTP-SL and STEP act as physiological regulators of the ERK1/2 signaling pathway.

Specific and reversible inactivation of protein tyrosine phosphatases by hydrogen peroxide: evidence for a sulfenic acid intermediate and implications for redox regulation

Protein tyrosine phosphatases (PTPs) catalyze the hydrolysis of phosphotyrosine from specific signal-transducing proteins. Although regulatory mechanisms for protein kinases have been described, no general mechanism for controlling PTPs has been demonstrated. Numerous reports have shown that cellular redox status plays an important role in tyrosine phosphorylation-dependent signal transduction pathways. This study explores the proposal that PTPs may be regulated by reversible reduction/oxidation involving cellular oxidants such as hydrogen peroxide (H2O2). Recent reports indicated that H2O2 is transiently generated during growth factor stimulation and that H2O2 production is concomitant with relevant tyrosine phosphorylation. By use of recombinant enzymes, the effects of H2O2 on three PTPs [PTP1, LAR (leukocyte antigen-related), and VHR (vaccinia H1-related)] and three distinct serine/threonine protein phosphatases (PPs: PP2Calpha, calcineurin, and lambda phosphatase) were determined. Hydrogen peroxide had no apparent effect on PP activity. In contrast, PTPs were rapidly inactivated (kinact = 10-20 M-1 s-1) with low micromolar concentrations of H2O2 but not with large alkyl hydroperoxides. PTP inactivation was fully reversible with glutathione and other thiols. Because of the slower rate of reduction, modification occurred even in the presence of physiological thiol concentrations. By utilization of a variety of biochemical techniques including chemical modification, pH kinetic studies, and mutagenesis, the catalytic cysteine thiolate of PTPs was determined to be the selective target of oxidation by H2O2. By use of the electrophilic reagent 7-chloro-4-nitrobenzo-2-oxa-1, 3-diazole (NBD-Cl), it was shown that a cysteine sulfenic acid intermediate (Cys-SOH) is formed after attack of the catalytic thiolate on H2O2. A chemical mechanism for reversible inactivation involving a cysteine sulfenic acid intermediate is proposed.

Nerve growth factor stimulates tyrosine phosphorylation and activation of Src homology-containing protein-tyrosine phosphatase 1 in PC12 cells

Rat PC12 cells respond to extracellular peptide growth factors in at least two distinct ways. When treated with nerve growth factor (NGF) PC12 cells exit the cell cycle and differentiate to a neuronal phenotype, whereas when treated with epidermal growth factor, they proliferate. We examined the potential role of Src homology 2 (SH2)-containing protein tyrosine phosphatases (PTPs) in the differentiation process. PC12 cells express substantial amounts of both SH-PTP1 and 2. SH-PTP1, but not SH-PTP2, becomes tyrosine phosphorylated following NGF, but not epidermal growth factor treatment. The enzymatic activity of SH-PTP1 toward an exogenous substrate following NGF treatment is increased 2-fold. We found that SH-PTP1 binds to the NGF receptor TrkA in vitro and that anti-TrkA immunoprecipitates have PTP activity. These results show that SH-PTP1 is differentially phosphorylated and activated by NGF in PC12 cells and suggest that this activation may play a role in NGF-induced differentiation.

Cloning and expression of PCPTP1 encoding protein tyrosine phosphatase

A novel cDNA encoding PTP (protein tyrosine phosphatase) was cloned from PC12h cells and designated as PCPTP1 (gene encoding PC12 protein Tyr phosphatase). The longest open reading frame (ORF) of this clone encodes a 656-amino-acid (aa) protein with a single PTP catalytic domain. Western blot analysis using a polyclonal Ab (antibody) raised against the cytoplasmic region of PCPTP1 detected two products, a major 65-kDa and minor 42-kDa protein, designated PCPTP1-MFI and PCPTP1-MVQ, respectively, in PC12h cells. These two proteins correspond to the products translated from the second and fifth methionine of PCPTP1, respectively. The bacterially expressed GST::PCPTP1-MVQ fusion protein had phosphatase activity with pNPP (p-nitrophenyl phosphate) as a substrate. Alignment of the aa sequence of PCPTP1-MVQ with those of other PTP showed the highest similarity to STEP and LC-PTP/HePTP, with 54 and 51% identity, respectively. Northern blot analysis showed only one 3.9-kb transcript in PC12h cells, indicating that PCPTP1 corresponds to this 3.9-kb transcript. The 3.9-kb PCPTP1 mRNA was detected in the brain and adrenal gland, but not in other non-neuronal tissues in adult rats. Two other transcripts of 3.3 and 1.7 kb were also detected in brain. NGF (nerve growth factor) and glucocorticoid are known to bimodally regulate the cell fate decision of sympathoadrenal precursors like PC12 cells, with NGF promoting the neuronal phenotype and glucocorticoid promoting the chromaffin phenotype. Still, both agents decreased the level of PCPTP1 mRNA in PC12h cells. Therefore, it is likely that the decrease in the level of PCPTP1 mRNA might be associated or correlated with cell differentiation in PC12h cells.

cDNA cloning and characterization of a novel receptor-type protein tyrosine phosphatase expressed predominantly in the brain

Protein tyrosine phosphatase has the potential to control various cellular events by negatively regulating the extent of tyrosine phosphorylation. Here, we report the isolation of a murine receptor protein tyrosine phosphatase, PTPBR7, which is expressed almost exclusively in the brain. Though the cytoplasmic portion of PTPBR7 reveals high similarity to HePTP/LC-PTP and STEP, these are, unlike PTPBR7, non-receptor protein tyrosine phosphatases. Unlike most receptor protein tyrosine phosphatases, PTPBR7 has only one cytoplasmic phosphatase domain, and its extracellular domain reveals no obvious structural similarity to known molecules. Thus, PTPBR7 defines a new subfamily of receptor-type protein tyrosine phosphatases. The putative extracellular domain of PTPBR7 was expressed in COS-7 cells as a chimeric fusion protein with an immunoglobulin Fc portion (PTPBR7-Fc). PTPBR7-Fc was secreted in the culture supernatant, confirming the capability of the extracellular domain of PTPBR7 to translocate across the cytoplasmic membrane. The cytoplasmic portion of PTPBR7 was expressed as a fusion protein in bacteria and was demonstrated to have catalytic activity. The expression of PTPBR7 was detectable in brain and especially in cerebellum but undetectable in liver, lung, heart, kidney, thymus, bone marrow, and spleen. In situ hybridization analysis revealed the most prominent signal in Purkinje cells. The predominant expression of PTPBR7 in the brain suggests that PTPBR7 may have role(s) in neuronal cells.

A novel receptor-type protein tyrosine phosphatase with a single catalytic domain is specifically expressed in mouse brain

Protein tyrosine phosphatases (PTPases) are important regulatory proteins that, together with protein tyrosine kinases, determine the phosphotyrosine levels in cell signalling proteins. By PCR amplification of mouse brain cDNA fragments encoding the catalytic domains of these enzymes, we identified three novel members of the PTPase gene family. Northern-blot analysis showed that two of these novel clones represent brain-specific PTPases, whereas the third originates from a large-sized mRNA that is more ubiquitously expressed. A full-length cDNA encoding one of the brain-specific PTPases, PTP-SL, was isolated. Sequence analysis revealed a transmembrane PTPase containing a single catalytic phosphatase domain that has 45% homology to a rat cytoplasmic brain-specific PTPase named STEP. This suggests a role for PTP-SL in cell-cell signalling processes in the brain.

A neuronal protein tyrosine phosphatase induced by nerve growth factor

A new protein tyrosine phosphatase (PC12-PTP1) was identified in nerve growth factor (NGF)-treated PC12 cells. The mRNA level of PC12-PTP1 is increased 9-fold over the initial 8 h of NGF treatment and then decreases dramatically after 24 h of treatment. In rat brain, three transcripts corresponding to 1.5, 2.6, and 3.0 kilobases (kb) in size are detected by Northern blot analysis. Although the 1.5- and 2.6-kb transcripts are present in brain and other tissues, the 3-kb transcript is exclusively expressed in brain and the expression of this transcript alone increases following NGF treatment. PC12-PTP1 is a non-receptor protein tyrosine phosphatase (PTP) with a 50% sequence homology in the phosphatase domain with several other non-receptor PTPs. PC12-PTP1 fusion protein exhibits tyrosine phosphatase activity, and in vitro translation of the PC12-PTP1 transcript produces a major protein of 39 kDa. The data presented suggest that NGF regulates the expression of PC12-PTP1 during periods of neuronal growth and differentiation.