Molecular cloning and chromosome mapping of the human gene encoding protein phosphotyrosyl phosphatase 1B

Inverse 15N-metabolic labeling/mass spectrometry for comparative proteomics and rapid identification of protein markers/targets

The inverse labeling/mass spectrometry strategy has been applied to protein metabolic (15)N labeling for gel-free proteomics to achieve the rapid identification of protein markers/targets. Inverse labeling involves culturing both the perturbed (by disease or by a drug treatment) and control samples each in two separate pools of normal and (15)N-enriched culture media such that four pools are produced as opposed to two in a conventional labeling approach. The inverse labeling is then achieved by combining the normal (14)N-control with the (15)N-perturbed sample, and the (15)N-control with the (14)N-perturbed sample. Both mixtures are then proteolyzed and analyzed by mass spectrometry (coupled with on-line or off-line separation). Inverse labeling overcomes difficulties associated with protein metabolic labeling with regard to isotopic peak correlation and data interpretation in the single-experiment approach (due to the non-predictable/variable mass difference). When two data sets from inverse labeling are compared, proteins of differential expression are readily recognized by a characteristic inverse labeling pattern or apparent qualitative mass shifts between the two inverse labeling analyses. MS/MS fragmentation data provide further confirmation and are subsequently used to search protein databases for protein identification. The methodology has been applied successfully to two model systems in this study. Utilizing the inverse labeling strategy, one can use any mass spectrometer of standard unit resolution, and acquire only the minimum, essential data to achieve the rapid and unambiguous identification of differentially expressed protein markers/targets. The strategy permits quick focus on the signals of differentially expressed proteins. It eliminates the detection ambiguities caused by the dynamic range of detection. Finally, inverse labeling enables the detection of covalent changes of proteins responding to a perturbation that one might fail to distinguish with a conventional labeling experiment.

Effects of adenovirus-mediated liver-selective overexpression of protein tyrosine phosphatase-1b on insulin sensitivity in vivo

AIM

Protein tyrosine phosphatase-1B (PTP-1B) is an intracellular PTP known to dephosphorylate and inactivate upstream tyrosine phosphoproteins in the insulin signalling cascade. We and others reported increased abundance of catalytically impaired PTP-1B in tissue lysates from obese human subjects with and without type 2 diabetes, while genetic knockout of PTP-1B improves insulin sensitivity and prevents nutritionally mediated insulin resistance and obesity. The aim of the present work was to further elucidate the role of PTP-1B in glucose metabolism in vivo.

METHODS

We used adenoviral constructs incorporating cDNAs for either wild-type (W/T) or a catalytically inactive C(215)S (C/S) mutant PTP-1B to achieve liver-selective PTP-1B overexpression in young Sprague-Dawley rats using tail vein injection, based on the high degree of hepatotropism of adenovirus 5 (Ad5). An Ad5-lacZ construct encoding beta-galactosidase was used as a control for viral effects alone. A hyperinsulinaemic euglycaemic clamp was used to study whole body glucose disposal and endogenous glucose production rates.

RESULTS

Control studies in HIRcB cells confirmed catalytic activity and inactivity of W/T and C/S respectively. Mean PTP-1B abundance was 2.24 +/- 0.02- and 2.33 +/- 0.04-fold of saline-treated control in liver lysates of W/T and C/S rats respectively. Liver selective overexpression was confirmed by analysis of tissue lysates from liver, fat and muscle tissues. Ad5 treatment did not result in a statistically or clinically significant liver injury, as determined by serum alanine aminotransferase and histological examination. Seven days post injection, no significant difference in rate of weight gain, fasting blood glucose or insulin levels were seen in any group. Similarly, under steady-state glucose clamp conditions, glucose disposal rate (R(d)), endogenous glucose production rate (EGP) and serum insulin levels were similar in all groups.

CONCLUSION

We conclude that moderate medium-term overabundance, to a degree resembling that seen in insulin-resistant states, of PTP-1B in liver tissue does not alter insulin action on glucose metabolism and that the major site of action of PTP-1B is presumably at insulin-responsive target tissue or tissues other than the liver.

Insulin stimulates tyrosine phosphorylation and inactivation of protein-tyrosine phosphatase 1B in vivo

Protein-tyrosine phosphatase (PTP) 1B has been implicated in negative regulation of insulin action, although little is known of the ability of insulin to regulate PTP1B itself. The ability of insulin to regulate phosphorylation and activation of PTP1B was probed in vivo. Challenge with insulin in vivo provoked a transient, sharp increase in the phosphotyrosine content of PTP1B in fat and skeletal muscle that peaked within 15 min. Insulin stimulated a decline of 60--70% in PTP1B activity. In mouse adipocytes, the inhibition of PTP1B activity and increased tyrosine phosphorylation of the enzyme were blocked by the insulin receptor tyrosine kinase inhibitor AG1024. Phosphoserine content of PTP1B declined in response to insulin stimulation. Elevation of intracellular cyclic AMP provokes a sharp increase in PTP1B activity and leads to increased phosphorylation of serine residues and decreased tyrosine phosphorylation. Suppression of cyclic AMP levels or inhibition of protein kinase A leads to a sharp decline in PTP1B activity, a decrease in phosphoserine content, and an increase in PTP1B phosphotyrosine content. PTP1B appears to be a critical point for insulin and catecholamine counter-regulation.

Attenuation of adhesion-dependent signaling and cell spreading in transformed fibroblasts lacking protein tyrosine phosphatase-1B

Previous biochemical evidence has yielded conflicting models for the role of protein tyrosine phosphatase-1B (PTP-1B) in the regulation of integrin signaling. Thus, to establish the physiological relevance for such a role, we employed a genetic approach by generating embryonic fibroblasts from PTP-1B knockout mice. Both primary fibroblasts and their derived cell lines were used in this study. Immortalization of wild-type primary cells with the SV40 Large T antigen resulted in a dramatic increase in the endogenous expression of PTP-1B, suggesting a role during transformation. Moreover, the absence of PTP-1B in the transformed cell lines led to a more pronounced effect on different pathways of fibronectin-mediated signaling compared with the untransformed state. Specifically, p130(Cas) phosphorylation, Erk activation as well as cell spreading were delayed in PTP-1B-deficient cells, compared with their wild-type counterparts. Interestingly, this attenuation in integrin-mediated events closely resembles that of Src-deficient fibroblasts. Indeed, PTP-1B deficient, transformed fibroblasts held in suspension do exhibit a hyperphosphorylation of the inhibitory site (Tyr-527) of Src, compared with their wild-type counterparts. These results establish PTP-1B as a positive physiological regulator of integrin signaling in transformed cells, acting upstream of Src Tyr-527 dephosphorylation that leads to several adhesion-dependent events.

The reciprocal role of Egr-1 and Sp family proteins in regulation of the PTP1B promoter in response to the p210 Bcr-Abl oncoprotein-tyrosine kinase

Protein-tyrosine phosphatase 1B (PTP1B) is an important regulator of protein-tyrosine kinase-dependent signaling pathways. Changes in expression and activity of PTP1B have been associated with various human diseases; however, the mechanisms by which PTP1B expression is regulated have yet to be characterized. Previously, we have shown that the expression of PTP1B is enhanced by p210 Bcr-Abl and that PTP1B is a specific antagonist of transformation induced by this oncoprotein protein-tyrosine kinase. Here we have characterized the PTP1B promoter and demonstrate that a motif with features of a stress-response element acts as a p210 Bcr-Abl-responsive sequence, termed PRS. We have shown that three C(2)H(2) zinc finger proteins, namely Sp1, Sp3, and Egr-1, bind to PRS. Whereas binding of either Sp1 or Sp3 induced promoter function, Egr-1 repressed Sp3-mediated PTP1B promoter activation. The binding of Egr-1 to PRS is suppressed by p210 Bcr-Abl due to the inhibition of Egr-1 expression, resulting in the enhancement of PTP1B promoter activity. Our data indicate that Egr-1 and Sp family proteins play a reciprocal role in the control of expression from the PTP1B promoter.

Genomic organization and alternative splicing of the human and mouse RPTPrho genes

BACKGROUND

Receptor protein tyrosine phosphatase rho (RPTPrho, gene symbol PTPRT) is a member of the type IIB RPTP family. These transmembrane molecules have been linked to signal transduction, cell adhesion and neurite extension. The extracellular segment contains MAM, Ig-like and fibronectin type III domains, and the intracellular segment contains two phosphatase domains. The human RPTPrho gene is located on chromosome 20q12-13.1, and the mouse gene is located on a syntenic region of chromosome 2. RPTPrho expression is restricted to the central nervous system.

RESULTS

The cloning of the mouse cDNA, identification of alternatively spliced exons, detection of an 8 kb 3'-UTR, and the genomic organization of human and mouse RPTPrho genes are described. The two genes are comprised of at least 33 exons. Both RPTPrho genes span over 1 Mbp and are the largest RPTP genes characterized. Exons encoding the extracellular segment through the intracellular juxtamembrane 'wedge' region are widely spaced, with introns ranging from 9.7 to 303.7 kb. In contrast, exons encoding the two phosphatase domains are more tightly clustered, with 15 exons spanning approximately 60 kb, and introns ranging in size from 0.6 kb to 13.1 kb. Phase 0 introns predominate in the intracellular, and phase 1 in the extracellular segment.

CONCLUSIONS

We report the first genomic characterization of a RPTP type IIB gene. Alternatively spliced variants may result in different RPTPrho isoforms. Our findings suggest that RPTPrho extracellular and intracellular segments originated as separate modular proteins that fused into a single transmembrane molecule during a later evolutionary period.

Combinatorial control of the specificity of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs), the enzymes that dephosphorylate tyrosyl phosphoproteins, were initially believed to be few in number and serve a 'housekeeping' role in signal transduction. Recent work indicates that this is totally incorrect. Instead, PTPs comprise a large superfamily whose members play critical roles in a wide variety of cellular processes. Moreover, PTPs exhibit exquisite substrate specificity in vivo. Recent evidence has led us to propose that members of the PTP family achieve selectivity through different combinations of specific targeting strategies and intrinsic catalytic domain specificity.

Analysis of in vitro interactions of protein tyrosine phosphatase 1B with insulin receptors

One strategy to treat the insulin resistance that is central to type II diabetes mellitus may be to maintain insulin receptors (IR) in the active (tyrosine phosphorylated) form. Because protein tyrosine phosphatase 1B (PTP1B) binds and subsequently dephosphorylates IR, inhibitors of PTP1B-IR binding are potential insulin 'sensitizers.' A Scintillation Proximity Assay (SPA) was developed to characterize and quantitate PTP1B-IR binding. Human IR were solubilized and captured on wheat germ agglutinin (WGA)-coated SPA beads. Subsequent binding of human, catalytically inactive [35S] PTP1B Cys(215)/Ser (PTP1B(C215S)) to the lectin-anchored IR results in scintillation from the SPA beads that can be quantitated. Binding of PTP1B to IR was pH- and divalent cation-sensitive. Ca(2+) and Mn(2+), but not Mg(2+), dramatically attenuated the loss of PTP1B-IR binding observed when pH was raised from 6.2 to 7.8. PTP1B binding to IR from insulin-stimulated cells was much greater than to IR from unstimulated cells and was inhibited by either an antiphosphotyrosine antibody or treatment of IR with alkaline phosphatase, suggesting that tyrosine phosphorylation of IR is required for PTP1B binding. Phosphopeptides modeled after various IR phosphotyrosine domains each only partially inhibited PTP1B-IR binding, indicating that multiple domains of IR are likely involved in binding PTP1B. However, competitive displacement of [35S]PTP1B(C215S) by PTP1B(C215S) fitted best to a single binding site with a K(d) in the range 100-1000 nM, depending upon pH and divalent cations. PNU-200898, a potent and selective inhibitor of PTP1B whose orientation in the active site of PTP1B has been solved, competitively inhibited catalysis and PTP1B-IR binding with equal potency. The results of this novel assay for PTP1B-IR binding suggest that PTP1B binds preferentially to tyrosine phosphorylated IR through its active site and that binding may be susceptible to therapeutic disruption by small molecules.

Genomic characterization of the human and mouse protein tyrosine phosphatase-1B genes

PTP-1B is a ubiquitously expressed intracellular protein tyrosine phosphatase (PTP) that has been implicated in the negative regulation of insulin signaling. Mice deficient in PTP-1B were found to have an enhanced insulin sensitivity and a resistance to diet-induced obesity. Interestingly, the human PTP-1B gene maps to chromosome 20q13.1 in a region that has been associated with diabetes and obesity. Although there has been a partial characterization of the 3' end of the human PTP-1B gene, the complete gene organization has not been described. In order to further characterize the PTP-1B gene, we have cloned and determined the genomic organization for both the human and mouse PTP-1B genes including the promoter. The human gene spans >74 kb and features a large first intron of >54 kb; the mouse gene likewise contains a large first intron, although the exact size has not been determined. The organization of the human and mouse PTP-1B genes is identical except for an additional exon at the 3' end of the human that is absent in the mouse. The mouse PTP-1B gene maps to the distal arm of mouse chromosome 2 in the region H2-H3. This region is associated with a mouse obesity quantitative trait locus (QTL) and is syntenic with human chromosome 20. The promoter region of both the human and mouse genes contain no TATA box but multiple GC-rich sequences that contain a number of consensus SP-1 binding sites. The basal activity of the human PTP-1B promoter was characterized in Hep G2 cells using up to 8 kb of 5' flanking sequence. A 432 bp promoter construct immediately upstream of the ATG was able to confer maximal promoter activity. Within this sequence, there are at least three GC-rich sequences and one CCAAT box, and deletion of any of these elements results in decreased promoter activity. In addition, the promoter in a number of mouse strains contains, 3.5 kb upstream of the start codon, an insertion of an intracisternal a particle (IAP) element that possibly could alter the expression of PTP-1B mRNA in these strains.

Protein tyrosine phosphatase-1B in diabetes

A role for protein tyrosine phosphatases in the negative regulation of insulin signaling and a putative involvement in the insulin resistance associated with type 2 diabetes have been postulated since their discovery. The recent demonstration that mice lacking the protein tyrosine phosphatase-1B (PTP-1B) have enhanced insulin sensitivity validates this. Furthermore, when fed a high fat diet, these mice maintained insulin sensitivity and were resistant to obesity, suggesting that inhibition of PTP-1B activity could be a novel way of treating type 2 diabetes and obesity. This commentary reviews our current knowledge of PTP-1B in insulin signaling and its role in diabetes and discusses the development of potent and selective PTP-1B inhibitors.

Expression of protein tyrosine phosphatases and its significance in esophageal cancer

Expression of mRNA protein tyrosine phosphatases (PTPs) was surveyed in an esophageal cancer cell line by RT-PCR using degenerate primers. The mRNAs for eight kinds of PTPs were expressed in the cell line. We examined mRNA expression of these PTPs in 12 cases of esophageal cancer by Northern analysis. Significant signals were obtained for three kinds of PTPs, PTP1B, PTPH1, and PTPD1. The magnitude of expression of each PTP was measured as the ratio of the signal intensity of each PTP to that of a control gene (NADPH), and the ratio was then compared to normal mucosa around the cancer lesion. Among the three kinds of PTPs, the expression of PTP1B mRNA was significantly depressed in cancer lesions compared with that in the surrounding normal mucosa. In contrast, the expression of PTPH1 mRNA was significantly increased in cancer lesions compared with that in normal mucosa. PTPD1 did not show any significant trend in comparisons of cancer and surrounding normal mucosa. The results suggest that PTP1B and PTPH1 are engaged in opposing signaling pathways, the tumor-suppressive and tumor-promoting pathways, respectively, in esophageal carcinogenesis.

Differential regulation of the cell wall integrity mitogen-activated protein kinase pathway in budding yeast by the protein tyrosine phosphatases Ptp2 and Ptp3

Mitogen-activated protein kinases (MAPKs) are inactivated by dual-specificity and protein tyrosine phosphatases (PTPs) in yeasts. In Saccharomyces cerevisiae, two PTPs, Ptp2 and Ptp3, inactivate the MAPKs, Hog1 and Fus3, with different specificities. To further examine the functions and substrate specificities of Ptp2 and Ptp3, we tested whether they could inactivate a third MAPK, Mpk1, in the cell wall integrity pathway. In vivo and in vitro evidence indicates that both PTPs inactivate Mpk1, but Ptp2 is the more effective negative regulator. Multicopy expression of PTP2, but not PTP3, suppressed growth defects due to the MEK kinase mutation, BCK1-20, and the MEK mutation, MKK1-386, that hyperactivate this pathway. In addition, deletion of PTP2, but not PTP3, exacerbated growth defects due to MKK1-386. Other evidence supported a role for Ptp3 in this pathway. Expression of MKK1-386 was lethal in the ptp2Delta ptp3Delta strain but not in either single PTP deletion strain. In addition, the ptp2Delta ptp3Delta strain showed higher levels of heat stress-induced Mpk1-phosphotyrosine than the wild-type strain or strains lacking either PTP. The PTPs also showed differences in vitro. Ptp2 was more efficient than Ptp3 at binding and dephosphorylating Mpk1. Another factor that may contribute to the greater effectiveness of Ptp2 is its subcellular localization. Ptp2 is predominantly nuclear whereas Ptp3 is cytoplasmic, suggesting that active Mpk1 is present in the nucleus. Last, PTP2 but not PTP3 transcript increased in response to heat shock in a Mpk1-dependent manner, suggesting that Ptp2 acts in a negative feedback loop to inactivate Mpk1.

The next wave: protein tyrosine phosphatases enter T cell antigen receptor signalling

Recent years have seen an exponentially increasing interest in the molecular mechanisms of signal transduction. Much of the focus has been on protein tyrosine kinase-mediated signalling, while the study of protein tyrosine phosphatases has lagged behind. We predict that the phosphatases will become a "hot topic" in the field within the next few years. This review summarizes the current state-of-the-art in our understanding of the structure, regulation and role of protein tyrosine phosphatases in T lymphocyte activation.

Insulin-inducible changes in the relative ratio of PTP1B splice variants

The skeletal muscle activity of protein tyrosine phosphates 1B (PTP1B), a modulator of insulin and IGF-1 signaling, is reduced in obese nondiabetic subjects and in subjects with type 2 diabetes in comparison with leaner, nondiabetic controls. PTP1B mRNA, like many other signaling molecules, including the insulin receptor, is alternatively spliced. Since we have shown that the ratio of the insulin receptor splice variants is modulated by insulin in vitro and is related to insulin levels in vivo, we hypothesized that the relative ratios of the alternatively spliced PTP1B mRNA might also vary in humans in proportion to the degree of hyperinsulinemia. This was tested in 21 nondiabetic Pima Indians, a population at increased risk for obesity and type 2 diabetes. The relative ratio of the PTP1B splice variants was quantified using RT-PCR of total RNA extracted from fractionated monocytes. The ratio of the splice variants was positively correlated with fasting plasma insulin concentration (r = 0.757; P = 0.0001), 2-h plasma insulin concentration following an oral glucose tolerance test (r = 0.614; P = 0.01, n = 16), and percentage of body fat (r = 0.746; P = 0.0001). These data indicate that variability in the ratio of the two splice variants is due, in part, to in vivo levels of chronic hyperinsulinemia. This simple, noninvasive assay is therefore a potential biomarker for chronic hyperinsulinemia, similar to the HbAlc assay in use to monitor glucose management in diabetic patients.

Retention of cytochrome b5 in the endoplasmic reticulum is transmembrane and luminal domain-dependent

Cytochrome b5 (b5), a typical tail-anchored protein of the endoplasmic reticulum (ER) membrane, is composed of three functionally different domains: amino-terminal heme-containing catalytic, central hydrophobic membrane-anchoring, and carboxyl-terminal ER-targeting domains (Mitoma, J., and Ito, A. (1992) EMBO J. 11, 4197-4203). To analyze the potential retention signal of b5, mutant proteins were prepared to replace each domain with natural or artificial sequences, and subcellular localizations were examined using immunofluorescence microscopy and cell fractionation. The transmembrane domain functioned to retain the cytochrome in the ER, and the mutation of all or part of the transmembrane domain with an artificial hydrophobic sequence had practically no effect on intracellular distribution of the cytochrome. However, when the transmembrane domain was extended systematically, a substantial portion of the protein with the domain of over 22 amino acid residues leaked from the organelle. Thus, the transmembrane length functions as the retention signal. When cytochromes with mutations at the carboxyl-terminal end were overexpressed in cells, a substantial portion of the protein was transported to the plasma membrane, indicating that the carboxyl-terminal luminal domain also has a role in retention of b5 in the ER. Carbohydrate moiety of the glycosylatably-mutated b5 was sensitive to endoglycosidase H but resistant to endoglycosidase D. Therefore, both transmembrane and carboxyl-terminal portions seems to function as the static retention signal.

Protein tyrosine phosphatase 1B antagonizes signalling by oncoprotein tyrosine kinase p210 bcr-abl in vivo

The p210 bcr-abl protein tyrosine kinase (PTK) appears to be directly responsible for the initial manifestations of chronic myelogenous leukemia (CML). In contrast to the extensive characterization of the PTK and its effects on cell function, relatively little is known about the nature of the protein tyrosine phosphatases (PTPs) that may modulate p210 bcr-abl-induced signalling. In this study, we have demonstrated that expression of PTP1B is enhanced specifically in various cells expressing p210 bcr-abl, including a cell line derived from a patient with CML. This effect on expression of PTP1B required the kinase activity of p210 bcr-abl and occurred rapidly, concomitant with maximal activation of a temperature-sensitive mutant of the PTK. The effect is apparently specific for PTP1B since, among several PTPs tested, we detected no change in the levels of TCPTP, the closest relative of PTP1B. We have developed a strategy for identification of physiological substrates of individual PTPs which utilizes substrate-trapping mutant forms of the enzymes that retain the ability to bind to substrate but fail to catalyze efficient dephosphorylation. We have observed association between a substrate-trapping mutant of PTP1B (PTP1B-D181A) and p210 bcr-abl, but not v-Abl, in a cellular context. Consistent with the trapping data, we observed dephosphorylation of p210 bcr-abl, but not v-Abl, by PTP1B in vivo. We have demonstrated that PTP1B inhibited binding of the adapter protein Grb2 to p210 bcr-abl and suppressed p210 bcr-abl-induced transcriptional activation that is dependent on Ras. These results illustrate selectivity in the effects of PTPs in a cellular context and suggest that PTP1B may function as a specific, negative regulator of p210 bcr-abl signalling in vivo.

Epidermal growth factor receptor and the adaptor protein p52Shc are specific substrates of T-cell protein tyrosine phosphatase

T-cell protein tyrosine phosphatase (TCPTP) exists as two forms generated by alternative splicing: a 48-kDa endoplasmic reticulum (ER)-associated form (TC48) and a 45-kDa nuclear form (TC45). To identify TCPTP substrates, we have generated substrate-trapping mutants, in which the invariant catalytic acid of TCPTP (D182) is mutated to alanine. The TCPTP D182A substrate-trapping mutants were transiently overexpressed in COS cells, and their ability to form complexes with tyrosine-phosphorylated (pTyr) proteins was assessed. No pTyr proteins formed complexes with wild-type TCPTP. In contrast, TC48-D182A formed a complex in the ER with pTyr epidermal growth factor receptor (EGFR). In response to EGF, TC45-D182A exited the nucleus and accumulated in the cytoplasm, where it bound pTyr proteins of approximately 50, 57, 64, and 180 kDa. Complex formation was disrupted by vanadate, highlighting the importance of the PTP active site in the interaction and supporting the characterization of these proteins as substrates. Of these TC45 substrates, the approximately 57- and 180-kDa proteins were identified as p52Shc and EGFR, respectively. We examined the effects of TC45 on EGFR signaling and observed that it did not modulate EGF-induced activation of p42Erk2. However, TC45 inhibited the EGF-induced association of p52Shc with Grb2, which was attributed to the ability of the PTP to recognize specifically p52Shc phosphorylated on Y239. These results indicate that TC45 recognizes not only selected substrates in a cellular context but also specific sites within substrates and thus may regulate discrete signaling events.

The noncatalytic C-terminal segment of the T cell protein tyrosine phosphatase regulates activity via an intramolecular mechanism

Human T cell protein tyrosine phosphatase (TCPTP) is a nontransmembrane enzyme, the first of the protein tyrosine phosphatase family to be cloned. Alternative mRNA splicing results in variation in the sequence at the extreme C terminus of TCPTP and generates a 45-kDa form (TC45) that is targeted to the nucleus and a 48-kDa variant (TC48) associated with membranes of the endoplasmic reticulum. In this report, we assessed the role of the C-terminal, noncatalytic segment of TCPTP in regulating activity, concentrating primarily on the TC45 variant. We have demonstrated that limited tryptic proteolysis of TC45 releases first a 42-kDa fragment, then a 33-kDa catalytic domain. Using reduced carboxyamidomethylated and maleylated lysozyme as substrate (RCML), the catalytic domain displays 20-100-fold more activity than the full-length enzyme. Analysis of the time course of limited trypsinolysis revealed that proteolytic activation occurred following cleavage of a protease-sensitive region (residues 353-387) located at the C terminus of TC45. The activity of truncation mutants illustrated that removal of 20 C-terminal residues was sufficient to activate the enzyme fully. The 33-kDa catalytic domain, but not the full-length enzyme, was inhibited in a concentration-dependent manner by addition of the noncatalytic C-terminal segment of TC45. A monoclonal antibody to TCPTP, CF4, which recognizes an epitope located between residues 350 and 363, was capable of fully activating TC45. These data indicate that the noncatalytic segment of TC45 contains an autoregulatory site that modulates activity via a reversible intramolecular interaction with the catalytic domain. These studies suggest that the C-terminal noncatalytic segment of TC45, and possibly TC48, may not only direct the enzyme to different subcellular locations but may also modulate activity in response to the binding of regulatory proteins and/or posttranslational modification.

Protein tyrosine phosphatase 1B interacts with and is tyrosine phosphorylated by the epidermal growth factor receptor

We used a substrate-trapping technique to search for substrates of protein tyrosine phosphatase (PTP) 1B. A catalytically inactive form of this enzyme forms a stable, phosphotyrosine-dependent complex with epidermal growth factor receptor (EGFR) both in vitro and in cells. PTP1B also interacts with activated platelet-derived growth factor receptor (PDGFR) but not with colony-stimulating factor 1 receptor (CSF-1R). After binding to EGFR, PTP1B becomes tyrosine-phosphorylated at Tyr-66, a site that conforms to the consensus binding sequence for the Src homology 2 (SH2) domains of the adapter protein Grb2. This tyrosine phosphorylation is correlated with a 3-fold increase in PTP catalytic activity. These findings suggest that PTP1B selectively regulates specific activated receptor protein tyrosine kinases (RPTKs) in vivo and might itself be regulated by such receptors.

Development of "substrate-trapping" mutants to identify physiological substrates of protein tyrosine phosphatases

The identification of substrates of protein tyrosine phosphatases (PTPs) is an essential step toward a complete understanding of the physiological function of members of this enzyme family. PTPs are defined by a conserved catalytic domain harboring 27 invariant residues. From a mutagenesis study of these invariant residues that was guided by our knowledge of the crystal structure of PTP1B, we have discovered a mutation of the invariant catalytic acid (Asp-181 in PTP1B) that converts an extremely active enzyme into a "substrate trap." Expression of this D181A mutant of PTP1B in COS and 293 cells results in an enzyme that competes with endogenous PTP1B for substrates and promotes the accumulation of phosphotyrosine primarily on the epidermal growth factor (EGF) receptor as well as on proteins of 120, 80, and 70 kDa. The association between the D181A mutant of PTP1B and these substrates was sufficiently stable to allow isolation of the complex by immunoprecipitation. As predicted for an interaction between the substrate-binding site of PTP1B and its substrates, the complex is disrupted by vanadate and, for the EGF receptor, the interaction absolutely requires receptor autophosphorylation. Furthermore, from immunofluorescence studies, the D181A mutant of PTP1B appeared to retain the endogenous EGF receptor in an intracellular complex. These results suggest that the EGF receptor is a bona fide substrate for PTP1B in vivo and that one important function of PTP1B is to prevent the inappropriate, ligand-independent, activation of newly synthesized EGF receptor in the endoplasmic reticulum. This essential catalytic aspartate residue is present in all PTPs and has structurally equivalent counterparts in the dual-specificity phosphatases and the low molecular weight PTPs. Therefore we anticipate that this method may be widely applicable to facilitate the identification of substrates of other members of this enzyme family.

Protein-tyrosine phosphatase 1B complexes with the insulin receptor in vivo and is tyrosine-phosphorylated in the presence of insulin

In response to insulin, protein-tyrosine phosphatase 1B (PTPase 1B) dephosphorylates 95- and 160-180-kDa tyrosine phosphorylated (PY) proteins (Kenner, K. A., Anyanwu, E., Olefsky, J. M., and Kusari, J. (1996) J. Biol. Chem. 271, 19810-19816). To characterize these proteins, lysates from control and insulin-treated cells expressing catalytically inactive PTPase 1B (CS) were immunoadsorbed and subsequently immunoblotted using various combinations of phosphotyrosine, PTPase 1B, and insulin receptor (IR) antibodies. Anti-PTPase 1B antibodies coprecipitated a 95-kDa PY protein from insulin-stimulated cells, subsequently identified as the IR beta-subunit. Similarly, anti-IR antibodies coprecipitated the 50-kDa PY-PTPase 1B protein from insulin-treated cells. To identify PTPase 1B tyrosine (Tyr) residues that are phosphorylated in response to insulin, three candidate sites (Tyr66, Tyr152, and Tyr153) were replaced with phenylalanine. Replacing Tyr66 or Tyr152 and Tyr153 significantly reduced insulin-stimulated PTPase 1B phosphotyrosine content, as well as its association with the IR. Studies using mutant IRs demonstrated that IR autophosphorylation is necessary for the PTPase 1B-IR interaction. These results suggest that PTPase 1B complexes with the autophosphorylated insulin receptor in intact cells, either directly or within a complex involving additional proteins. The interaction requires multiple tyrosine phosphorylation sites within both the receptor and PTPase 1B.

Expression of cell regulatory proteins in ovarian borderline tumors

BACKGROUND

Tumors of borderline malignancy are still a controversial subgroup of ovarian neoplasms. The expression of several cell regulatory proteins was studied to characterize the molecular phenotype of these tumors, and to compare them with their benign and malignant counterparts.

METHODS

Specimens from 22 patients with tumors of borderline malignancy (11 serous and 11 mucinous tumors), 12 patients with benign tumors, and 16 patients with invasive ovarian carcinomas were evaluated for expression of epidermal growth factor receptor (EGFR), HER-2/neu, PTP1B, and p53 by immunohistochemical techniques.

RESULTS

One or both of the tyrosine kinase growth factor receptors EGFR and HER-2/neu was expressed by 42% of benign, 59% of borderline, and 81% of malignant ovarian tumors. EGFR was expressed in a significantly greater fraction of malignant lesions (69%) than borderline lesions (18%) (P< 0.004). EGFR expression was not observed among the 11 mucinous borderline tumors. HER-2/neu was expressed by 50% of borderline tumors and was not a marker for malignancy. The tyrosine phosphatase PTP1B was expressed by a similar fraction of benign (17%), borderline (27%), and malignant (19%) tumors. The number of cases studied precluded correlation of kinase and phosphatase activity. However, among 12 tumors with PTP1B expression, 9 also expressed EGFR or HER-2/neu. Overexpression of p53 was observed only in malignant serous tumors and was not found in malignant mucinous, borderline, or benign lesions.

CONCLUSIONS

Either EGFR or HER-2/neu was detected in a majority of borderline cancers. PTP1B was present only in a minority of these cancers. Frankly malignant serous lesions differed from borderline and benign tumors with regard to expression of EGFR and overexpression of p53.

Alternative splicing generates four different forms of a non-transmembrane protein tyrosine phosphatase mRNA

PTP-S is a widely expressed non-transmembrane protein tyrosine phosphatase (PTPase), which binds to DNA in vitro. The cellular PTP-S gene product is present mainly in the nucleus in association with chromatin. cDNAs related to PTP-S have been described from human and mouse cells. To establish the origin of molecular diversity in these cDNAs, genomic clones of rat PTP-S were isolated that span over 40 kb of the gene and contain 7 axons. The exon-intron splice sites in the catalytic domain are conserved between PTP-S and human PTP1B. Sequences specific to and homologous to human T-cell PTPase (TC-PTP) were found in the genomic clones of PTP-S, which are expressed in rat cells, as determined by using a specific probe and Northern blot analysis. Analysis of RNA from different rat tissues by reverse transcription-polymerase chain reaction (RT-PCR) showed the presence of four different forms of PTP-S mRNA (named PTP-S1, PTP-S2, PTP-S3, and PTP-S4). PTP-S1 is same as PTP-S reported previously by us. PTP-S2, which is the major form, differs from PTP-S1 in having additional 19 amino acids corresponding to exon E1. PTP-S4 is similar to human T-cell phosphatase. PTP-S3 differs from PTP-S4 in having a deletion of 19 amino acids corresponding to exon E1. Our results suggest that four different forms of PTP-S mRNA arise from a single gene by differential splicing. Two of these forms, PTP-S1 and PTP-S3, were not found in human cells, possibly due to the loss of an internal splice acceptor site in one of the exons, suggesting the occurrence of species-specific splicing in this gene.

Selective down-regulation of the insulin receptor signal by protein-tyrosine phosphatases alpha and epsilon

Binding of insulin to its receptor (IR) causes rapid autophosphorylation with concomitant activation of its tyrosine kinase which transmits the signal by phosphorylating cellular substrates. The IR activity is controlled by protein-tyrosine phosphatases, but those directly involved in regulating the insulin receptor and its signaling pathways have not yet been identified. Using baby hamster kidney cells overexpressing the IR and a novel insulin-based selection principle, we established stable cell lines with functionally coupled expression of the IR and protein-tyrosine phosphatases. The two closely related protein-tyrosine phosphatases alpha and epsilon were identified as negative regulators of IR tyrosine kinase.

Gene expressions of protein tyrosine phosphatases in regenerating rat liver and rat ascites hepatoma cells

mRNA levels for ten protein tyrosine phosphatases (PTPs), PTP-S, PTPH1, PTP-1, GLEPP1, LRP, PTP1D, PTPG1, PTP gamma, PTP delta, and LAR, were determined during regeneration of rat liver, and mRNA levels for 5 PTPs, PTP-S, PTP-1, PTP gamma, PTP delta, and LRP, were determined in three lines of rat ascites hepatoma cells. In regenerating rat liver, the expression patterns of PTP genes after partial hepatectomy could be classified into four groups. In group 1 (PTP-S and PTPH1), the mRNA levels increased rapidly, reached a maximum 7 h after partial hepatectomy, remained at a plateau for 1-2 days and then decreased gradually. In group 2 (PTP-1, GLEPP1, and LRP), the mRNA levels showed two peaks on days 1 and 5, and then decreased gradually. In group 3 (PTP1D and PTPG1), the mRNA levels increased rapidly, reached a maximum at 7 h, remained high for several days, and then did not decrease but rather increased after day 7. In group 4 (PTP gamma, PTP delta, and LAR), the mRNA levels remained constant for the first 5 days and increased over the control levels after day 7. In rat ascites hepatomas, gene expression of non-receptor-like PTPs (PTP-S and PTP-1) showed various neoplastic alterations, whereas mRNAs of receptor-like PTPs (PTP gamma, PTP delta, and LRP) were lost or drastically decreased.

Characterization of 20q deletions in patients with myeloproliferative disorders or myelodysplastic syndromes

Deletions of the long arm of chromosome 20 are associated with several myeloid malignancies. We have analyzed the structure of the del(20q) in 30 patients and two cell lines. Twenty-one of the patients presented with a myeloproliferative disorder and nine with a myelodysplastic syndrome. Two categories of deletions were identified. Eighteen patients had a large deletion with loss of both G(+) bands from the long arm of chromosome 20. Twelve patients had small deletions with loss of one G(+) band from the long arm of chromosome 20. A chromosome paint was generated from a del 20q marker carrying a small deletion. This probe was hybridized to normal metaphases (reverse chromosome painting) and also to metaphases from patients with a del 20q (comparative reverse chromosome painting). All six small deletions analyzed were characterized by loss of the proximal G(+) band (q12) and retention of the distal G(+) band (q13.2). These data define a minimal deleted region extending from 20q11.2-20q13.1.

Thirteen genes (Cebpb, E2f1, Tcf4, Cyp24, Pck1, Acra4, Edn3, Kcnb1, Mc3r, Ntsr, Cd40, Plcg1 and Rcad) that probably lie in the distal imprinting region of mouse chromosome 2 are not monoallelically expressed

Seven imprinted genes are currently known in the mouse but none have been identified yet in the distal imprinting region of mouse Chromosome (Chr) 2, a region which shows striking linkage conservation with human chromosome 20q13. Both maternal duplication/paternal deficiency and its reciprocal for distal Chr 2 lead to mice with abnormal body shapes and behavioural abnormalities. We have tested a number of candidate genes, that are either likely or known to lie within the distal imprinting region, for monoallelic expression. These included 3 genes (Cebpb, E2f1 and Tcf4) that express transcription factors, 2 genes (Cyp24 and Pck1) that are involved in growth, 5 genes (Acra4, Edn3, Kcnb1, Mc3r and Ntsr) where a defect could lead to neurological and probably behavioural problems, and 3 genes (Cd40, Plcg1 and Rcad) that are less obvious candidates but sequence information was available for designing primers to test their expression. On/off expression of each gene was tested by reverse transcription-polymerase chain reaction (RT-PCR) analysis of RNA extracted from tissues of mice with maternal duplication/paternal deficiency and its reciprocal for the distal region of Chr 2. None of the 13 genes is monoallelically expressed in the appropriate tissues before and shortly after birth which suggests that these genes are not imprinted later in development. This study has narrowed down the search for imprinted genes, and valuable information on which genes have been tested for on/off expression is provided. Since there is considerable evidence of conservation of imprinting between mouse and human, we would predict that the 13 genes are not imprinted in human. Five of the genes: E2f1, Tcf4, Kcnb1, Cd40 and Rcad, have not yet been mapped in human. However, because of the striking linkage conservation observed between mouse Chr 2 and human chromosome 20, we would expect these genes to map on human chromosome 20q13.

Protein tyrosyl phosphatase-1B is expressed by normal human epidermis, keratinocytes, and A-431 cells and dephosphorylates substrates of the epidermal growth factor receptor

In the epidermis tyrosine kinases such as those found in the epidermal growth factor receptor (EGF-R) phosphorylate regulatory molecules on tyrosine and play an important role in controlling epidermal growth. Phosphotyrosyl phosphatases (PTPase) that dephosphorylate EGF-R and other proteins phosphorylated on tyrosine must also play an important role in controlling epidermal growth. The presence and metabolism of one such PTPase, PTP-1B, was detected and studied in human skin using biochemical, immunochemical, and molecular biologic methods. The message for PTP-1B was expressed in human epidermis, in keratinocytes cultured from human epidermis, and in human keratinocyte cell lines. The 49-kDa but not the 37-kDa form of PTP-1B was identified in membranes prepared from these cells and tissues by immunodetection on Western blots. Nearly all of the labeled proteins identified by gel electrophoresis of an A-431 particulate fraction phosphorylated with [gamma-32P] ATP in the presence of epidermal growth factor are substrates for PTP-1B because their labeling decreased after incubation with a catalytically active and purified PTP-1B fusion protein. Immunohistochemical methods were used to show that PTP-1B was primarily localized to the basal cell layers in normal thick epidermis. The presence of PTP-1B in intact human epidermis suggests that this molecule is not an artifact limited to cultured cells but is an important molecule in the in vivo regulation of epidermal functions.

Molecular cloning and expression of a unique receptor-like protein-tyrosine-phosphatase in the leucocyte-common-antigen-related phosphate family

Protein-tyrosine-phosphatases (PTPases) have been implicated in the regulation of certain tyrosine kinase growth factor receptors in that they dephosphorylate the activated (autophosphorylated) form of the receptors. In order to identify PTPases that potentially act on receptor targets in liver, we used the human leucocyte common antigen-related PTPase (LAR) cDNA [Streuli, Krueger, Hall, Schlossman and Saito (1988) J. Exp. Med. 168, 1523-1530] and isolated two closely related transmembrane PTPase homologues from a rat hepatic cDNA library. Both PTPases had large extracellular domains that contained three immunoglobulin-like repeats and eight type-III fibronectin repeats. Both enzymes had tandem homologous PTPase domains following a single hydrophobic transmembrane domain. One sequence encoded the rat homologue of LAR. The second PTPase, designated LAR-PTP2, had 79 and 90% identity with rat LAR in the respective cytoplasmic PTPase domains, with only 57% sequence similarity in the extracellular domain. The catalytic domains of LAR and LAR-PTP2 prepared by bacterial expression were active in dephosphorylating a variety of phosphotyrosyl substrates but did not hydrolyse phosphoserine or phosphothreonine residues of labelled casein. Both enzymes exhibited rapid turnover numbers of 4-7 s-1 for myelin basic protein and 78-150 s-1 for derivatized lysozyme. LAR and LAR-PTP2 displayed similar PTPase activity towards the simultaneous dephosphorylation of receptors of intact insulin and epidermal growth factor from liver membranes. These data indicate that there is a family of LAR-related PTPases that may regulate the phosphorylation state of receptor tyrosine kinases in liver and other tissues.

Expression, purification and crystallization of human phosphotyrosine phosphatase 1B

Protein phosphotyrosine phosphatases are believed to be involved in the regulation of the activity of cellular proteins, such as receptor tyrosine kinases, by controlling their phosphorylation status. One of the best described and characterized protein of this class of enzymes is the phosphotyrosine phosphatase 1B. To obtain sufficient quantities for structural investigations, truncated forms of PTP1B encompassing the catalytic domain were over-expressed in Escherichia coli and purified to apparent homogeneity by conventional chromatography. The activity of these purified enzymes has been compared with the wild-type enzyme expressed in mammalian cells. By measuring the activities against p-nitrophenyl phosphate, the pH dependence of this activity, and responses to different modulators, it could be demonstrated that the truncated forms of PTP1B retained the same characteristics as the full-length mammalian enzyme, but are not subject to inhibition of enzymic activity mediated by the C-terminus. Due to their improved solubility, it can be assumed that the catalytic domains are advantageous for crystallization studies in comparison to the natural enzyme. In a screening for crystallization conditions, we obtained protein crystals indicating that the quality of the purified protein is sufficient for crystallographic studies.

KAP: a dual specificity phosphatase that interacts with cyclin-dependent kinases

The cyclin-dependent kinases are key cell cycle regulators whose activation is required for passage from one cell cycle phase to the next. In mammalian cells, CDK2 has been implicated in control of the G1 and S phases. We have used a two-hybrid protein interaction screen to identify cDNAs encoding proteins that can interact with CDK2. Among those identified was a protein (KAP), which contained the HCXX-XXGR motif characteristic of protein tyrosine phosphatases. KAP showed phosphatase activity toward substrates containing either phosphotyrosine or phosphoserine residues. Since KAP is not significantly similar to known phosphatases beyond the catalytic core motif, it represents an additional class of dual specificity phosphatase. KAP interacted with cdc2 and CDK2 in yeast. In mammalian cells, KAP also associated with cdc2 and CDK2 but showed a preference for cdc2. The ability of KAP to bind multiple cyclin-dependent kinases suggests that it may play a role in cell cycle regulation.

Induction of protein-tyrosine phosphatase LC-PTP by IL-2 in human T cells. LC-PTP is an early response gene

Tyrosine phosphorylation has been implicated in interleukin 2 (IL-2)-induced growth signaling and the phosphorylation levels are regulated by the balance of tyrosine kinase and tyrosine phosphatase activities. Here, we demonstrate the rapid activation of a leukocyte tyrosine phosphatase LC-PTP (HePTP) gene expression by IL-2 in an IL-2 dependent human T cell ILT-Mat. Accumulation of LC-PTP mRNA appeared at 1 h and peaked at 6 h after IL-2 stimulation, simultaneous with the G1 to early S phase, and the induction of LC-PTP mRNA did not require protein synthesis. LC-PTP protein increased approximately 6-fold at 8 h after IL-2 stimulation. Nuclear run-on assays showed that the induction of LC-PTP mRNA expression is mostly due to transcriptional activation. These data suggest that LC-PTP is an early response gene and its protein seems to be a crucial molecule which regulates the tyrosine phosphorylation level during T cell proliferation.

Comparison of the biochemical and biological functions of tyrosine phosphatases from fission yeast, budding yeast and animal cells

In a previous communication, we have shown that two protein tyrosine tyrosine phosphatases (PTPases) from fission yeast, pyp1+ and pyp2+, act as novel inhibitors of mitosis upstream of the wee1+/mik1+ pathway (Ottilie et al., 1992). Here we describe that both genes possess intrinsic PTPase activity as judged by in vitro PTPase assays using 32P-labeled Raytide as a substrate, and that 32P-labeled p107wee1 is an in vitro substrate for pyp1. To compare the biological activity of pyp1 and pyp2 to that of other known PTPases, we expressed the budding yeast PTP1 and human placental phosphatase 1B (PTP1B) genes in either a cdc25-22 or wee1-50 genetic background and established that, in contrast to pyp1+ and pyp2+, Saccharomyces cerevisiae PTP1 and human PTP1B complement the cdc25 mutant, opposing the wee1+/mik1+ pathway.

Receptor protein tyrosine phosphatase alpha activates pp60c-src and is involved in neuronal differentiation

Here we report that protein tyrosine phosphatases (PTPases), like their enzymatic counterpart the protein tyrosine kinases, can play an important role in cell differentiation. Expression of the transmembrane PTPase receptor protein tyrosine phosphatase alpha (RPTP alpha) is transiently enhanced during neuronal differentiation of embryonal carcinoma (EC) and neuroblastoma cells. Retinoic acid induces wild type P19 cells to differentiate into endoderm- and mesoderm-like cells. By contrast, retinoic acid treatment leads to neuronal differentiation of P19 cells, ectopically expressing functional RPTP alpha, as illustrated by their ability to generate action potentials. Endogenous pp60c-src kinase activity is enhanced in the RPTP alpha-transfected cells, which may be due to direct dephosphorylation of the regulatory Tyr residue at position 527 in pp60c-src by RPTP alpha. Our results demonstrate that RPTP alpha is involved in neuronal differentiation and imply a role for pp60c-src in the differentiation process.

Binding of a protein-tyrosine phosphatase to DNA through its carboxy-terminal noncatalytic domain

The noncatalytic domain of a non-receptor-type protein-tyrosine phosphatase (the T-cell phosphatase or PTP-S) isolated from a rat spleen cDNA library shows homology with the basic domains of transcription factors Fos and Jun [Swarup, G., Kamatkar, S., Radha, V., & Rema, V. (1991) FEBS Lett. 280,65-69]. We have expressed this phosphatase in Escherichia coli under the control of T7 promoter. The PTP-S gene product expressed in E. coli shows protein-tyrosine phosphatase activity and binds to DNA at pH 7.4 as determined by DNA affinity chromatography, Southwestern blotting, and gel retardation methods. The carboxy-terminal region of this phosphatase was fused with glutathione S-transferase by constructing expression vectors. Experiments using fusion proteins with glutathione S-transferase suggest that the carboxy-terminal 57 amino acids of PTP-S are sufficient for DNA binding. Deletion of the C-terminal 57 amino acids of PTP-S protein abolished its DNA binding property, as determined by Southwestern blotting, but not its enzymatic activity. This suggests that the C-terminal 57 amino acids are essential for the DNA binding function of this protein but not for its enzymatic activity. Another non-receptor-type protein-tyrosine phosphatase, PTP-1, when expressed in enzymatically active form in E. coli did not bind to DNA. These results suggest that a nontransmembrane protein-tyrosine phosphatase, PTP-S, binds to DNA in vitro through its carboxy-terminal noncatalytic region.

The fission yeast genes pyp1+ and pyp2+ encode protein tyrosine phosphatases that negatively regulate mitosis

We have used degenerate oligonucleotide probes based on sequences conserved among known protein tyrosine phosphatases (PTPases) to identify two Schizosaccharomyces pombe genes encoding PTPases. We previously described the cloning of pyp1+ (S. Ottilie, J. Chernoff, G. Hannig, C. S. Hoffman, and R. L. Erikson, Proc. Natl. Acad. Sci. USA 88:3455-3459, 1991), and here we describe a second gene, called pyp2+. The C terminus of each protein contains sequences conserved in the apparent catalytic domains of all known PTPases. Disruption of pyp2+ results in viable cells, as was the case for pyp1+, whereas disruption of pyp2+ and pyp1+ results in synthetic lethality. Overexpression of either pyp1+ or pyp2+ in wild-type strains leads to a delay in mitosis but is suppressed by a wee1-50 mutation at 35 degrees C or a cdc2-1w mutation. A pyp1 disruption suppresses the temperature-sensitive lethality of a cdc25-22 mutation. Our data suggest that pyp1+ and pyp2+ act as negative regulators of mitosis upstream of the wee1+/mik1+ pathway.

Molecular cloning, nucleotide sequence and expression of a cDNA encoding an intracellular protein tyrosine phosphatase, PTPase-2, from mouse testis and T-cells

The PTP-2 cDNA encoding an intracellular protein tyrosine phosphatase (PTPase-2) was isolated and sequenced from mouse testis and T-cell cDNA libraries. This PTP-2 cDNA was found to be homologous to human PTP-TC and rat PTP-S, and contained 1,551 nucleotides, including 1,146 nucleotides encoding 382 amino acids as well as 5' (61 nucleotides) and 3' (344 nucleotides) non-coding regions. Northern blot analysis indicated that PTP-2 mRNA of 1.9 Kb was most abundant in testis and kidney, although it was also present in spleen, muscle, liver, heart and brain.

Identification of a human src homology 2-containing protein-tyrosine-phosphatase: a putative homolog of Drosophila corkscrew

src homology 2 (SH2) domains direct binding to specific phosphotyrosyl proteins. Recently, SH2-containing protein-tyrosine-phosphatases (PTPs) were identified. Using degenerate oligonucleotides and the PCR, we have cloned a cDNA for an additional PTP, SH-PTP2, which contains two SH2 domains and is expressed ubiquitously. When expressed in Escherichia coli, SH-PTP2 displays tyrosine-specific phosphatase activity. Strong sequence similarity between SH-PTP2 and the Drosophila gene corkscrew (csw) and their similar patterns of expression suggest that SH-PTP2 is the human corkscrew homolog. Sequence comparisons between SH-PTP2, SH-PTP1, corkscrew, and other SH2-containing proteins suggest the existence of a subfamily of SH2 domains found specifically in PTPs, whereas comparison of the PTP domains of the SH2-containing PTPs with other tyrosine phosphatases suggests the existence of a subfamily of PTPs containing SH2 domains. Since corkscrew, a member of the terminal class signal transduction pathway, acts in concert with D-raf to positively transduce the signal generated by the receptor tyrosine kinase torso, these findings suggest several mechanisms by which SH-PTP2 may participate in mammalian signal transduction.

The fission yeast genes pyp1+ and pyp2+ encode protein tyrosine phosphatases that negatively regulate mitosis

We have used degenerate oligonucleotide probes based on sequences conserved among known protein tyrosine phosphatases (PTPases) to identify two Schizosaccharomyces pombe genes encoding PTPases. We previously described the cloning of pyp1+ (S. Ottilie, J. Chernoff, G. Hannig, C. S. Hoffman, and R. L. Erikson, Proc. Natl. Acad. Sci. USA 88:3455-3459, 1991), and here we describe a second gene, called pyp2+. The C terminus of each protein contains sequences conserved in the apparent catalytic domains of all known PTPases. Disruption of pyp2+ results in viable cells, as was the case for pyp1+, whereas disruption of pyp2+ and pyp1+ results in synthetic lethality. Overexpression of either pyp1+ or pyp2+ in wild-type strains leads to a delay in mitosis but is suppressed by a wee1-50 mutation at 35 degrees C or a cdc2-1w mutation. A pyp1 disruption suppresses the temperature-sensitive lethality of a cdc25-22 mutation. Our data suggest that pyp1+ and pyp2+ act as negative regulators of mitosis upstream of the wee1+/mik1+ pathway.

Purification and characterization of a rat liver protein-tyrosine phosphatase with sequence similarity to src-homology region 2

Utilizing three proteins plus tyrosine-glutamate copolymer as substrates, all of which are subjected to (near) stoichiometrical phosphorylation exclusively on tyrosine residues, we partially purified four different protein-tyrosine phosphatases (PTPases) from rat liver cytosol which differed in substrate preference. Of the four PTPases, tentatively termed L1, L2, L3, and L4, PTPase L1 was purified to apparent homogeneity by a procedure involving chromatography on DEAE-cellulose at pH 7.0, Blue Sepharose, DEAE-cellulose at pH 7.6, hydroxyapatite, Phenyl Sepharose, Mono Q, and TSKgel Heparin. PTPase L1 was purified about 7000-fold from the extract and 0.27 mg was isolated from 1000 g liver corresponding to a yield of 13% from the Blue Sepharose step where it had become freed from any other PTPases detectable by our assay procedure. The purified PTPase L1 showed a major protein band of 67 kDa on SDS/PAGE. Catalytically, PTPase L1 had a specific activity of about 6500 nmol Pi released min-1mg-1 toward tyrosine-glutamate copolymer phosphorylated on tyrosine residues. PTPase L1 exhibited very low sensitivities to PTPase inhibitors such as zinc acetate, sodium vanadate, and acidic compounds as compared with those of most of the PTPases purified thus far. Amino acid sequence analysis of the purified PTPase L1 revealed a partial peptide sequence showing similarity to the catalytic domain core sequences conserved in the PTPase family. PTPase L1 was most similar to a PTPase termed PTP1C encoded by a human breast carcinoma cDNA but the identity was 55% over 117 residues spanning nearly half of the catalytic domain of PTP1C. The analysis also revealed another partial peptide sequence (113 residues) 70% identical with the sequence corresponding to 68% of two adjacent copies of the src homology region 2(SH-2 domain) identified in PTP1C. Besides those peptide sequences, PTPase L1 had regional sequences which were 70-90% identical with the residues lying between the two SH-2 domains or between the more C-terminal SH-2 domain and the catalytic domain of the carcinoma PTPase.

Dephosphorylation of human insulin-like growth factor I (IGF-I) receptors by membrane-associated tyrosine phosphatases

The insulin-like growth factor-I (IGF-I) receptor exhibits structural and functional similarities to the insulin receptor. Although the regulation of the insulin-receptor tyrosine kinase has been extensively investigated, the mechanisms involved in phosphorylation/dephosphorylation of the IGF-I receptor have received only little attention. To obtain a better understanding of the mode of IGF-I action, we have investigated the effects of protein phosphotyrosine phosphatases (PTPases) on the phosphorylation status of the IGF-I receptor. The dephosphorylation of the human IGF-I receptor by membrane-associated tyrosine phosphatases was studied by an immuno-enzymic assay based on the recognition of phosphotyrosine residues by anti-phosphotyrosine antibodies. Using intact IGF-I receptors as substrates, we show that they could be completely dephosphorylated by different cellular PTPases. Three pieces of evidence indicate that receptor dephosphorylation takes place on phosphotyrosine, i.e. the inhibition profile of phosphatase activity by zinc and vanadate, its absolute requirement for thiol compounds and the diminution of [32P]phosphotyrosine labelling of the beta subunit assessed by SDS/PAGE and phosphoamino acid analysis. Tyrosine kinase activity and autophosphorylation of the IGF-I receptor were decreased in a dose-dependent manner by PTPases, indicating that partial dephosphorylation of the receptor was associated with a decrease in its intrinsic activity. The sensitivity of the activated human IGF-I receptor to dephosphorylation on tyrosine leads to the speculation that IGF-I receptor activity might be regulated by mechanisms such as those described for the insulin receptor. Further investigation of the pathways of IGF-I receptor dephosphorylation will contribute to define the role(s) of PTPases in the overall mechanism of IGF-I signalling.

Expression of a protein tyrosine phosphatase in normal and v-src-transformed mouse 3T3 fibroblasts

A rat cDNA encoding a 51-kD protein tyrosine phosphatase (PTP1) was cloned into a mammalian expression vector and transfected into normal and v-src-transformed mouse NIH 3T3 fibroblasts. In the stable subclones isolated, PTP1 expression at the mRNA level was elevated twofold to 25-fold. The highest constitutive level of phosphotyrosine- specific dephosphorylating activity observed without cytotoxic effects or significant clonal instability was approximately 10-fold over the endogenous activity. The expressed PTP1 was found to be associated with the particulate fraction of the fibroblasts. Subcellular fractionation and immunofluorescent microscopic examination of PTP1-overexpressing cells has shown the phosphatase to be localized to the reticular network of the ER. PTP1 was readily solubilized by detergents, but not by high salt. Limited proteolysis of membrane-associated PTP1 resulted in the release of lower molecular mass (48 and 37 kD) forms of the enzyme to the cytosol. Thermal phase partitioning of isolated membranes with Triton X-114 indicated that the full-length PTP1 was strongly integrated into the membrane in contrast to the proteolytically derived fragments of PTP1. Overexpression of PTP1 caused little apparent change in the rate of cell proliferation, but did induce changes in fibroblast morphology. A substantial increase in the proportion of bi- and multinucleate cells in PTP1-expressing cell populations was observed, and, in the case of the v-src-transformed cells, cell flattening and loss of refractibility occurred. Although no apparent difference in the tyrosine phosphorylation of pp60v-src was noted in v-src-transformed control and PTP1-overexpressing fibroblasts, the phosphotyrosine content of a 70-kD polypeptide was decreased in PTP1-overexpressing cells.