Protein tyrosine phosphatases: characterization of extracellular and intracellular domains

Structural and biochemical characterization of the vaccinia virus envelope protein D8 and its recognition by the antibody LA5

Smallpox vaccine is considered a gold standard of vaccines, as it is the only one that has led to the complete eradication of an infectious disease from the human population. B cell responses are critical for the protective immunity induced by the vaccine, yet their targeted epitopes recognized in humans remain poorly described. Here we describe the biochemical and structural characterization of one of the immunodominant vaccinia virus (VACV) antigens, D8, and its binding to the monoclonal antibody LA5, which is capable of neutralizing VACV in the presence of complement. The full-length D8 ectodomain was found to form a tetramer. We determined the crystal structure of the LA5 Fab-monomeric D8 complex at a resolution of 2.1 Å, as well as the unliganded structures of D8 and LA5-Fab at resolutions of 1.42 Å and 1.6 Å, respectively. D8 features a carbonic anhydrase (CAH) fold that has evolved to bind to the glycosaminoglycan (GAG) chondroitin sulfate (CS) on host cells. The central positively charged crevice of D8 was predicted to be the CS binding site by automated docking experiments. Furthermore, sequence alignment of various poxvirus D8 orthologs revealed that this crevice is structurally conserved. The D8 epitope is formed by 23 discontinuous residues that are spread across 80% of the D8 protein sequence. Interestingly, LA5 binds with a high-affinity lock-and-key mechanism above this crevice with an unusually large antibody-antigen interface, burying 2,434 Å(2) of protein surface.

Form, function, and regulation of protein tyrosine phosphatases and their involvement in human diseases

Protein tyrosine phosphatases (PTPs) are a family of enzymes that modulate the cellular level of tyrosine phosphorylation. Based on cellular location, they are classified as receptor like or intracellular PTPs. Structure and function studies have led to the understanding of the enzymatic mechanism of this class of enzymes. Proper targeting of PTPs is essential for many cellular signalling events including antigen induced proliferative responses of B and T cells. The physiological significance of PTPs is further unveiled through mice gene knockout studies and human genome sequencing and mapping projects. Several PTPs are shown to be critical in the pathogenesis of human diseases.

Regulation of microglial tyrosine phosphorylation in response to neuronal injury

The regulation and substrate specificity of microglial phosphotyrosine (ptyr) increases accompanying motor neuron degeneration in the rat spinal cord induced by injection of the cytotoxic lectin, ricin, into sciatic nerve were examined using specific enzyme inhibitors, immunohistochemistry, and Western blot analyses. Optical density measurements of immunostained sections show that microglial ptyr levels are elevated at 3 days postinjection. This period coincides with initial stages of neuronal degeneration, and ptyr levels are maximal at 7 days. We next asked whether this increase is due to increased tyrosine kinase or decreased tyrosine phosphatase activities by assaying ptyr immunostaining in animals that received osmotic pump infusion of the nonreceptor tyrosine kinase inhibitor, herbimycin A, for the 7-day survival period. When compared to the control ventral horn, microglial ptyr on the experimental side was attenuated by at least 45% in the presence of herbimycin A. In order to identify microglial substrates undergoing increased tyrosine phosphorylation, Western blot analysis was performed on hemicord and punch biopsy samples from control and experimental sides following ricin injection. A subset of two proteins was identified whose increased ptyr was almost completely attenuated in the herbimycin-A-treated animals. We conclude that the data support earlier indications that upregulation of microglial tyrosine phosphorylation is a key early event in response to neuronal injury. Further, this upregulation is due to turning on tyrosine kinase activities, particularly nonreceptor kinases, and the end product is phosphorylation of a very limited number of substrates. This suggests the activation of specific tyrosine phosphorylation pathways, which may represent critical therapeutic intervention points, rather than a global response. The results are discussed in terms of recent cell culture models of microglial activation and earlier data demonstrating elevated microglial ptyr in neurodegenerative disease.

Germline mosaicism in X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM; OMIM310400) is a congenital muscle disorder characterized by severe hypotonia and respiratory insufficiency. The disorder was mapped to Xq28 by linkage studies and the MTM1 gene was isolated by positional cloning. The gene product is a 603 amino acid protein named myotubularin. A small domain in its sequence shows high homology to a consensus active site of tyrosine phosphatases, a diverse class of proteins involved in signal transduction, control of cell growth, and differentiation. In this report, two brothers affected with XLMTM are shown to have a point mutation (G1187A) in exon 11 of the MTM1 gene. Surprisingly, their mother does not have this mutation in her lymphocytes. Therefore, she likely has a germline mosaicism. As this is the third report of germline mosaicism in XLMTM, the finding has important implications for genetic counseling.

Molecular dynamics simulations of protein-tyrosine phosphatase 1B. I. ligand-induced changes in the protein motions

Activity of enzymes, such as protein tyrosine phosphatases (PTPs), is often associated with structural changes in the enzyme, resulting in selective and stereospecific reactions with the substrate. To investigate the effect of a substrate on the motions occurring in PTPs, we have performed molecular dynamics simulations of PTP1B and PTP1B complexed with a high-affinity peptide DADEpYL, where pY stands for phosphorylated tyrosine. The peptide sequence is derived from the epidermal growth factor receptor (EGFR988-993). Simulations were performed in water for 1 ns, and the concerted motions in the protein were analyzed using the essential dynamics technique. Our results indicate that the predominately internal motions in PTP1B occur in a subspace of only a few degrees of freedom. Upon substrate binding, the flexibility of the protein is reduced by approximately 10%. The largest effect is found in the protein region, where the N-terminal of the substrate is located, and in the loop region Val198-Gly209. Displacements in the latter loop are associated with the motions in the WPD loop, which contains a catalytically important aspartic acid. Estimation of the pKa of the active-site cysteine along the trajectory indicates that structural inhomogeneity causes the pKa to vary by approximately +/-1 pKa unit. In agreement with experimental observations, the active-site cysteine is negatively charged at physiological pH.

Binding of a diphosphotyrosine-containing peptide that mimics activated platelet-derived growth factor receptor beta induces oligomerization of phosphatidylinositol 3-kinase

Phosphatidylinositol 3-kinase (PI3K) is a heterodimeric enzyme comprising a p110 catalytic subunit and a p85 regulatory subunit. We have recently shown that the isolated p85 subunit exists as a dimer; therefore, we examined whether the heterodimeric enzyme was capable of further self-association. Size-exclusion chromatography demonstrated that PI3K was a 1:1 complex of p85 and p110 under native conditions. However, binding of a diphosphotyrosine-containing peptide that mimics an activated platelet-derived growth factor receptor beta induced an increase in the apparent molecular mass of PI3K. This increase was due to dimerization of PI3K and was dependent on PI3K concentration but not diphosphopeptide concentration. Dimer formation was also observed directly using fluorescence resonance energy transfer. Diphosphopeptide-induced activation of PI3K (Carpenter, C. L., Auger, K. R., Chanudhuri, M., Yoakim, M., Schaffhausen, B., Shoelson, S., and Cantley, L. C. (1993) J. Biol. Chem. 268, 9478-9483; Rordorf-Nikolic, T., Van Horn, D. J., Chen, D., White, M. F., and Backer, J. M. (1995) J. Biol. Chem. 270, 3662-3666) was not a direct result of dimerization and occurred only when phosphatidylinositol, and not phosphatidylinositol-4,5-diphosphate, was the phosphorylation substrate. Binding of the tandem SH2 domains of the p85 regulatory subunit to activated receptor tyrosine kinases therefore induces dimerization of PI3K, which may be an early step in inositol lipid-mediated signal transduction.

Protein tyrosine phosphatase-eta expression is upregulated by the PKA-dependent and is downregulated by the PKC-dependent pathways in thyroid cells

We have recently reported the isolation of a rat cDNA encoding a receptor-type tyrosine phosphatase, which appears to be a marker of thyroid differentiation. To elucidate the molecular mechanisms underlying r-PTPeta expression in normal thyroid cells both in vitro and in vivo, we investigated the regulation of r-PTPeta expression in cultured thyrocytes (the rat cell line PC Cl 3) and in an animal model of TSH-dependent thyroid goitrogenesis. In vitro studies showed that mRNA expression of r-PTPeta in thyroid cells is induced in a time- and dose-dependent manner by the activation of growth- and differentiation-linked PKA pathways (TSH and forskolin), whereas it is down-regulated by the activation of the proliferative dedifferentiating PKC-dependent transduction pathway (TPA). However, the regulation of r-PTPeta expression by TSH and TPA, respectively, is observed only in normal thyroid cells, but is lost in transformed thyroid cells. In vivo studies with thiouracil-fed rats demonstrated that increased serum levels of TSH up-regulated r-PTPeta mRNA expression in parallel with the stimulation of thyroid growth and function. The reduction of blood TSH levels due to iodide refeeding to goitrous rats determined a marked down-regulation of r-PTPeta expression, in parallel with involution of thyroid hyperplasia. Taken together these results demonstrate that the phosphatase r-PTPeta is regulated by the two main thyroid regulatory pathways and suggest that it may play an important role in the growth and differentiation of thyroid cells.

Identification and characterization of RPTP rho, a novel RPTP mu/kappa-like receptor protein tyrosine phosphatase whose expression is restricted to the central nervous system

We describe the cloning, chromosomal localization and characterization of RPTPrho, a new member of the RPTPmu/kappa phosphatase subfamily. Receptor tyrosine phosphatases in this subfamily are comprised of a MAM domain near the N-terminal, an immunoglobulin-like domain, four fibronectin type III repeats, a single transmembrane domain, and a large juxtamembrane segment followed by two intracellular phosphatase domains. An alternatively spliced mini-exon was identified in the extracellular segment of RPTPrho, between the fourth fibronectin type III repeat and the transmembrane domain. The RPTPrho gene was mapped to human chromosome 20 and mouse chromosome 2. Northern blot analysis demonstrated that RPTPrho expression was restricted to the central nervous system, and in situ hybridization studies showed that the RPTPrho transcript was distributed throughout the murine brain and spinal cord. Exceptionally high levels of the transcript were present in the cortex and olfactory bulbs during perinatal development, but were down-regulated during postnatal week two. The motifs found in the extracellular segment of type II receptor protein tyrosine phosphatases are commonly found in neural cell adhesion molecules, suggesting that RPTPrho may be involved in both signal transduction and cellular adhesion in the central nervous system.

Protein tyrosine phosphatases in the developing nervous system

Protein tyrosine phosphatases (PTPs) constitute a diverse family of intracellular and transmembrane proteins. Expression data and recent genetic analyses indicate that many PTPs play important roles in different aspects of nervous system development. Although PTP mechanisms are still poorly understood, current data suggest considerable complexity in these signaling pathways.

Novel receptor protein tyrosine phosphatase (RPTPrho) and acidic fibroblast growth factor (FGF-1) transcripts delineate a rostrocaudal boundary in the granule cell layer of the murine cerebellar cortex

We have identified a novel receptor-like protein tyrosine phosphatase (RPTPrho) transcript whose expression in the cerebellar cortex is restricted to the granule cell layer of lobules 1-6. Acidic fibroblast growth factor (FGF-1) mRNA follows a similar cerebellar expression pattern. Together, the two markers define a sharp boundary in lobule 6, slightly caudal to the primary fissure. Anterior and posterior compartments became discernible only during postnatal weeks two and six, for RPTPrho and FGF-1, respectively. A rostrocaudal boundary in lobule 6 of the murine cerebellar cortex has also been identified morphologically by the effects of the meander tail mutation. The position of the RPTPrho and FGF-1 boundary on the rostrocaudal axis of the cerebellar cortex was close to, but not coincident with, the caudal extent of the disorganized anterior lobe of meander tail and the rostral extent of Otx-2 expression. The restricted pattern of FGF-1 and RPTPrho implies that these molecules may have specific signaling roles in the tyrosine phosphorylation/dephosphorylation pathway in the anterior compartment of the adult cerebellar cortex.

Dimerization-induced inhibition of receptor protein tyrosine phosphatase function through an inhibitory wedge

The function and regulation of the receptorlike transmembrane protein tyrosine phosphatases (RPTPs) are not well understood. Ligand-induced dimerization inhibited the function of the epidermal growth factor receptor (EGFR)-RPTP CD45 chimera (EGFR-CD45) in T cell signal transduction. Properties of mutated EGFR-CD45 chimeras supported a general model for the regulation of RPTPs, derived from the crystal structure of the RPTPalpha membrane-proximal phosphatase domain. The phosphatase domain apparently forms a symmetrical dimer in which the catalytic site of one molecule is blocked by specific contacts with a wedge from the other.

Thyroid cell transformation inhibits the expression of a novel rat protein tyrosine phosphatase

We have isolated a rat thyroid cDNA encoding a novel rat receptor-type tyrosine phosphatase protein. This gene, on the basis of its homology to another tyrosine phosphatase, the recently isolated human DEP-1/HPTPeta, has been named r-PTPeta. In rat thyroid cells the r-PTPeta gene acts as a differentiation marker. Indeed, the block of thyroid cell differentiation induced by viral and cellular oncogenes is associated with the inhibition or marked reduction of the expression of this gene, and its expression is positively regulated by thyrotropin, the physiological stimulator of thyroid cell growth.

Impaired mammary gland development and function in mice lacking LAR receptor-like tyrosine phosphatase activity

The LAR receptor-like protein tyrosine phosphatase is composed of two intracellular tyrosine phosphatase domains and a cell adhesion molecule-like extracellular region containing three immunoglubulin-like domains in combination with eight fibronectin type-III-like repeats. This architecture suggests that LAR may function in cellular signalling by the regulation of tyrosine phosphorylation through cell-cell or cell-matrix interactions. We used gene targeting in mouse embryonic stem cells to generate mice lacking sequences encoding both LAR phosphatase domains. Northern blot analysis of various tissues revealed the presence of a truncated LAR mRNA lacking the cytoplasmic tyrosine phosphatase domains and indicated that this LAR mutation is not accompanied by obvious changes in the expression levels of one of the LAR-like receptor tyrosine phosphatases PTPdelta or PTPsigma. LAR-/- mice develop and grow normally and display no appreciable histological tissue abnormalities. However, upon breeding we observed an abnormal neonatal death rate for pups from LAR-/- females. Mammary glands of LAR-/- females were incapable of delivering milk due to an impaired terminal differentiation of alveoli at late pregnancy. As a result, the glands failed to switch to a lactational state and showed a rapid involution postpartum. In wild-type mice, LAR expression is regulated during pregnancy reaching maximum levels around Day 16 of gestation. Taken together, these findings suggest an important role for LAR-mediated signalling in mammary gland development and function.

Molecular cloning of a novel receptor-type protein tyrosine phosphatase from murine fetal liver

A cDNA fragment encoding a novel tyrosine phosphatase (PTPase), termed ptpf, was isolated from day 11.5 mouse fetal liver using reverse transcription-polymerase chain reaction (RT-PCR) with degenerate primers. The 5.5-kb cDNA encoding the complete coding region was isolated from an adult mouse kidney cDNA library. This cDNA contained a single open reading frame (ORF) encoding a predicted 1436-amino-acid protein with a molecular mass of 161,150 Da. Sequence analysis revealed that PTPf was homologous to PTPmu and PTPkappa, and a putative receptor-type PTPase. Northern blotting analysis of adult mouse mRNA indicated the existence of four major ptpf transcripts of approximately 10, 6, 3 and 2.7 kb, and these transcripts were expressed in a tissue-specific manner. During embryogenesis, only the 6-kb transcript was detected.

Differential accumulation of DPTP61F alternative transcripts: regulation of a protein tyrosine phosphatase by segmentation genes

DPTP61F is a non-receptor protein tyrosine phosphatase that is expressed during Drosophila oogenesis and embryogenesis. DPTP61F transcripts are alternatively spliced to produce two isoforms of the protein which are targeted to different subcellular locations. DPTP61Fn accumulates in the nucleus, and DPTP61Fm associates with the membranes of the reticular network and the mitochondria. We have examined the spatial and temporal expression of the two alternative transcripts of dptp61F during Drosophila embryogenesis. Our observations indicate that the two isoforms are expressed in distinct patterns. The DPTP61Fn transcript is expressed in the mesoderm and neuroblast layer during germband extension and later in the gut epithelia. In comparison, the transcript encoding DPTP61Fm accumulates in 16 segmentally repeated stripes in the ectoderm during germband extension. These stripes are flanked by, and adjacent to, the domains of engrailed and wingless gene expression in the anterior/posterior axis. In stage 10 embryos, the domains of DPTP61Fm transcript accumulation are wedge shaped and roughly coincide with the area lateral to the denticle belts that will give rise to naked cuticle. The DPTP61Fm transcript is also expressed later in embryogenesis in the central nervous system. The segmental modulation of DPTP61Fm transcript accumulation in the A/P axis of the germband is regulated by the pair-rule genes, and the intrasegmental pattern of transcript accumulation is regulated by the segment polarity genes.

Translocation of the Csk homologous kinase (Chk/Hyl) controls activity of CD36-anchored Lyn tyrosine kinase in thrombin-stimulated platelets

Chk/Hyl is a recently isolated non-receptor tyrosine kinase with greatest homology to a ubiquitous negative regulator of Src family kinases, Csk. To understand the significance of co-expression of Chk and Csk in platelets, we examined the subcellular localization of each protein. Chk, but not Csk, was completely translocated from the Triton X-100-soluble to the Triton X-100-insoluble cytoskeletal fraction within 10 s of thrombin stimulation. Chk and Lyn, but not Csk and c-Src, co-fractionated in the higher density lysate fractions of resting platelets, with Chk being found to localize close to CD36 (membrane glycoprotein IV)-anchored Lyn. The kinase activity of co-fractionated Lyn was suppressed 3-fold. In vitro phosphorylation assays showed that Chk suppressed Lyn activity by phosphorylating its C-terminal negative regulatory tyrosine. Upon stimulation of platelets with thrombin, the rapid and complete translocation of Chk away from Lyn caused concomitant activation of Lyn. This activation was accompanied by dephosphorylation of Lyn at its C-terminal negative regulatory tyrosine in cooperation with a protein tyrosine phosphatase. These results suggest that Chk, but not Csk, may function as a translocation-controlled negative regulator of CD36-anchored Lyn in thrombin-induced platelet activation.

Identification of a receptor-type protein tyrosine phosphatase expressed in postmitotic maturing neurons: its structure and expression in the central nervous system

We have isolated a rat cDNA encoding a receptor-type protein-tyrosine-phosphatase (RTP) expressed in brain and kidney (RPTP-BK) and characterized its expression in the developing central nervous system. RPTP-BK has seven fibronectin type III-like repeats in the extracellular region and a unique catalytic phosphatase domain in the cytoplasmic region. Bacterial expression of its phosphatase domain showed that the dephosphorylation of phosphotyrosine residues was mediated by the cytoplasmic catalytic domain. Sequence comparison revealed that RPTP-BK is homologous with GLEPP1, a rabbit PTP expressed in renal glomerular epithelia, and has the same phosphatase domain as murine PTPphi expressed in macrophages. RPTP-BK has also significant homology with Drosophila DPTP10D in the phosphatase domain, whose expression is localized exclusively in growth cones of the embryonal brains. The gene for RPTP-BK is well conserved among other species, and the expression in the brain but not in the kidney is developmentally regulated during the neonatal stage. Hybridization in situ showed that RPTP-BK is highly expressed in the postmitotic maturing neurons of the olfactory bulb, developing neocortex, hippocampus and thalamus. Because the expression of RPTP-BK in the developing neocortex is correlated with the stage of axonogenesis in cortical neurons, RPTP-BK might be crucial in neural cell development of the mammalian central nervous system.

Cloning and characterization of rat density-enhanced phosphatase-1, a protein tyrosine phosphatase expressed by vascular cells

We have cloned from cultured vascular smooth muscle cells a protein tyrosine phosphatase, rat density-enhanced phosphatase-1 (rDEP-1), which is a probable rat homologue of DEP-1/HPTP eta. rDEP-1 is encoded by an 8.7-kb transcript and is expressed as a 180- to 220-kD protein. The rDEP-1 gene is located on human chromosome 11 (region p11.2) and on mouse chromosome 2 (region 2E). The cDNA sequence predicts a transmembrane protein consisting of a single phosphatase catalytic domain in the intracellular region, a single transmembrane domain, and eight fibronectin type III repeats in the extracellular region (GenBank accession number U40790). In situ hybridization analysis demonstrates that rDEP-1 is widely expressed in vivo but that expression is highest in cells that form epithelioid monolayers. In cultured cells with epitheliod morphology, including endothelial cells and newborn smooth muscle cells, but not in fibroblast-like cells, rDEP-1 transcript levels are dramatically upregulated as population density increases. In vivo, quiescent endothelial cells in normal arteries express relatively high levels of rDEP-1. During repair of vascular injury, expression of rDEP-1 is downregulated in migrating and proliferating endothelial cells. In vivo, rDEP-1 transcript levels are present in very high levels in megakaryocytes, and circulating plates have high levels of the rDEP-1 protein. In vitro, initiation of differentiation of the human megakaryoblastic cell line CHRF-288-11 with phorbol 12-myristate 13-acetate leads to a very strong upregulation of rDEP-1 transcripts. The deduced structure and the regulation of expression of rDEP-1 suggest that it may play a role in adhesion and/or signaling events involving cell-cell and cell-matrix contact.

Structural basis for inhibition of receptor protein-tyrosine phosphatase-alpha by dimerization

Receptor-like protein-tyrosine phosphatases (RPTPs), like their non-receptor counterparts, regulate the level of phosphotyrosine-containing proteins derived from the action of protein-tyrosine kinases. RPTPs are type-I integral membrane proteins which contain one or two catalytic domains in their cytoplasmic region. It is not known whether extracellular ligands regulate the activity of RPTPs. Here we describe the crystal structure of the membrane-proximal catalytic domain (D1) of a typical RPTP, murine RPTP alpha. Significant structural deviations from the PTP1B fold reside within the amino-terminal helix-turn-helix segment of RPTPalphaD1 (residues 214 to 242) and a distinctive two-stranded beta-sheet formed between residues 211-213 and 458-461. The turn of the N-terminal segment inserts into the active site of a dyad-related D1 monomer. On the basis of two independent crystal structures, sequence alignments, and the reported biological activity of EGF receptor/CD45 chimaeras, we propose that dimerization and active-site blockage is a physiologically important mechanism for downregulating the catalytic activity of RPTPalpha and other RPTPs.

Molecular cloning of phogrin, a protein-tyrosine phosphatase homologue localized to insulin secretory granule membranes

An insulin granule membrane protein-tyrosine phosphatase (PTP) homologue, phogrin, was cloned by expression screening of a rat insulinoma cDNA library. The 3723-base pair cDNA encoded a transmembrane glycoprotein of 1004 amino acids (Mr 111876) that underwent post-translational proteolysis to 60-64-kDa products after a 30-min delay. The kinetics of proteolytic conversion (t1/2 = 45 min) and turnover (t1/2 = 12 h) were consistent with sorting and conversion in a late compartment of the secretory pathway. Studies on the native beta-cell protein suggested that the COOH-terminal PTP domain was on the cytosolic face of the secretory granule. The lumenal segment was comprised of a protease-resistant globular domain of around 25 kDa. Its localization and topology is thus consistent with a transmembrane receptor function related to granule biogenesis, exocytosis, or subsequent membrane recovery, and it should prove to be a useful cell biological marker for the granule membrane. High expression of the mRNA (5.4 kilobases) and protein was evident in islets, pancreatic alpha- and beta-cell tumor lines, brain cells, and other cells of neuroendocrine lineage. It is closely related to the diabetic autoantigen ICA512 (IA-2) (42% identity overall; 80% in the 260-amino acid PTP domain) and thus a potential target of autoimmunity in diabetes mellitus.

Cloning and Expression of Chicken Protein Tyrosine Phosphatase Gamma

A 5,403 bp cDNA encoding chicken protein tyrosine phosphatase gamma (PTPgamma) was isolated and sequenced. The predicted open reading frame of 1,422 amino acids (aa) includes 742 aa of extracellular (EC) domain, 26 aa of transmembrane (TM) domain and 634 aa of intracellular domain. The chicken PTPgamma has a 86.7% aa identity to its human homolog and contains the carbonic anhydrase-like domain and fibronectin type III homologous regions in the EC domain, as well as the tandem linked catalytic sequences in the cytoplasmic domain. However, the chicken PTPgamma lacks 29 aa immediate downstream of the putative TM domain in comparison with its human counterpart. Northern analysis revealed the presence of two transcripts of 6.3 and 9.5 kb in various tissues. The cytoplasmic domain of the PTPgamma could be expressed as an enzymatically active form in SF9 insect cells. PTPgamma could also be expressed in normal and rsc-transformed NIH3T3 and Rat 1 cells as a gag-PTP fusion protein, but no detectable effects on growth and colony formation of these cells were observed. Copyright 1996 S. Karger AG, Basel

Binary expression of olfactory bulb-protein tyrosine phosphatase in rat central nervous system: developmental gene regulation in neonate cerebral cortex and constitutive expression in olfactory-rhinencephalon

Olfactory bulb-protein tyrosine phosphatase (OB-PTP) is a receptor type PTPase dominantly expressed in olfactory bulb. Previously, we isolated and molecularly cloned a rat OB-PTP cDNA from an embryonal brain cDNA library. In the present study, we investigated its temporal and spatial gene expression by Northern blot and in situ hybridization analysis. The expression of OB-PTP gene was firstly detected in day 16 post coitum embryo and significantly increased during the late-gestational stage, attaining the highest level in the first week of neonate. The OB-PTP transcript was then down-regulated postnatally and was detected barely in an adult brain. In situ hybridization analysis showed that the transcript was characteristically localized in the postmitotic neurons of cerebral cortex and subcortical structures, and was down-regulated by day 28 when the cortical and subcortical structures have been organized. In the olfactory-rhinencephalon system including olfactory bulb and piriform cortex, the OB-PTP was preferentially expressed in the postmitotic neurons, and in contrast continuously expressed in the matured brain. Based on the evidence that DPTP10D, the Drosophila homolog of OB-PTP, is localized in the axons of specific pioneer neurons in Drosophila embryo, the OB-PTP is presumably involved in the axonogenesis of cortical and subcortical neurons as well as olfactory neurons in mammalian central nervous system. The biological significance of transcriptional regulation in olfactory system is discussed in terms of continuous axonal connections by regenerating olfactory neurons.

A gene mutated in X-linked myotubular myopathy defines a new putative tyrosine phosphatase family conserved in yeast

X-linked recessive myotubular myopathy (MTM1) is characterized by severe hypotonia and generalized muscle weakness, with impaired maturation of muscle fibres. We have restricted the candidate region to 280 kb and characterized two candidate genes using positional cloning strategies. The presence of frameshift or missense mutations (of which two are new mutations) in seven patients proved that one of these genes is indeed implicated in MTM1. The protein encoded by the MTM1 gene is highly conserved in yeast, which is surprising for a muscle specific disease. The protein contains the consensus sequence for the active site of tyrosine phosphatases, a wide class of proteins involved in signal transduction. At least three other genes, one located within 100 kb distal from the MTM1 gene, encode proteins with very high sequence similarities and define, together with the MTM1 gene, a new family of putative tyrosine phosphatases in man.

The multidomain protein Trio binds the LAR transmembrane tyrosine phosphatase, contains a protein kinase domain, and has separate rac-specific and rho-specific guanine nucleotide exchange factor domains

rho-like GTP binding proteins play an essential role in regulating cell growth and actin polymerization. These molecular switches are positively regulated by guanine nucleotide exchange factors (GEFs) that promote the exchange of GDP for GTP. Using the interaction-trap assay to identify candidate proteins that bind the cytoplasmic region of the LAR transmembrane protein tyrosine phosphatase (PT-Pase), we isolated a cDNA encoding a 2861-amino acid protein termed Trio that contains three enzyme domains: two functional GEF domains and a protein serine/threonine kinase (PSK) domain. One of the Trio GEF domains (Trio GEF-D1) has rac-specific GEF activity, while the other Trio GEF domain (Trio GEF-D2) has rho-specific activity. The C-terminal PSK domain is adjacent to an Ig-like domain and is most similar to calcium/calmodulin-dependent kinases, such as smooth muscle myosin light chain kinase which similarly contains associated Ig-like domains. Near the N terminus, Trio has four spectrin-like repeats that may play a role in intracellular targeting. Northern blot analysis indicates that Trio has a broad tissue distribution. Trio appears to be phosphorylated only on serine residues, suggesting that Trio is not a LAR substrate, but rather that it forms a complex with LAR. As the LAR PTPase localizes to the ends of focal adhesions, we propose that LAR and the Trio GEF/PSK may orchestrate cell-matrix and cytoskeletal rearrangements necessary for cell migration.

Protein aspartate phosphatases control the output of two-component signal transduction systems

Phosphorylation or dephosphorylation of an aspartate regulates the output activity of the response regulator of two-component signaling systems. Signal input in these systems is dependent on signal-transducing kinases, which can respond to a variety of signal ligands and, in some cases, to small phosphorylated metabolic intermediates. The kinase component of many two-component signaling systems also displays a response regulator-phosphate phosphatase activity that inactivates the response regulator in response to signals. Newly discovered kinase-independent phosphatases allow additional signals to influence the extent of response-regulator phosphorylation. Such phosphatases are prevalent in signal transduction systems controlling complex processes, such as the initiation of development in microorganisms.

Molecular cloning and characterization of Byp, a murine receptor-type tyrosine phosphatase similar to human DEP-1

Novel murine cDNAs encoding a receptor-like protein tyrosine phosphatase, termed Byp (HPTP beta-like tyrosine phosphatase) were cloned. The putative Byp protein consists of 1238 amino acids, which possesses a single catalytic domain in the cytoplasmic region. The extracellular region comprises eight repeats of a fibronectin type III module and contains multiple N-glycosylation sites. The byp mRNA was 7.7-kb long and expressed in every tissue examined, its level being high in the brain and kidney. Transfection of the byp cDNA expression plasmid into COS7 cells resulted in the expression of a 220-kDa tyrosine phosphorylated protein. Furthermore, co-expression of Byp and the Src family kinase Fyn increased the level of tyrosine phosphorylation of Byp, suggesting that Byp was tyrosine-phosphorylated by Fyn. Finally, the byp gene was located to mouse chromosome 2E1-2 and rat chromosome 3q32-33.

The yeast immunophilin Fpr3 is a physiological substrate of the tyrosine-specific phosphoprotein phosphatase Ptp1

The tyrosine-specific phosphoprotein phosphatase encoded by the Saccharomyces cerevisiae PTP1 gene dephosphorylates artificial substrates in vitro, but little is known about its functions and substrates in vivo. The presence of Ptp1 resulted in dephosphorylation of multiple tyrosine-phosphorylated proteins in yeast expressing a heterologous tyrosine-specific protein kinase, indicating that Ptp1 can dephosphorylate a broad range of substrates in vivo. Correspondingly, several proteins phosphorylated at tyrosine by endogenous protein kinases exhibited a marked increase in tyrosine phosphorylation in ptp1 mutant cells. One of these phosphotyrosyl proteins (p70) was also dephosphorylated in vitro when incubated with recombinant Ptp1. p70 was purified to homogeneity; analysis of four tryptic peptides revealed that p70 is identical to the recently described FPR3 gene product, a nucleolarly localized proline rotamase of the FK506- and rapamycin-binding family. The identity of p70 with Fpr3 was confirmed in the demonstration that the abundance of tyrosine-phosphorylated p70 in ptp1 mutants was strictly correlated with the level of FPR3 expression; immobilized phosphotyrosyl Fpr3 was directly dephosphorylated by recombinant Ptp1. Site-directed mutagenesis demonstrated that the site of tyrosine phosphorylation is Tyr-184, which resides within the nucleolin-like amino-terminal domain of Fpr3. Protein kinase activities from yeast cell extracts can bind to and phosphorylate the immobilized amino-terminal domain of Fpr3 on serine, threonine, and tyrosine. Fpr3 represents the first phosphotyrosyl protein identified in S. cerevisiae that is not itself a protein kinase and is as yet the only known physiological substrate of Ptp1.

Expression of protein tyrosine phosphatase genes during oogenesis in Drosophila melanogaster

The spatial and temporal expression of seven Drosophila protein tyrosine phosphatase genes during oogenesis was examined by whole mount in-situ hybridization of antisense RNA probes to ovaries. Our observations indicate diverse expression patterns consistent with multiple roles for protein tyrosine phosphatases in the ovary. DPTP99A and corkscrew transcripts are expressed in follicle cells, consistent with possible roles in the EGF receptor signaling pathway. Transcripts from corkscrew and DPTP10D are detected in the germline during oogenesis and localized to the oocyte during egg chamber development. Localization of the two transcripts is disrupted by mutations in egalitarian and Bicaudal D. DLAR and DPTP4E transcripts are found in the germline during the same developmental stages as DPTP10D transcripts, but their transcripts are not localized to the oocyte. DPTP61F transcription is detected only after stage 6 of oogenesis. After stage 10B these transcripts are transported to the oocyte; thus ovarian transcription of DPTP61F may reflect a maternal contribution of the mRNA for later use during embryogenesis. DPTP69D transcripts are sequestered in the nucleus from stage 7 to stage 10, and then released to the cytoplasm. Our observations suggest that the export of DPTP69D mRNA from the nucleus is temporally regulated during oogenesis.

Murine protein tyrosine phosphatase-PEST, a stable cytosolic protein tyrosine phosphatase

We have isolated the murine cDNA homologue of the human protein tyrosine phosphatase PTP-PEST (MPTP-PEST) from an 18.5-day mouse embryonic kidney library. The cDNA isolated has a single open reading frame predicting a protein of 775 amino acids. When expressed in vitro as a glutathione S-transferase fusion protein, the catalytic domain (residues 1-453) shows intrinsic phosphatase activity. Reverse transcriptase PCR and Northern-blot analysis show that MPTP-PEST mRNA is expressed throughout murine development. Indirect immunofluorescence in COS-1 cells against a heterologous epitope tag attached to the N-terminus of MPTP-PEST, together with cellular fractionation and Western-blot experiments from different murine cell lines, indicate that MPTP-PEST is a free cytosolic protein of 112 kDa. Finally, sequence analysis indicates that the C-terminal portion of the protein contains four regions rich in proline, glutamate, serine and threonine, otherwise known as PEST sequences. These are characteristic of proteins that display very short intracellular half-lives. Despite the presence of these motifs, pulse-chase labelling experiments demonstrate that MPTP-PEST has a half-life of more than 4 h.

Molecular cloning and tissue-specific RNA processing of a murine receptor-type protein tyrosine phosphatase

The molecular cloning of a murine receptor-type protein tyrosine phosphatase, termed PTP NU-3, with an extracellular cell-adhesion-molecule-like domain is reported. NU-3 was isolated from 11.5-day total mouse embryonic RNA by reverse-transcriptase PCR using degenerate oligonucleotides flanking the conserved protein tyrosine phosphatase catalytic domain. This produced a 280-bp DNA probe which was subsequently employed to screen a mouse embryonic kidney library. Several overlapping cDNA clones were isolated, collectively forming a cDNA of 6.0 kb that encodes a putative 211-kDa protein. Northern-blot analysis of total RNA from adult and embryonic mouse tissues indicates the existence of two major PTP NU-3 transcripts of approximately 6 kb and 7 kb. Both messages are expressed predominantly in brain tissues and neuronal-derived cell lines, although detectable levels of the 7-kb message were found in other non-neuronal tissues. We have identified a unique 132-bp exon segment that is present in the 7-kb message but is completely absent in the 6-kb transcript, suggesting tissue-specific levels of expression and RNA processing. Analysis of the amino acid sequence encoded by the 132-bp segment reveals that it completes a partial fibronectin type-III element resulting in a protein with a total of nine such elements. Bacterial expression of the two catalytic domains demonstrated that only the first domain possesses enzymic activity towards a tyrosine phosphorylated substrate.

Protein tyrosine phosphatases in disease processes

Given the importance of tyrosine phosphorylation of proteins in signalling pathways, it is perhaps not surprising that protein tyrosine phosphatases (PTPs) are involved in the pathogenesis of certain human diseases. A PTP produced by the Yersinia bacteria (which can cause bubonic plague, septicemia and enteric diseases) is thought to be used as a 'weapon' against host cell functions. In addition, dysfunction of cells' endogenous PTPs may contribute to defective immune function, to cancer and to diabetes.