Low molecular weight protein tyrosine phosphatases: small, but smart.
ABSTRACT Low molecular weight protein tyrosine phosphatases (LMW-PTPs) are a family of 18-kDa enzymes involved in cell growth regulation. Despite very limited sequence similarity to the PTP superfamily, they display a conserved signature motif in the catalytic site. LMW-PTP associates and dephosphorylate many growth factor receptors, such as platelet-derived growth factor receptor (PDGF-r), insulin receptor and ephrin receptor, thus downregulating many of the tyrosine kinase receptor functions that lead to cell division. In particular, LMW-PTP acts on both growth-factor-induced mitosis, through dephosphorylation of activated PDGF-r, and on cytoskeleton rearrangement, through dephosphorylation of p190RhoGAP and the consequent regulation of the small GTPase Rho. LMW-PTP activity is modulated by tyrosine phosphorylation on two specific residues, each of them with specific characteristics. LMW-PTP activity on specific substrates depends also on its localization. Moreover, LMW-PTP is reversibly oxidized during growth factor signaling, leading to inhibition of its enzymatic activity. Recovery of phosphatase activity depends on the availability of reduced glutathione and involves the formation of an S-S bridge between the two catalytic site cysteines. Furthermore, studies on the redox state of LMW-PTP in contact-inhibited cells and in mature myoblasts suggest that LMW-PTP is a general and versatile modulator of growth inhibition.
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ABSTRACT: RT-PCR experiments on RNA from K562 and HepG2 cells and from human placenta led to the isolation of a novel cDNA, a further alternative splicing product of the primary transcript of low Mr phosphotyrosine phosphatase (LMW-PTP), already known to produce isoforms 1 and 2. This new transcript represents 15-20% of the total LMW-PTP mRNA in the cell. This novel cDNA codifies for a protein that we have named SV3 (splicing variant 3): the deduced protein sequence presents the first 49 residues identical to those of isoform 1, followed by 24 unrelated amino acids, due to a frameshift introduced at the novel exon-exon boundary. The SV3 protein, expressed in E. coli is enzymatically inactive, most probably because unfolded, as suggested by far-UV circular dichroism (CD) experiments. SV3 protein appears to possess the characteristics of an unstructured polypeptide chain lacking the packing of side chain residues and the secondary structure level that are typical of globular proteins. This protein could represent an inactive variant of the human LMW-PTP.FEBS Letters 08/1998; 431(1):111-5. · 3.58 Impact Factor
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ABSTRACT: The low molecular weight phosphotyrosine phosphatases (PTPases) constitute a distinctive class of phosphotyrosine phosphatases that is widely distributed among vertebrate and invertebrate organisms. In vertebrates, two isoenzymes of these low molecular weight PTPases are commonly expressed. The two human isoenzymes, HCPTPA and HCPTPB, differ in an alternatively spliced sequence (residues 40-73) referred to as the variable loop, resulting in isoenzymes that have different substrate specificities and inhibitor/activator responses. We present here the x-ray crystallographic structure of a human low molecular weight PTPase solved by molecular replacement to 2.2 A. The structure of human low molecular weight PTPase is compared with a structure representing the other isoenzyme in this PTPase class, in each case with a sulfonate inhibitor bound to the active site. Possible aromatic residue interactions with the phosphotyrosine substrate are proposed from an examination of the binding site of the inhibitors. Differences are observed in the variable loop region, which forms one wall and the floor of a long crevice leading from the active-site loop. A set of residues lying along this crevice (amino acids 49, 50, and 53) is suggested to be responsible for differences in substrate specificity in these two enzymes.Journal of Biological Chemistry 09/1998; 273(34):21714-20. · 4.65 Impact Factor
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ABSTRACT: Stimulation of various cells with growth factors results in a transient increase in the intracellular concentration of H2O2 that is required for growth factor-induced protein tyrosine phosphorylation. The effect of H2O2 produced in response to epidermal growth factor (EGF) on the activity of protein-tyrosine phosphatase 1B (PTP1B) was investigated in A431 human epidermoid carcinoma cells. H2O2 inactivated recombinant PTP1B in vitro by oxidizing its catalytic site cysteine, most likely to sulfenic acid. The oxidized enzyme was reactivated more effectively by thioredoxin than by glutaredoxin or glutathione at their physiological concentrations. Oxidation by H2O2 prevented modification of the catalytic cysteine of PTP1B by iodoacetic acid, suggesting that it should be possible to monitor the oxidation state of PTP1B in cells by measuring the incorporation of radioactivity into the enzyme after lysis of the cells in the presence of radiolabeled iodoacetic acid. The amount of such radioactivity associated with PTP1B immunoprecipitated from A431 cells that had been stimulated with EGF for 10 min was 27% less than that associated with PTP1B from unstimulated cells. The amount of iodoacetic acid-derived radioactivity associated with PTP1B reached a minimum 10 min after stimulation of cells with EGF and returned to base line values by 40 min, suggesting that the oxidation of PTP1B is reversible in cells. These results indicate that the activation of a receptor tyrosine kinase by binding of the corresponding growth factor may not be sufficient to increase the steady state level of protein tyrosine phosphorylation in cells and that concurrent inhibition of protein-tyrosine phosphatases by H2O2 might also be required.Journal of Biological Chemistry 07/1998; 273(25):15366-72. · 4.65 Impact Factor