Structure and function of the low Mr phosphotyrosine protein phosphatases

All intermediates of the arsenate reductase mechanism, including an intramolecular dynamic disulfide cascade

The mechanism of pI258 arsenate reductase (ArsC) catalyzed arsenate reduction, involving its P-loop structural motif and three redox active cysteines, has been unraveled. All essential intermediates are visualized with x-ray crystallography, and NMR is used to map dynamic regions in a key disulfide intermediate. Steady-state kinetics of ArsC mutants gives a view of the crucial residues for catalysis. ArsC combines a phosphatase-like nucleophilic displacement reaction with a unique intramolecular disulfide bond cascade. Within this cascade, the formation of a disulfide bond triggers a reversible "conformational switch" that transfers the oxidative equivalents to the surface of the protein, while releasing the reduced substrate.

Low-molecular-weight protein tyrosine phosphatase is a positive component of the fibroblast growth factor receptor signaling pathway

Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) has been implicated in the regulation of cell growth and actin rearrangement mediated by several receptor tyrosine kinases, including platelet-derived growth factor and epidermal growth factor. Here we identify the Xenopus laevis homolog of LMW-PTP1 (XLPTP1) as an additional positive regulator in the fibroblast growth factor (FGF) signaling pathway during Xenopus development. XLPTP1 has an expression pattern that displays substantial overlap with FGF receptor 1 (FGFR1) during Xenopus development. Using morpholino antisense technology, we show that inhibition of endogenous XLPTP1 expression dramatically restricts anterior and posterior structure development and inhibits mesoderm formation. In ectodermal explants, loss of XLPTP1 expression dramatically blocks the induction of the early mesoderm gene, Xbrachyury (Xbra), by FGF and partially blocks Xbra induction by Activin. Moreover, FGF-induced activation of mitogen-activated protein (MAP) kinase is also inhibited by XLPTP1 morpholino antisense oligonucleotides; however, introduction of RNA encoding XLPTP1 is able to rescue morphological and biochemical effects of antisense inhibition. Inhibition of FGF-induced MAP kinase activity due to loss of XLPTP1 is also rescued by an active Ras, implying that XLPTP1 may act upstream of or parallel to Ras. Finally, XLPTP1 physically associates only with an activated FGFR1, and this interaction requires the presence of SNT1/FRS-2 (FGFR substrate 2). Although LMW-PTP1 has been shown to participate in other receptor systems, the data presented here also reveal XLPTP1 as a new and important component of the FGF signaling pathway.

ACP1 and human adaptability: association with past malarial morbidity in the Sardinian population

Acid Phosphatase locus 1 (ACP1) is a polymorphic enzyme controlled by a locus on chromosome 2 with three common codominant alleles: *A, *B, and *C. ACP1 shows two major isoforms, F and S. The ratio of their concentration differs markedly among genotypes. Two functions have been proposed for the enzyme: flavin-mononucleotide phosphatase and tyrosine phosphatase activity. An association between ACP1 polymorphism and past malarial morbidity in Sardinia and the Po Valley has been described. Genetic polymorphisms could contribute to natural resistance or susceptibility to the disease. On the other hand, malaria pressure may select for genes that increase susceptibility to common diseases of modern civilization. Thus, the association between ACP1 and malaria in Sardinia in the light of recent understanding of the function of ACP1 and the molecular basis of malaria pathophysiology, especially aspects of the structure of band 3 protein (B3P) and the role of cytokines have been revisited. There is a significant negative correlation between ACP1 S isoform concentration, directly related to the ACP1*C allele, and past malarial morbidity in Sardinia. Populations subjected in the past to a heavy malarial burden show, at present, a lower concentration of the S isoform compared to a nearby malaria-free population, suggesting that genotypes with high S isoform concentration have been subjected to negative selection in a malarial environment. Correlation analysis and analysis of the joint G-6-PD/ACP1 distribution suggest that the relationship between past endemic malaria and the S isoform has not been mediated by glucose-6-phosphate dehydrogenase (G-6-PD) deficiency, thus pointing to a direct effect of malaria on ACP1.

18 kDa protein tyrosine phosphatase in the ocular lens

Protein tyrosyl phosphorylation and dephosphorylation play essential roles in regulating cellular events such as proliferation and differentiation, and their involvement in the lens development and transparency is also suggested. The level of tyrosine phosphorylation in a given protein is regulated by the opposing actions of protein-tyrosine kinases (Tyr kinases) and protein-tyrosine phosphatases (TyrPases). Recent studies have revealed that some Tyr kinases, such as platelet-derived growth factor receptor and fibroblast growth factor receptor, are present in the lens, however, little is known about TyrPases in the lens. In this study, we found a 18 kDa protein tyrosine phosphatase (18 kDa TyrPase) predominantly present in the ocular lens of various animals. We purified the phosphatase from the lens of chick embryo and characterized its activity.Phosphatase activity was determined in chick embryo, mouse, rabbit and bovine lenses using p -nitrophenyl phosphate (p NPP) as substrate. All lenses examined dephosphorylated p NPP under acidic conditions, and a large portion of the activity resided in a low molecular weight protein, ca. 18 kDa, following high-resolution gel permeation column chromatography. The brain and liver showed high dephosphorylation activities, but most of their activity was present in high molecular weight fractions, unlike that in the lens. The 18 kDa phosphatase was purified from the lens of 17 day old chick embryos to near-homogeneity with two-step rapid chromatography. This phosphatase showed strict substrate specificity for phosphotyrosine and phosphotyrosyl peptides, suggesting that it was a kind of protein tyrosine phosphatases (TyrPases). Several known inhibitors of TyrPases, such as SH blockers, vanadate and phenylarsine oxide, strongly inhibited the enzyme activity. The molecular weight, substrate specificity, and responses to various inhibitors and activators coincide well with those reported for the low molecular weight protein tyrosine phosphatase (LMW-TyrPase), belonging to the TyrPase superfamily. These results suggest that the 18 kDa phosphatase found in the lens is a LMW-TyrPase. The 18 kDa TyrPase is the predominant phosphatase in the ocular lens. It may be involved in regulation of lens cell proliferation, differentiation and/or lens transparency.

LMW-PTP exerts a differential regulation on PDGF- and insulin-mediated signaling

Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) is able to specifically bind and dephosphorylate activated PDGF and insulin receptors, modulating the onset of mitogenic process. LMW-PTP is present in two distinct intracellular locations. While the cytosolic LMW-PTP pool interacts directly with activated insulin or PDGF receptors, the cytoskeleton-associated LMW-PTP is tyrosine phosphorylated upon PDGF stimulation and is involved in cytoskeleton rearrangement acting on p190Rho-GAP. We investigated the differential role of LMW-PTP in PDGF- or insulin-induced mitogenesis and cytoskeleton rearrangement. Dominant negative LMW-PTP influences both PDGF- and insulin-induced mitogenesis with a different extent and it induces a decrease in cellular adhesion and chemotaxis after PDGF but not insulin treatment. PDGF but not insulin stimulation leads to tyrosine phosphorylation of LMW-PTP. We propose that the differential effect of LMW-PTP on PDGF and insulin signaling is mainly due to the fact that during insulin signaling LMW-PTP does not become phosphorylated and thus does not act on its cytoskeleton-associated substrate/s.

The low Mr phosphotyrosine protein phosphatase behaves differently when phosphorylated at Tyr131 or Tyr132 by Src kinase

The low molecular weight phosphotyrosine protein phosphatase (LMW-PTP) is phosphorylated by Src and Src-related kinases both in vitro and in vivo; in Jurkat cells, and in NIH-3T3 cells, it becomes tyrosine-phosphorylated upon stimulation by PDGF. In this study we show that pp60Src phosphorylates in vitro the enzyme at two tyrosine residues, Tyr131 and Tyr132, previously indicated as the main phosphorylation sites of the enzyme, whereas phosphorylation by the PDGF-R kinase is much less effective and not specific. The effects of LMW-PTP phosphorylation at each tyrosine residue were investigated by using Tyr131 and Tyr132 mutants. We found that the phosphorylation at either residue has differing effects on the enzyme behaviour: Tyr131 phosphorylation is followed by a strong (about 25-fold) increase of the enzyme specific activity, whereas phosphorylation at Tyr132 leads to Grb2 recruitment. These differing effects are discussed on the light of the enzyme structure.

Low molecular weight acid phosphatase/phosphotyrosyl protein phosphatase in the developing chick brain: partial characterization and levels during development

Low molecular weight acid phosphatase/phosphotyrosyl protein phosphatase is largely expressed in chick brain tissue during development. The enzyme was purified from brain extract prepared from 19-day-old chick embryos and from adult chickens using ammonium sulfate fractionation, gel filtration on Sephadex G-75 and two DEAE-Cellulose ion-exchange chromatography steps. The purified enzymes from embryo and adult chick brains show identical molecular weight values (about 18-20 kDa) and biochemical and structural properties such as substrate specificity, sensitivity to inhibitors, and number of free reactive sulphydryl groups. These data suggest that they are the same enzyme protein. Although the total acid phosphatase activity does not change appreciably during development, the activity associated with the low molecular weight acid phosphatase/phosphotyrosyl protein phosphatase markedly increases after birth and reaches the adult values within the first week of life. Taken together, our results suggest an involvement of the low molecular weight acid phosphatase/phosphotyrosyl protein phosphatase in postnatal development and maturation of chick brain tissue. The variations in tyrosine phosphorylation profile of chick brain polypeptides analyzed by Western blotting at the same developmental stages are also reported.

The low-molecular-weight phosphotyrosine protein phosphatase, when overexpressed, reduces the mitogenic response to macrophage colony-stimulating factor and tyrosine phosphorylation of its receptor

The interference of low-molecular-weight phosphotyrosine protein phosphatase with the macrophage response to macrophage colony-stimulating factor was investigated. This paper shows that this phosphatase, already known to be involved in platelet-derived growth factor receptor signaling, is physiologically expressed in murine macrophages and dephosphorylates in vitro macrophage colony-stimulating factor receptor molecules immunoprecipitated from macrophage colony-stimulating factor-stimulated macrophages. We obtained the first demonstration that a phosphotyrosine-specific protein phosphatase dephosphorylates the macrophage colony-stimulating factor receptor in vivo and reduces the mitogenic response to macrophage colony-stimulating factor. The data indicate that low-molecular-weight phosphotyrosine protein phosphatase is a negative regulator of macrophage colony-stimulating factor receptor signaling.

Cloning of murine low molecular weight phosphotyrosine protein phosphatase cDNA: identification of a new isoform

The low molecular weight phosphotyrosine protein phosphatase (LMW-PTP) is a 18 kDa cytosolic enzyme, involved in the negative regulation of cell proliferation. In different mammalian species LMW-PTPs are expressed in two molecular forms produced from a single primary transcript through an alternative splicing mechanism. In this paper we report the cloning, expression and characterization of mouse isoforms of LMW-PTPs (called m-IF1 and m-IF2), very similar to the corresponding rat and human isoenzymes. Moreover we have identified a third cDNA encoding a protein (m-IF2P) that presents three substitutions compared to m-IF2. This new isoform is still active on pNPP, although to a lower extent: this reduction is mainly due to the leucine to proline substitution in position 13, within the catalytic loop. The mRNA expression level of this isoform is comparable to those of m-IF1 and m-IF2. It is likely that a gene duplication process followed by mutations has generated this new gene.

Cloning, expression and characterisation of a new human low Mr phosphotyrosine protein phosphatase originating by alternative splicing

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.

Crystal structure of the catalytic domain of the human cell cycle control phosphatase, Cdc25A

Cdc25 phosphatases activate the cell division kinases throughout the cell cycle. The 2.3 A structure of the human Cdc25A catalytic domain reveals a small alpha/beta domain with a fold unlike previously described phosphatase structures but identical to rhodanese, a sulfur-transfer protein. Only the active-site loop, containing the Cys-(X)5-Arg motif, shows similarity to the tyrosine phosphatases. In some crystals, the catalytic Cys-430 forms a disulfide bond with the invariant Cys-384, suggesting that Cdc25 may be self-inhibited during oxidative stress. Asp-383, previously proposed to be the general acid, instead serves a structural role, forming a conserved buried salt-bridge. We propose that Glu-431 may act as a general acid. Structure-based alignments suggest that the noncatalytic domain of the MAP kinase phosphatases will share this topology, as will ACR2, a eukaryotic arsenical resistance protein.

Expression, purification and preliminary crystal analysis of the human low Mr phosphotyrosine protein phosphatase isoform 1

The genes of the human low Mr phosphotyrosine protein phosphatase (PTPase) isoforms 1 (IF1) and 2 (IF2) were isolated by screening a human placenta cDNA library, cloned in pGEX and expressed in E. coli as fusion proteins with glutathione S-transferase. The recombinant proteins were purified by a rapid one-step procedure allowing each enzyme to purify with high final yield and specific activity. This result is important for IF1, whose purification from natural sources is difficult, due to precipitation propensity, thus hindering structural studies. The enzymes obtained showed kinetic parameters very similar to those previously determined for the enzymes purified by classical procedures from both human erythrocytes and rat liver. These recombinant enzymes can therefore be used in place of those purified from natural sources for every purpose. IF1 and IF2 crystals were also grown. IF1 crystals were X-ray-grade, diffracted to better than 2.4 A and were suitable for high resolution X-ray structure determination.

Sequence-specific recognition of peptide substrates by the low Mr phosphotyrosine protein phosphatase isoforms

A number of phosphotyrosine-containing peptides derived from the PDGF receptor phosphorylation sites have been synthesised. The peptides were assayed as substrates of the two isoforms (IF1 and IF2) of the low Mr PTPase. The calculated k(cat), Km, and k(cat)/Km values indicate that only one peptide is best hydrolysed by IF2 (but not IF1), whose catalytic efficiency averages those previously reported for most PTPases (except the Yersinia enzyme). This peptide is the only one containing a couple of no bulky hydrophobic residues at the phosphotyrosine N-side. The determination of the same catalytic parameters in the presence of analogues of the best hydrolysed peptide in which one or both hydrophobic residues were replaced by Asp or Lys residues confirmed the importance of the hydrophobic cluster at the phosphotyrosine N-side for optimal enzymatic hydrolysis. These findings are discussed in the light of the known IF2 X-ray structure.