Are Protein-tyrosine Phosphatases Specific for Phosphotyrosine?

New aspects of the phosphatase VHZ revealed by a high-resolution structure with vanadate and substrate screening

The recently discovered 150-residue human VHZ (VH1-related protein, Z member) is one of the smallest protein tyrosine phosphatases (PTPs) known and contains only the minimal structural elements common to all PTPs. We report a substrate screening analysis and a crystal structure of the VHZ complex with vanadate at 1.1 Å resolution, with a detailed structural comparison with other members of the protein tyrosine phosphatase family, including classical tyrosine-specific protein tyrosine phosphatases (PTPs) and dual-specificity phosphatases (DSPs). A screen with 360 phosphorylated peptides shows VHZ efficiently catalyzes the hydrolysis of phosphotyrosine (pY)-containing peptides but exhibits no activity toward phosphoserine (pS) or phosphothreonine (pT) peptides. The new structure reveals a deep and narrow active site more typical of the classical tyrosine-specific PTPs. Despite the high degrees of structural and sequence similarity between VHZ and classical PTPs, its general acid IPD-loop is most likely conformationally rigid, in contrast to the flexible WPD counterpart of classical PTPs. VHZ also lacks substrate recognition domains and other domains typically found on classical PTPs. It is therefore proposed that VHZ is more properly classified as an atypical PTP rather than an atypical DSP, as has been suggested.

MALDI mass sequencing and biochemical characterization of Setaria cervi protein tyrosine phosphatase

A 30-kDa acid phosphatase with protein tyrosine phosphatase activity was identified in Setaria cervi (ScPTP). The enzyme was purified to homogeneity using three-step column chromatography. Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) analysis of purified ScPTP yielded a total of eight peptides matching most closely to phosphoprotein phosphatase of Ricinus communis (RcPP). A hydrophilicity plot of RcPP revealed the presence of these peptides in the hydrophilic region, suggesting their antigenic nature. The substrate specificity of ScPTP with ortho-phospho-L-tyrosine and inhibition with sodium orthovanadate and ammonium molybdate affirmed it as a protein tyrosine phosphatase. ScPTP was also found to be tartrate resistant. The Km and Vmax were 6.60 mM and 83.3 μM/ml/min, respectively, with pNPP and 8.0 mM and 111 μM/ml/min, respectively, with ortho-phospho-L-tyrosine as the substrate. The Ki value with sodium orthovanadate was calculated to be 16.10 mM. Active site modification with DEPC, EDAC and pHMB suggested the presence of histidine, cysteine and aspartate at its active site. Thus, on the basis of MALDI-TOF and biochemical studies, it was confirmed that purified acid phosphatase is a PTP.

Ecto-phosphatases in protozoan parasites: possible roles in nutrition, growth and ROS sensing

The cellular plasma membrane contains enzymes whose active sites face the external medium rather than the cytoplasm. The activities of these enzymes, referred to as ecto-enzymes, can be measured using living cells. Ecto-phosphatases are ecto-enzymes that presumably hydrolyze extracellular phosphorylated substrates, releasing free inorganic phosphate. Although, several alternative functions have been suggested for these enzymes, such as participation in proliferation, differentiation, adhesion, virulence, and infection, little is known about the physiological roles of these enzymes in protozoa parasites. In this review, we discuss the principal features of ecto-phosphatases in protozoan parasites that are causative agents of important diseases such as Chagas' disease, leishmaniasis, amoebiasis, giardiasis, trichomoniasis and, sleeping sickness.

Mycobacterium tuberculosis secreted antigen (MTSA-10) modulates macrophage function by redox regulation of phosphatases

Macrophages are the primary host cells for Mycobacterium tuberculosis (Mtb). Although macrophages can mount a strong inflammatory response to dispose of invading microbial pathogens, the immune dysfunction of the Mtb-infected macrophage constitutes the hallmark of mycobacterial pathogenesis. A 10-kDa, Mtb secretory antigen (MTSA-10), encoded by ORF Rv3874, is one of the predominant members of the 'region of difference 1' locus of Mtb genome that has been strongly implicated in mycobacterial virulence. In this study, we investigated the possible role of MTSA-10 in modulating the macrophage dysfunction in a mouse macrophage cell line J774.1. We found that recombinant MTSA-10 caused extensive protein dephosphorylation in J774.1 cells as revealed by two-dimensional gel electrophoresis analysis. We also observed that MTSA-10 treatment downregulated the reactive oxygen species levels in the cells leading to activation of cellular protein phosphatases putatively responsible for the dephosphorylation phenomenon. This implied a direct role of MTSA-10 in the disruption of host cell signaling, resulting in downregulation of transcription of several genes essential for macrophage function.

Free-energy profiles for catalysis by dual-specificity phosphatases

PTPs (protein tyrosine phosphatases) are fundamental enzymes for cell signalling and have been linked to the pathogenesis of several diseases, including cancer. Hence, PTPs are potential drug targets and inhibitors have been designed as possible therapeutic agents for Type II diabetes and obesity. However, a complete understanding of the detailed catalytic mechanism in PTPs is still lacking. Free-energy profiles, obtained by computer simulations of catalysis by a dual-specificity PTP, are shown in the present study and are used to shed light on the catalytic mechanism. A highly accurate hybrid potential of quantum mechanics/molecular mechanics calibrated specifically for PTP reactions was used. Reactions of alkyl and aryl substrates, with different protonation states and PTP active-site mutations, were simulated. Calculated reaction barriers agree well with experimental rate measurements. Results show the PTP substrate reacts as a bi-anion, with an ionized nucleophile. This protonation state has been a matter of debate in the literature. The inactivity of Cys-->Ser active-site mutants is also not fully understood. It is shown that mutants are inactive because the serine nucleophile is protonated. Results also clarify the interpretation of experimental data, particularly kinetic isotope effects. The simulated mechanisms presented here are better examples of the catalysis carried out by PTPs.

[Synthesis of phosphonic acid and phosphinic acid derivatives for development of biologically active compounds]

This paper covers recent publications from our laboratory on the synthesis of a variety of phosphonate and phosphinate derivatives. New methods for the enantioselective synthesis of alpha-hydroxyphosphonates were established by Lewis acid-mediated cleavage of homochiral 1,3-dioxaneacetals with P(OEt)(3) and chiral metal ligand-mediated hydrophosphonylation of aldehydes. Two diastereomers of HPmp derivatives were prepared by an application of these methods. The HPmp derivatives were convered to FPmp derivatives but with low diastereoselectivity. Hydrophosphonylation of alpha-aminoaldehydes afforded threo- and erythro-beta-amino-alpha-hydroxyphosphonates under chelation and nonchelation controlled conditions, respectively. The asymmetric dihydroxylation of alpha, beta-, and beta, gamma-unsaturated phosphonates with AD-mix-alpha and AD-mix-beta reagents gave alpha, beta- and beta, gamma-dihydroxyphosphonates with high enantioselectivity. The method was applied to the kinetic resolution of racemic alpha-oxygetated beta, gamma-unsaturated phosphonates. Treatment of allyloxymethylphosphonates with the base afforded alpha-hydroxyphosphonates via the [2,3]-Wittig reaction. Threo- and erythro-beta-amino-alpha-hydroxyphosphinates were obtained with high diastereoselectivity by phosphinylation of alpha-aminoaldehydes in the presence of (R)- and (S)-ALB, respectively. The phosphinylation of alpha-oxygenated aldehydes afforded the corresponding alpha, beta-dioxygenated phosphinates, but with low diastereoselectivity. Sphingomyelin analogues containing CF(2)PO(OH)(2) were synthesized starting from (S)- and (R)-Garner aldehyde for the purpose of obtaining potent sphyngomyelinase inhibitors. A useful method for the synthesis of alpha, alpha-difluorobenzylphosphonates was established based on the cross coupling reaction of an iodobenzene derivative with ZnCuBr(2)CF(2)PO(OEt)(2). The synthetic utility of ZnCuBr(2)CF(2)PO(OEt)(2) was examined to obtain alpha, alpha-difluoromethylenenphosphonates. The method was applied to a synthesis of PNP-inhibitory active compounds by combination of the purine base and alcohols containing difluoromethylenephosphonate. The methodology for the beta-selective N-glycosylation of 2,3-dideoxy glucoside was established by introducing phosphonothioates at the 3-position of glycosyl doners instead of phosphonate. Synthesis of new acylic nucleotide analogues designed based on the structural modification of ARS2267 is also described. Finally, kiral synthesis of some phosphonates was achieved using lipase through kinetic resolution.

Mechanistic studies on protein tyrosine phosphatases

The human genome encodes approximately 100 phosphatases that belong to the protein tyrosine phosphatase (PTP) superfamily. The hallmark for this superfamily is the active site sequence C(X)5R, also known as the PTP signature motif. The PTPs are key regulatory components in signal transduction pathways and the importance of PTPs in the control of cellular signaling is well established. Based on structure and substrate specificity, the PTP superfamily is divided into four distinct subfamilies: (1) pTyr-specific PTPs, (2) dual specificity phosphatases, (3) Cdc25 phosphatases, and (4) LMW PTPs. The PTPs have similar core structures made of a central parallel beta-sheet with flanking a-helices containing a beta-loop-alpha-loop that encompasses the PTP signature motif. Site-directed mutagenesis of conserved amino acids in the Yersinia PTP and several other phosphatases in the PTP superfamily combined with detailed kinetic and mechanistic analyses have revealed a common chemical mechanism for phosphate hydrolysis despite the differences in substrate specificity. This article reviews our current knowledge of the common features important for PTP catalysis, the nature of the enzymatic transition state, and the roles of essential residues in transition stabilization. Future mechanistic studies of PTPs will focus on the use of physiological substrates to determine the molecular basis of substrate recognition and regulation, which is essential for understanding the specific functional role of PTPs in cellular signaling.

A translocated protein tyrosine phosphatase of Pseudomonas syringae pv. tomato DC3000 modulates plant defence response to infection

Pseudomonas syringae strains translocate effector proteins into host cells via the hrp-encoded type III protein secretion system (TTSS) to facilitate pathogenesis in susceptible plants. However, the mechanisms by which pathogenesis is favoured by these effectors are not well understood. Individual strains express multiple effectors with apparently distinct activities that are co-ordinately regulated by the alternative sigma factor HrpL. Genes for several effectors were identified in the P. syringae pv. tomato DC3000 genome using a promoter trap assay to identify HrpL-dependent promoters. In addition to orthologues of avrPphE and hrpW, an unusual allele of avrPphD was detected that carried an IS52 insertion. Using this avrPphD::IS52 allele as a probe, a wild-type allele of avrPphD, hopPtoD1, and a chimeric homologue were identified in the DC3000 genome. This chimeric homologue, identified as HopPtoD2 in the annotated DC3000 genome, consisted of an amino terminal secretion domain similar to that of AvrPphD fused to a potential protein tyrosine phosphatase domain. Culture filtrates of strains expressing HopPtoD2 were able to dephosphorylate pNPP and two phosphotyrosine peptides. HopPtoD2 was shown to be translocated into Arabidopsis thaliana cells via the hrp-encoded TTSS. A DeltahopPtoD2 mutant of DC3000 exhibited strongly reduced virulence in Arabidopsis thaliana. Ectopic expression of hopPtoD2 in P. syringae Psy61 that lacks a native hopPtoD2 orthologue delayed the development of several defence-associated responses including programmed cell death, active oxygen production and transcription of the pathogenesis-related gene PR1. The results indicate that HopPtoD2 is a translocated effector with protein tyrosine phosphatase activity that modulates plant defence responses.

Glial cells as targets and producers of neurotrophins

Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

Assessment of protein-tyrosine phosphatase 1B substrate specificity using "inverse alanine scanning"

An "inverse alanine scanning" peptide library approach has been developed to assess the substrate specificity of protein-tyrosine phosphatases (PTPases). In this method each Ala moiety in the parent peptide, Ac-AAAApYAAAA-NH(2), is separately and sequentially replaced by the 19 non-Ala amino acids to generate a library of 153 well defined peptides. The relatively small number of peptides allows the acquisition of explicit kinetic data for all library members, thereby furnishing information about the contribution of individual amino acids with respect to substrate properties. The approach was applied to protein-tyrosine phosphatase 1B (PTP1B) as a first example, and the highly potent peptide substrate Ac-ELEFpYMDYE-NH(2) (k(cat)/K(m) 2.2 +/- 0.05 x 10(7) M(-1) s(-1)) has been identified. More importantly, several heretofore unknown features of the substrate specificity of PTP1B were revealed. This includes the ability of PTP1B to accommodate acidic, aromatic, and hydrophobic residues at the -1 position, a strong nonpreference for Lys and Arg residues in any position, and the first evidence that residues well beyond the +1 position contribute to substrate efficacy.

Stimulated phosphorylation of intracellular connexin43

A monoclonal antibody, Zymed 13-8300, was previously reported to only detect nonphosphorylated connexin43 (Nagy et al., Exp. Cell Res. 236, 127-136, 1997). We show that 13-8300 can detect several phosphorylated species of connexin43 in Western blots after stimulation of two fibroblast cell systems with fresh growth medium, 12-O-tetradecanoyl phorbol-13-acetate, pervanadate, or permolybdate. In one of the cell systems, at least three forms of phosphorylated connexin43 could migrate at the same position during electrophoresis. The comigration of differentially phosphorylated species may complicate the molecular and functional analysis of phosphorylation sites in Cx43. Immunofluorescence experiments indicated that the newly generated phosphorylated Cx43 forms mainly had a perinuclear location. Also, in cells treated with brefeldin A for 8 h, in which the majority of connexin43 was intracellular, phosphorylation was induced by the agents. Phosphorylation of intracellular connexin43 can therefore be induced by several stimuli.

Identification of a second aryl phosphate-binding site in protein-tyrosine phosphatase 1B: a paradigm for inhibitor design

The structure of the catalytically inactive mutant (C215S) of the human protein-tyrosine phosphatase 1B (PTP1B) has been solved to high resolution in two complexes. In the first, crystals were grown in the presence of bis-(para-phosphophenyl) methane (BPPM), a synthetic high-affinity low-molecular weight nonpeptidic substrate (Km = 16 microM), and the structure was refined to an R-factor of 18. 2% at 1.9 A resolution. In the second, crystals were grown in a saturating concentration of phosphotyrosine (pTyr), and the structure was refined to an R-factor of 18.1% at 1.85 A. Difference Fourier maps showed that BPPM binds PTP1B in two mutually exclusive modes, one in which it occupies the canonical pTyr-binding site (the active site), and another in which a phosphophenyl moiety interacts with a set of residues not previously observed to bind aryl phosphates. The identification of a second pTyr molecule at the same site in the PTP1B/C215S-pTyr complex confirms that these residues constitute a low-affinity noncatalytic aryl phosphate-binding site. Identification of a second aryl phosphate binding site adjacent to the active site provides a paradigm for the design of tight-binding, highly specific PTP1B inhibitors that can span both the active site and the adjacent noncatalytic site. This design can be achieved by tethering together two small ligands that are individually targeted to the active site and the proximal noncatalytic site.

Structure and function of the low Mr phosphotyrosine protein phosphatases

Phosphotyrosine protein phosphatases (PTPases) catalyse the hydrolysis of phosphotyrosine residues in proteins and are hence implicated in the complex mechanism of the control of cell proliferation and differentiation. The low Mr PTPases are a group of soluble PTPases displaying a reduced molecular mass; in addition, a group of low molecular mass dual specificity (ds)PTPases which hydrolyse phosphotyrosine and phosphoserine/threonine residues in proteins are known. The enzymes belonging to the two groups are unrelated to each other and to other PTPase classes except for the presence of a CXXXXXRS/T sequence motif containing some of the catalytic residues (active site signature) and for the common catalytic mechanism, clearly indicating convergent evolution. The low Mr PTPases have a long evolutionary history since microbial (prokaryotic and eukaryotic) counterparts of both tyrosine-specific and dsPTPases have been described. Despite the relevant number of data reported on the structural and catalytic features of a number of low Mr PTPases, only limited information is presently available on the substrate specificity and the true biological roles of these enzymes, in prokaryotic, yeast and eukaryotic cells.

Effect of extracellular AMP on cell proliferation and metabolism of breast cancer cell lines with high and low glycolytic rates

In differentiated tissues, such as muscle and brain, increased adenosine monophosphate (AMP) levels stimulate glycolytic flux rates. In the breast cancer cell line MCF-7, which characteristically has a constantly high glycolytic flux rate, AMP induces a strong inhibition of glycolysis. The human breast cancer cell line MDA-MB-453, on the other hand, is characterized by a more differentiated metabolic phenotype. MDA-MB-453 cells have a lower glycolytic flux rate and higher pyruvate consumption than MCF-7 cells. In addition, they have an active glycerol 3-phosphate shuttle. AMP inhibits cell proliferation as well as NAD and NADH synthesis in both MCF-7 and MDA-MB-453 cells. However, in MDA-MB-453 cells glycolysis is slightly activated by AMP. This disparate response of glycolytic flux rate to AMP treatment is presumably caused by the fact that the reduced NAD and NADH levels in AMP-treated MDA-MB-453 cells reduce lactate dehydrogenase but not cytosolic glycerol-3-phosphate dehydrogenase reaction. Due to the different enzymatic complement in MCF-7 cells, proliferation is inhibited under glucose starvation, whereas MDA-MB-453 cells grow under these conditions. The inhibition of cell proliferation correlates with a reduction in glycolytic carbon flow to synthetic processes and a decrease in phosphotyrosine content of several proteins in both cell lines.

Structure and function of the protein tyrosine phosphatases

The tyrosine and dual-specificity phosphatases are involved in signaling, cell growth and differentiation, and the cell cycle. The enzymes share a common catalytic mechanism mediated by an active site cysteine, arginine and aspartic acid. Supplementary domains assist in targeting and substrate specificity.

The active site specificity of the Yersinia protein-tyrosine phosphatase

Yersinia protein-tyrosine phosphatase substrates have been synthesized employing an expedient methodology that incorporates phosphorylated non-amino acid residues into an active site-directed peptide. While the peptidic portion of these compounds serves an enzyme targeting role, the nonpeptidic component provides a critical assessment of the range of functionality that can be accommodated within the active site region. We have found that the Yersinia phosphatase hydrolyzes both L- and D-stereoisomers of phosphotyrosine in active site-directed peptides, with the former serving as a 10-fold more efficient substrate than the latter. In addition, this enzyme catalyzes the hydrolysis of a variety of aromatic and aliphatic phosphates. Indeed, a peptide bearing the achiral phosphotyrosine analog, phosphotyramine, is not only the most efficient substrate described in this study, it is also one of the most efficient substrates ever reported for the Yersinia phosphatase. Straight chain peptide-bound aliphatic phosphates of the general structure, (Glu)4-NH-(CH2)n-OPO3(2-) (n = 2-8), are also hydrolyzed, where the most efficient substrate contains seven methylene groups. Finally, a comparison of the substrate efficacy of the peptide-bound species with that of the corresponding non-peptidic analogs, reveals that the peptide component enhances kcat/Km by up to nearly 3 orders of magnitude.

Protein tyrosine phosphatases take off

Protein tyrosine phosphatases (PTPs) are a family of signal transduction enzymes that dephosphorylate phosphotyrosine containing proteins. Structural and kinetic studies provide a molecular understanding of how these enzymes regulate a wide range of intracellular processes.