Human dual specificity phosphatase VHR activates maturation promotion factor and triggers meiotic maturation in Xenopus oocytes

Involvement of the Mitochondrial Protein Tyrosine Phosphatase PTPM1 in the Promotion of Conidiation, Development, and Pathogenicity in Colletotrichum graminicola

The phosphorylation status of proteins, which is determined by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs), governs many cellular actions. In fungal pathogens, phosphorylation-mediated signal transduction has been considered to be one of the most important mechanisms in pathogenicity. Colletotrichum graminicola is an economically important corn pathogen. However, whether phosphorylation is involved in its pathogenicity is unknown. A mitochondrial protein tyrosine phosphatase gene, designated CgPTPM1, was deduced in C. graminicola through the use of bioinformatics and confirmed by enzyme activity assays and observation of its subcellular localization. We then created a CgPTPM1 deletion mutant (ΔCgPTPM1) to analyze its biological function. The results indicated that the loss of CgPTPM1 dramatically affected the formation of conidia and the development and differentiation into appressoria. However, the colony growth and conidial morphology of the ΔCgPTPM1 strains were unaffected. Importantly, the ΔCgPTPM1 mutant strains exhibited an obvious reduction of virulence, and the delayed infected hyphae failed to expand in the host cells. In comparison with the wild-type, ΔCgPTPM1 accumulated a larger amount of H2O2 and was sensitive to exogenous H2O2. Interestingly, the host cells infected by the mutant also exhibited an increased accumulation of H2O2 around the infection sites. Since the expression of the CgHYR1, CgGST1, CgGLR1, CgGSH1 and CgPAP1 genes was upregulated with the H2O2 treatment, our results suggest that the mitochondrial protein tyrosine phosphatase PTPM1 plays an essential role in promoting the pathogenicity of C. graminicola by regulating the excessive in vivo and in vitro production of H2O2.

The Putative Protein Phosphatase MoYvh1 Functions Upstream of MoPdeH to Regulate the Development and Pathogenicity in Magnaporthe oryzae

Protein phosphatases are critical regulators in eukaryotic cells. For example, the budding yeast Saccharomyces cerevisiae dual specificity protein phosphatase (DSP) ScYvh1 regulates growth, sporulation, and glycogen accumulation. Despite such importance, functions of Yvh1 proteins in filamentous fungi are not well understood. In this study, we characterized putative protein phosphatase MoYvh1, an Yvh1 homolog in the rice blast fungus Magnaporthe oryzae. Deletion of the MoYVH1 gene resulted in significant reductions in vegetative growth, conidial production, and virulence. The ΔMoyvh1 mutant also displayed defects in cell-wall integrity and was hyposensitive to the exogenous osmotic stress. Further examination revealed that the ΔMoyvh1 mutant had defects in appressorium function and invasive hyphae growth, resulting attenuated pathogenicity. Interestingly, we found that MoYvh1 affects the scavenging of host-derived reactive oxygen species that promotes M. oryzae infection. Finally, overexpression of the phosphodiesterase MoPDEH suppressed the defects in conidia formation and pathogenicity of the ΔMoyvh1 mutant, suggesting MoYvh1 could regulate MoPDEH for its function. Our study reveals not only the importance of MoYvh1 proteins in growth, differentiation, and virulence of the rice blast fungus but, also, a genetic link between MoYvh1 and MoPDEH-cAMP signaling in this fungus.

A zinc-binding dual-specificity YVH1 phosphatase in the malaria parasite, Plasmodium falciparum, and its interaction with the nuclear protein, pescadillo

Biochemical evidence revealed protein tyrosine kinase and phosphatase activities in the human malarial parasite Plasmodium falciparum, a member of the Apicomplexa. A novel cDNA sequence of a dual-specificity phosphatase was identified in both sexual and asexual stages of P. falciparum, and named PfYVH1, since the predicted primary structure of the 278-amino acid polypeptide showed significant similarity to the human and yeast YVH1 phosphatases. The N-terminal half of PfYVH1 contained a conserved tyrosine phosphatase catalytic domain within a dual-specificity phosphatase domain. The C-terminal region, consisting of one histidine and eight cysteines, represented a zinc-binding domain with a potentially unconventional architecture. Recombinant PfYVH1 contained 2mol of zinc per mol protein and dephosphorylated both phosphoserine and phosphotyrosine residues. Mutation of specific Cys residues in the putative zinc finger region abolished zinc binding and drastically reduced phosphatase activity, suggesting an allosteric role of zinc in catalysis. PfYVH1 was expressed in essentially all erythrocytic stages of the parasite, and shuttled between the nucleus and the cytoplasm in a stage-specific manner. A Plasmodium ortholog of the nuclear pescadillo protein (PfPES) was also characterized and shown to interact with PfYVH1, thus implicating PfYVH1 in the regulation of parasitic development. PfYVH1 represents the first dual-specificity zinc-finger phosphatase characterized in the protozoan kingdom.

Synthesis of a tetronic acid library focused on inhibitors of tyrosine and dual-specificity protein phosphatases and its evaluation regarding VHR and cdc25B inhibition

Selective inhibitors of protein tyrosine phosphatases (PTPs) and dual-specificity phosphatases (DSPs) are expected to be useful tools for clarifying the biological functions of the PTPs themselves and also to be candidates for novel therapeutics. We planned a library approach for the identification of PTP/DSP inhibitors in which 3-acyltetronic acid is used as a "core" phosphate mimic. A series of novel tetronic acid derivatives were synthesized and evaluated as inhibitors of the dual-specificity protein phosphatases VHR and cdc25B. Several compounds are found to be potent inhibitors of cdc25B, which is a key enzyme for cell-cycle progression. The promising results described herein strongly indicated that this tetronic acid library is potent as a library focused on the PTP/DSP-selective inhibitor.

Recombinant C1b domain of PKCdelta triggers meiotic maturation upon microinjection in Xenopus laevis oocytes

The C1 domains are 50 amino acid sequences present in protein kinase C (PKC) isozymes that are responsible for binding of phorbol esters and the lipid second messenger diacylglycerol (DAG). We found that bacterially expressed C1b domain of PKCdelta induces germinal vesicle breakdown (GVBD) when microinjected into Xenopus laevis oocytes. Injection of the C1b domain of PKCdelta significantly enhanced insulin- but not progesterone-induced maturation. Interestingly, the PKCdelta C1b domain markedly synergized with normal Ras protein to induce oocyte maturation when both proteins were co-injected in oocytes. Our results demonstrate that the purified C1b domain of PKCdelta is sufficient to promote meiotic maturation of X. laevis oocytes probably through activation of components of the insulin/Ras signaling pathway.

Oncogenic Ras-induced germinal vesicle breakdown is independent of phosphatidylinositol 3-kinase in Xenopus oocytes

A number of reports have identified phosphatidylinositol 3-kinase as a downstream effector of Ras in various cellular settings, in contrast to others supporting the notion that phosphatidylinositol 3-kinase acts upstream of Ras. Here, we used Xenopus oocytes, a model of Ras-mediated cell cycle progression (G2/M transition) to analyze the contribution of phosphatidylinositol 3-kinase to insulin/Ras-dependent signaling pathways leading to germinal vesicle breakdown and to ascertain whether phosphatidylinositol 3-kinase acts upstream or downstream of Ras in those signaling pathways. We analyzed the process of meiotic maturation induced by progesterone, insulin or micro-injected oncogenic Ras (Lys12) proteins in the presence and absence of specific inhibitors of phosphatidylinositol 3-kinase activity. As expected, the progesterone-induced maturation was independent of phosphatidylinositol 3-kinase since similar rates of germinal vesicle breakdown were produced by the hormone in the presence and absence of wortmannin and LY294002. In contrast, insulin-induced germinal vesicle breakdown was completely blocked by pre-incubation with the inhibitors prior to insulin treatment. Interestingly, similar rates of germinal vesicle breakdown were obtained in Ras (Lys12)-injected oocytes, independently of whether or not they had been pre-treated with phosphatidylinositol 3-kinase inhibitors. The effect of wortmannin or LY294002 on MAPK and Akt activation by progesterone, insulin or Ras was also analyzed. Whereas insulin activated those kinases in a phosphatidylinositol 3-kinase-dependent manner, progesterone and Ras were able to activate those kinases in the absence of phosphatidylinositol 3-kinase activity. Since Ras is a necessary and sufficient downstream component of insulin signaling pathways leading to germinal vesicle breakdown, these observations demonstrate that phosphatidylinositol 3-kinase is not a downstream effector of Ras in insulin/Ras-dependent signaling pathways leading to entry into the M phase in Xenopus oocytes.

Airway nerves and protein phosphatases

Much attention has focused on the important role played by phosphatases in the control of gene transcription, cell differentiation and memory regulation. It is also clear that phosphatases may regulate a number of biochemical pathways which can modulate cellular function. Of particular interest is the role of phosphatases in the control of neuronal function. Alterations in neuronal function may contributed to the heightened airways responsiveness observed in asthma to a number of physiological stimuli including distilled water, sulfur dioxide, metabisulfite, hypertonic saline, exercise, allergens, viruses and cold air. An understanding of the mechanisms which regulate the function of sensory nerves could have important clinical implications. In this review we will highlight a number of studies that have investigated the role of phosphatases in the regulation of airway nerve function.

Protein-tyrosine phosphatases: biological function, structural characteristics, and mechanism of catalysis

The protein-tyrosine phosphatases (PTPases) superfamily consists of tyrosine-specific phosphatases, dual specificity phosphatases, and the low-molecular-weight phosphatases. They are modulators of signal transduction pathways that regulate numerous cell functions. Malfunction of PTPases have been linked to a number of oncogenic and metabolic disease states, and PTPases are also employed by microbes and viruses for pathogenicity. There is little sequence similarity among the three subfamilies of phosphatases. Yet, three-dimensional structural data show that they share similar conserved structural elements, namely, the phosphate-binding loop encompassing the PTPase signature motif (H/V)C(X)5R(S/T) and an essential general acid/base Asp residue on a surface loop. Biochemical experiments demonstrate that phosphatases in the PTPase superfamily utilize a common mechanism for catalysis going through a covalent thiophosphate intermediate that involves the nucleophilic Cys residue in the PTPase signature motif. The transition states for phosphoenzyme intermediate formation and hydrolysis are dissociative in nature and are similar to those of the solution phosphate monoester reactions. One strategy used by these phosphatases for transition state stabilization is to neutralize the developing negative charge in the leaving group. A conformational change that is restricted to the movement of a flexible loop occurs during the catalytic cycle of the PTPases. However, the relationship between loop dynamics and enzyme catalysis remains to be established. The nature and identity of the rate-limiting step in the PTPase catalyzed reaction requires further investigation and may be dependent on the specific experimental conditions such as temperature, pH, buffer, and substrate used. In-depth kinetic and structural analysis of a representative number of phosphatases from each group of the PTPase superfamily will most likely continue to yield insightful mechanistic information that may be applicable to the rest of the family members.

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.

VHR and PTP1 protein phosphatases exhibit remarkably different active site specificities toward low molecular weight nonpeptidic substrates

The dual-specificity protein phosphatases have recently been shown to act as key regulators of mitogenic signaling pathways as well as of the cell cycle process. The are unusual catalysts in that they can utilize protein substrates containing phosphotyrosine as well as phosphoserine/threonine. The dual-specificity phosphatases and the protein tyrosine phosphatases (PTPase) share the active site motif (H/V)C(X)5R(S/T) but display little amino acid sequence identity outside of the active site. Although the dual-specificity phosphatases and the PTPases appear to bring about phosphate monoester hydrolysis through a similar mechanism, there is very limited information about the structural features that control the substrate specificity for the two groups of enzymes. As a first step in the development of selective dual-specificity phosphatase inhibitors, we have examined the active site substrate specificity of the human dual-specificity phosphatase, VHR [for VH1-Related; Ishibashi et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 12170-12174]. Like the tyrosine-specific PTP1, VHR also preferentially catalyzes the hydrolysis of aromatic phosphates. However, we demonstrate herein that relatively modest changes in the substitution patterns on the phosphorylated aromatic nucleus generates dramatic, and differential, swings in substrate selectivity for VHR and PTP1. For example, VHR appears to be significantly more accommodating than PTP1 toward sterically-demanding substrates. Thus, the active site specificity of these two protein phosphatases is decidedly dissimilar. In addition, we have also identified several low molecular weight compounds that are more efficient substrates than the most potent peptidic substrates ever reported for VHR. Finally, we have shown that the Michaelis constants exhibited by these substrates are accurate assessments of enzyme affinity. Consequently, it should be possible to develop phospatase-selective inhibitors based upon the distinct substrate specificities of these enzymes.

Crystal structure of the dual specificity protein phosphatase VHR

Dual specificity protein phosphatases (DSPs) regulate mitogenic signal transduction and control the cell cycle. Here, the crystal structure of a human DSP, vaccinia H1-related phosphatase (or VHR), was determined at 2.1 angstrom resolution. A shallow active site pocket in VHR allows for the hydrolysis of phosphorylated serine, threonine, or tyrosine protein residues, whereas the deeper active site of protein tyrosine phosphatases (PTPs) restricts substrate specificity to only phosphotyrosine. Positively charged crevices near the active site may explain the enzyme's preference for substrates with two phosphorylated residues. The VHR structure defines a conserved structural scaffold for both DSPs and PTPs. A "recognition region," connecting helix alpha1 to strand beta1, may determine differences in substrate specificity between VHR, the PTPs, and other DSPs.

A single mutation converts a novel phosphotyrosine binding domain into a dual-specificity phosphatase

Dual-specificity protein-tyrosine phosphatases (dsPTPases) have been implicated in the inactivation of mitogen-activated protein kinases (MAPKs). We have identified a novel phosphoserine/threonine/tyrosine-binding protein (STYX) that is related in amino acid sequence to dsPTPases, except for the substitution of Gly for Cys in the conserved dsPTPase catalytic loop (HCXXGXXR(S/T)). cDNA subcloning and Northern blot analysis in mouse shows poly(A+) hybridization bands of 4.6, 2.4, 1.5, and 1.2 kilobases, with highest abundance in skeletal muscle, testis, and heart. Polymerase chain reaction amplification of reverse-transcribed poly(A+) RNA revealed an alternatively spliced form of STYX containing a unique carboxyl terminus. Bacterially expressed STYX is incapable of hydrolyzing Tyr(P)-containing substrates; however, mutation of Gly120 to Cys (G120C), which structurally mimics the active site of dsPTPases, confers phosphatase activity to this molecule. STYX-G120C mutant hydrolyzes p-nitrophenyl phosphate and dephosphorylates both Tyr(P) and Thr(P) residues of peptide sequences of MAPK homologues. The kinetic parameters of dephosphorylation are similar to human dsPTPase, Vaccinia H1-related, including inhibition by vanadate. We believe this is the first example of a naturally occurring "dominant negative" phosphotyrosine/serine/threonine-binding protein which is structurally related to dsPTPases.