Protein tyrosine phosphatases in higher plants

Calcium imaging: a technique to monitor calcium dynamics in biological systems

Calcium ion (Ca2+) is a multifaceted signaling molecule that acts as an important second messenger. During the course of evolution, plants and animals have developed Ca2+ signaling in order to respond against diverse stimuli, to regulate a large number of physiological and developmental pathways. Our understanding of Ca2+ signaling and its components in physiological phenomena ranging from lower to higher organisms, and from single cell to multiple tissues has grown exponentially. The generation of Ca2+ transients or signatures for various stress factor is a well-known mechanism adopted in plant and animal systems. However, the decoding of such remarkable signatures is an uphill task and is always an interesting goal for the scientific community. In the past few decades, studies on the concentration and dynamics of intracellular Ca2+ are significantly increasing and have become a trend in modern biology. The advancement in approaches from Ca2+ binding dyes to in vivo Ca2+ imaging through the use of Ca2+ biosensors to achieve spatio-temporal resolution in micro and milliseconds range, provide us phenomenal opportunities to study live cell Ca2+ imaging or dynamics. Here, we describe the usage, improvement and advancement of Ca2+ based dyes, genetically encoded probes and sensors to achieve extraordinary Ca2+ imaging in plants and animals.

Graphical abstract

Involvement of plasma membrane H+-ATPase in diamide-induced extracellular alkalization by roots from pea seedlings

MAIN CONCLUSION

The plasma membrane H+-ATPase can be considered as a redox-dependent enzyme, because diamide-mediated inhibition of its hydrolytic and transport activities is accompanied by alkalization of the rhizosphere and retardation of root growth. Plasma membranes were isolated from roots of etiolated pea seedlings treated in the presence of an oxidant-diamide and an inhibitor of redox-sensitive protein phosphatase-phenylarsine oxide. Hydrolytic and proton transport activities of H+-ATPase were determined. The effects of diamide appeared in inhibition of both ATP hydrolysis and the proton transport. However, root treatment with phenylarsine oxide only slightly reduced Vmax, but did not affect ATP-dependent proton transport. The thiol groups of cysteines in the proteins can act as molecular targets for both compounds. However, treatment of isolated membranes with diamide or dithiothreitol did not have any effect on the H+ transport. It can be assumed that water-soluble diamide acts indirectly and its effects are not associated with oxidation of H+-ATPase cysteines. Therefore, plasmalemma was subjected to PEGylation-process where reduced cysteines available for PEG maleimide (5 kDa) were alkylated. Detection of such cysteines was carried out by Western blot analysis with anti-ATPase antibodies. It was found that shifts in the apparent molecular weight were detected only for denaturated proteins. These data suggest that available thiols are not localized on the enzyme surfaces. BN-PAGE analysis showed that the molecular weights of the ATPase complexes are almost identical in all samples. Therefore, oligomerization is probably not the reason for the inhibition of ATPase activity. Roots treated with these inhibitors in vivo exhibited stunted growth; however, a strong alkaline zone around the roots was formed only in the presence of diamide. Involvement of H+-ATPase redox regulation in this process is discussed.

MAP-kinase activity in etiolated Cucumis sativus cotyledons: the effect of red and far-red light irradiation

Phytochrome (phy) signalling in plants may be transduced through protein phosphorylation. Mitogen-activated protein kinase (MAP-kinase, MAPK) activity and the effect of R (red) and FR (far-red) light irradiation on MAPK activity were studied in etiolated Cucumis sativus L. cotyledons. By in vitro protein phosphorylation and in-gel assays with myelin basic protein (MBP), a protein band (between 48 and 45 kDa) with MAPK-like activity was detected. The addition to the phosphorylation buffer of specific protein phosphatase (PTP) inhibitors (Na(3)VO(4) and NaF) and genistein, apigenin or PD98059 as MAPK inhibitors allowed us to confirm the MAPK activity of the protein band. Irradiation of etiolated cotyledons with FR light for 5, 10 or 60 min rapidly and transiently stimulated the MAPK activity of the protein band. This suggests that there was a very low fluence response (VLFR) of phys. In addition, 15 min of R light irradiation or a sequential treatment of 15 min of R plus 5 min of FR also increased MAPK activity. The stimulatory effect of R light was also attributed to the same photoreceptor, which suggests that MAPKs are involved in phytochrome signal transduction. Protein immunoprecipitation and immunoblotting analysis with the polyclonal antibody anti-pERK1/2 (Tyr 204) and the monoclonal antibody anti-phosphotyrosine PY20 allowed us to recognize the above mentioned protein band as two proteins with molecular masses (M(r)) of approximately 47 and 45 kDa, and MAPK activity. The biochemical and immunological properties showed by the proteins detected indicated that they were members of the MAPK family phosphorylated in tyrosine residues.

Purification and characterization of ZmRIP1, a novel reductant-inhibited protein tyrosine phosphatase from maize

Protein tyrosine phosphatases (PTPases) have long been thought to be activated by reductants and deactivated by oxidants, owing to the presence of a crucial sulfhydryl group in their catalytic centers. In this article, we report the purification and characterization of Reductant-Inhibited PTPase1 (ZmRIP1) from maize (Zea mays) coleoptiles, and show that this PTPase has a unique mode of redox regulation and signaling. Surprisingly, ZmRIP1 was found to be deactivated by a reductant. A cysteine (Cys) residue (Cys-181) near the active center was found to regulate this unique mode of redox regulation, as mutation of Cys-181 to arginine-181 allowed ZmRIP1 to be activated by a reductant. In response to oxidant treatment, ZmRIP1 was translocated from the chloroplast to the nucleus. Expression of ZmRIP1 in Arabidopsis (Arabidopsis thaliana) plants and maize protoplasts altered the expression of genes encoding enzymes involved in antioxidant catabolism, such as At1g02950, which encodes a glutathione transferase. Thus, the novel PTPase identified in this study is predicted to function in redox signaling in maize.

In vivo influence of cyanobacterial toxins on enzyme activity and gene expression of protein phosphatases in Alfalfa (Medicago sativa)

Irrigation of crop plants with surface water can be a threat if cyanobacterial toxins are present in the water. Cyanotoxins are known to cause adverse effects in plants. Microcystin (MC), a cyclic heptapeptide, with more than 70 structural variants, is a frequently occurring toxin. MC is a specific inhibitor of serine/threonine protein phosphatases 1 and 2A (PP1 and 2A), important regulatory enzymes in eukaryotic cells. Protein phosphatases consist of a catalytic subunit and one or more regulatory subunits. In Alfalfa several isoforms of the catalytic subunit of PP1 (MsPP1alpha, MsPP1beta, MsPP1gamma, MsPP1delta, MsPP1varepsilon) and PP2A (MsPP2A Calpha/beta/gamma) are known along with isoforms of the regulatory subunits of PP2A (MsPP2A Aalpha/beta, MsPP2A Balpha/beta). The in vivo effect of environmentally relevant concentrations of cyanobacterial components on the mRNA transcript level of the subunits of protein phosphatases 1 and 2A in Alfalfa (Medicago sativa) was examined using semi-quantitative RT-PCR. Plants were exposed for one week to 5 microg L(-1) microcystin-LR, microcystin-LW, okadaic acid and to cell-free cyanobacterial crude extracts from Microcystis aeruginosa containing 5 microg L(-1) microcystin-LR and a toxin-free crude extract from Synechocystis spp. The protein phosphatase activity in vivo was inhibited when exposed to toxins and crude extract containing microcystin-LR, no change was induced by Synechocystis crude extract. The gene expression of the MsPP1gamma subunit and the MsPP1varepsilon subunit was induced in plants exposed to MC-LW.

Effect of brassinolide on tyrosine phosphorylation of pea leaf proteins

Brassinosteroid-induced phosphorylation of tyrosine residues in proteins was studied. Proteins of crude extract of pea leaves were analyzed by one- and two-dimensional electrophoresis followed by Western blotting with monoclonal antibodies PY20 to phosphotyrosine proteins. One- and two-dimensional electrophoresis revealed 7 and 13 tyrosine-phosphorylated proteins, respectively. Brassinolide increased the phosphorylation level of most of these proteins. With inhibitors of tyrosine protein phosphatases, such as phenylarsine oxide and orthovanadate, the level of tyrosine phosphorylation of these proteins increased.

Tyrosine phosphorylation inhibits the interaction of 14-3-3 proteins with the plant plasma membrane H+-ATPase

Interaction of 14-3-3 proteins with their targets depends not only on the phosphorylation status of the target but also on that of 14-3-3 (Fu et al., 2000). In this work we demonstrated that the maize 14-3-3 isoform GF14-6 is a substrate of the tyrosine kinase insulin growth factor receptor 1. By means of site-directed mutants of GF14-6, we identified Tyr-137 as the specific tyrosine residue phosphorylated by the insulin growth factor receptor 1. Phosphorylation of GF14-6 on Tyr-137 lowered its affinity for a peptide mimicking the 14-3-3 binding site of the plant plasma membrane H+-ATPase. Moreover, phosphorylation in planta of 14-3-3 tyrosine residues, resulting from incubation with the tyrosine phosphatase inhibitor, phenylarsine oxide, decreased their association to the H+-ATPase.

Of weeds and men: what genomes teach us about plant cell biology

It has generally been assumed that fundamental cellular processes are conserved at the molecular level. Genome comparisons, however, suggest that the molecular mechanisms underlying programmed cell death, defense, adaptation and development may differ considerably between the plant and animal kingdoms. Phylogenetic analyses have revealed a great deal of novelty in the plant genes that are implicated in conserved processes such as transcription, cytoskeletal dynamics and vesicle trafficking. The Arabidopsis genome highlights the highly dynamic and regulated nature of the plant cell, which is fine-tuned to light, water, nutrient availability, temperature, touch and wind.