Isolation and characterization of phosphoprotein phosphatase 1 from alfalfa

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.

Protein phosphatase 1 catalytic subunit isoforms from alfalfa: biochemical characterization and cDNA cloning

The catalytic subunit of protein phosphatase 1 (PP1c) was purified from an alfalfa (Medicago sativa) microcallus cell culture. The preparation was inhibited by rabbit muscle inhibitor-2 and okadaic acid and had a molecular mass of 35 kDa. Five distinct cDNAs termed MsPP1alpha, -beta, -gamma, -delta, and -epsilon were cloned from a M. sativa somatic embryo library. MsPP1alpha was identical to a cDNA reported earlier [A. Páy, M. Pirck, L. Bögre, H. Hirt, and E. Heberle-Bors Mol. Gen. Genet. 244, 176-182, 1994], while the others represented novel isoforms encoded by separate genes. The predicted amino acid sequences of MsPP1alpha, -beta, -gamma, -delta, and -epsilon were highly similar to each other and to other known PP1c sequences. The GST-MsPP1ss fusion protein expressed in Escherichia coli was catalytically active and was inhibited by inhibitor-2 and okadaic acid. Affinity-purified polyclonal MsPP1antipeptide antibody detected a protein of 36 kDa in crude cell extracts. These results proved that the cDNA clone encoded an active PP1c which was very similar to the purified enzyme. The mRNA and protein concentrations of PP1c as well as the specific activity of protein phosphatase 1 did not change during the cell cycle in a synchronized alfalfa cell culture. On the other hand, the isoforms exhibited different steady-state mRNA levels in different plant organs suggesting tissue-specific functions.

Genomic organization and functional analysis of the murine protein phosphatase 1c gamma (Ppp1cc) gene

Protein phosphatase 1 holoenzymes are composed of catalytic subunits in combination with various regulatory subunits. In rodents, four different catalytic isoforms are known, PP1c alpha, -delta, -gamma 1, and -gamma 2. Here we describe the genomic organization of the murine Ppp1cc gene that encodes the PP1c gamma 1 and PP1c gamma 2 isoforms. We determined that Ppp1cc maps to F1.2-G1.2 on chromosome 5 by FISH mapping. Southern hybridization and analysis of cross-hybridizing genomic clones revealed four Ppp1cc-related pseudogenes in the mouse genome. The authentic Ppp1cc gene encodes two isoforms, PP1c gamma 1 and PP1c gamma 2, that arise from alternative splicing and differ by retention of the last intron. The introns of Ppp1cc are flanked by short direct repeats, the significance of which is not clear. Both isoforms retain phosphatase function since they are able to complement the cold-sensitive PP1 defect caused by the dis2-11 mutation in the fission yeast Schizosaccharomyces pombe.

[Protein phosphatases and protein kinases in higher plants]

The recent gain in knowledge concerning enzymes involved in signal transduction pathways is a direct consequence of the considerable advances made in molecular biology. Protein kinases and protein phosphatases, the two major enzymes implicated in post-translational modifications, have been studied in particular. The number of characterized plant genes and/or cDNAs encoding these enzymes is increasing everyday. Since 1991, 26 genes and cDNAs coding for plant protein phosphatases have been isolated and characterized. The huge number of protein kinases (estimated at several thousands) makes it impossible to give an exhaustive list of the genes already identified, but a classification of these enzymes, based on phylogenetic criteria, allows us to appreciate the range of functions this protein family may play in plants.

PLANT PROTEIN PHOSPHATASES

Posttranslational modification of proteins by phosphorylation is a universal mechanism for regulating diverse biological functions. Recognition that many cellular proteins are reversibly phosphorylated in response to external stimuli or intracellular signals has generated an ongoing interest in identifying and characterizing plant protein kinases and protein phosphatases that modulate the phosphorylation status of proteins. This review discusses recent advances in our understanding of the structure, regulation, and function of plant protein phosphatases. Three major classes of enzymes have been reported in plants that are homologues of the mammalian type-1, -2A, and -2C protein serine/threonine phosphatases. Molecular genetic and biochemical studies reveal a role for some of these enzymes in signal transduction, cell cycle progression, and hormonal regulation. Studies also point to the presence of additional phosphatases in plants that are unrelated to these major classes.

Molecular cloning of the cDNA encoding a stress-inducible protein phosphatase 1 (PP1) catalytic subunit from French bean (Phaseolus vulgaris L.)

A cDNA showing high sequence similarity (> 70%) to plant protein phosphatase 1 catalytic subunit variants from other species has been isolated from a cDNA library derived from mRNAs expressed in elicitor-treated suspension-cultured cells. The clone appears to be a near full-length 1431 bp with a 172 bp 5'-untranslated region and a 317 bp 3'-untranslated region. The open reading frame, determined by sequence similarity, codes for a protein with predicted M(r) of 35,552. Alternatively an ATG situated to the 5' end of the putative start site would increase the protein size by 6 amino acids. The mRNA for Pvpp1 was shown to be rapidly induced by elicitor treatment of suspension-cultured cells of French bean. The cloned cDNA represents one of the few examples of a gene product that is probably involved in dephosphorylation events arising after the initial responses to biotic stress.