Enhanced UV sensitivity of yeast cells induced by overexpression of Mg(2+)-dependent protein phosphatase alpha (type 2C alpha)

A rapid and simple spectroscopic method for the determination of yeast cell viability using methylene blue

Determination of cell viability is important in various microbiological studies. The microscopic method, counting dead cells stained by methylene blue (MB), has often been used for the determination of viability, although it is not efficient for the measurement of a large number of samples. Alternatively, some spectroscopic methods have been proposed to avoid tedious cell counting. One of these proposed methods detects the decrease in MB absorbance in the supernatant of cell suspension, because dead cells incorporate MB more efficiently than viable cells. However, at present, this spectroscopic method is rarely used due to its low throughput. Therefore, we devised a small-scale, rapid and simple method by improving several points as follows. (1) The peak wavelength of MB absorbance, 665 nm, was used to detect MB efficiently at the microtube scale. (2) The composition of the MB solution was improved by adding trisodium citrate. (3) The reaction time was shortened. And (4) the concentration ranges of both MB and cells, with which absorbance is linearly related to cell viability, were determined. The improved method enabled us to evaluate the dose-dependent toxicities of alcohols, antifungal/antimalarial quinacrine, and UV-C irradiation. The results were compatible with those of conventional microscopic counting and colony formation. The method would be applicable to automated determination and to various organisms such as bacteria and filamentous fungi which are difficult to be counted microscopically.

Protein phosphatase 2C inactivates F-actin binding of human platelet moesin

During activation of platelets by thrombin phosphorylation of Thr(558) in the C-terminal domain of the membrane-F-actin linking protein moesin increases transiently, and this correlates with protrusion of filopodial structures. Calyculin A enhances phosphorylation of moesin by inhibition of phosphatases. To measure this moesin-specific activity, a nonradioactive enzyme-linked immunosorbent assay method was developed with the synthetic peptide Cys-Lys(555)-Tyr-Lys-Thr(P)-Leu-Arg(560) coupled to bovine serum albumin as the substrate and moesin phosphorylation state-specific polyclonal antibodies for the detection and quantitation of dephosphorylation. Calyculin A-sensitive and -insensitive protein-threonine phosphatase activities were detected in platelet lysates and separated by DEAE-cellulose chromatography. The calyculin A-sensitive enzyme was identified as a type 1 protein phosphatase. The calyculin A-insensitive enzyme activity was purified to homogeneity by phenyl- Sepharose, protamine-, and phosphonic acid peptide-agarose chromatography and characterized biochemically and immunologically as a 53-kDa protein(s) and a type 2C protein phosphatase (PP2C). Phosphorylation of Thr(558) is necessary for F-actin binding of moesin in vitro. The purified enzyme, as well as bacterially made PP2Calpha and PP2Cbeta, efficiently dephosphorylate(s) highly purified platelet phospho-moesin. This reverses the activating effect of phosphorylation, and moesin no longer co-sediments with actin filaments. In vivo, regulation of these phosphatase activities are likely to influence dynamic interactions between the actin cytoskeleton and membrane constituents linked to moesin.

Alternative promoters direct tissue-specific expression of the mouse protein phosphatase 2Cbeta gene

Type 2C protein phosphatases (PP2Cs), a class of ubiquitous and evolutionally conserved serine/threonine protein phosphatases, are encoded in at least four distinct genes and implicated in the regulation of various cellular functions. Of these four PP2C genes, the expression of the PP2Cbeta gene has been reported to be tissue-specific and development-dependent. To understand more precisely the regulatory mechanism of this expression, we have isolated and characterized overlapping mouse genomic lambda clones. A comparison of genomic sequences with PP2Cbeta cDNA sequences provided information on the structure and localization of intron/exon boundaries and indicated that PP2Cbeta isoforms with different 5' termini were generated by alternative splicing of its pre-mRNA. The 5'-flanking region of exon 1 had features characteristic of a housekeeping gene: it was GC-rich, lacked TATA boxes and CAAT boxes in the standard positions, and contained potential binding sites for the transcription factor SP1. In the 5'-flanking region of exon 2, several consensus sequences were found, such as a TATA-like sequence and negative regulatory element box-1, -2 and -3. Subsequent analysis by transient transfection assay with a reporter gene showed that these regions act as distinct promoters. Analysis of PP2Cbeta transcripts by reverse transcriptase-PCR showed that exon-1 transcripts were expressed ubiquitously in all of the tissues examined, whereas exon-2 transcripts were predominantly expressed in the testis, intestine and liver. These results suggest that the alternative usage of two promoters within the PP2Cbeta gene regulates tissue-specific expression of PP2Cbeta mRNA.

Isoform specific phosphorylation of protein phosphatase 2C expressed in COS7 cells

Of the six distinct isoforms of mouse protein phosphatase 2C (PP2C) (alpha, beta-1, beta-2, beta-3, beta-4 and beta-5), PP2C alpha was specifically phosphorylated on the serine residue(s) when expressed in COS7 cells. Analysis of phosphorylation sites using site-directed mutagenesis demonstrated that Ser-375 and/or Ser-377 were phosphorylated in vivo. These serine residues were the sites of phosphorylation by casein kinase II in vitro. Phosphorylation of PP2C alpha was enhanced two-fold by the addition of okadaic acid to the culture medium, but addition of cyclosporin A had no such effect. These results suggest that the expressed PP2C alpha is phosphorylated by a casein kinase II-like protein kinase and dephosphorylated by PP1 and/or PP2A in COS7 cells.

Mutational analysis of the domain structure of mouse protein phosphatase 2Cbeta

The structures of five distinct isoforms of mammalian protein phosphatase 2Cbeta (PP2Cbeta-1, -2, -3, -4 and -5) have previously been found to differ only at their C-terminal regions. In the present study, we performed mutational analysis of recombinant mouse PP2Cbeta-1 to determine the functional domains of the molecule and elucidate the biochemical significance of the structural differences in the isoforms. Differences in affinity for [32P]phosphohistone but not for [32P]phosphocasein were observed among the five PP2Cbeta isoforms. Deletion of 12 amino acids from the C-terminal end, which form a unique sequence for PP2Cbeta-1, caused a 35% loss of activity against [32P]phosphohistone but no loss of activity against [32P]phosphocasein. Deletion of up to 78 amino acids from this end did not cause any further alteration in activity, whereas deletion of 100 amino acids totally eliminated the activity against both [32P]phosphohistone and [32P]phosphocasein. On the other hand, deletion of 11 amino acids from the N-terminal end caused a 97% loss of enzyme activity, and further deletions caused a total loss of activity. Substitution of any of the six specific amino acids among 16 tested in this study, which were located among the 250 N-terminal residues, caused 98-100% loss of enzyme activity. Among these amino acids, three (Glu-38, -60 and -243) have recently been reported to be essential for the binding of metal ions in the catalytic site of the PP2C molecule [Das, Helps, Cohen and Barford (1996) EMBO J. 15, 6798-6809]. These observations indicate that PP2Cbeta is composed of at least two distinct functional domains, an N-terminal catalytic domain of about 310 amino acids and the remaining C-terminal domain, which is involved in determination of substrate specificity.

Differentiation-dependent enhanced expression of protein phosphatase 2Cbeta in germ cells of mouse seminiferous tubules

The presence of five distinct isoforms of protein phosphatase 2Cbeta (PP2Cbeta-1 approximately -5) is known. In this study, we demonstrate that the mRNA levels of PP2Cbeta-3, -4 and -5 and PP2Cbeta protein level increased during the course of the first wave of spermatogenesis in neonatal mouse testis. Northern blot and in situ hybridization analyses revealed that PP2Cbeta-3, -4 and -5 were expressed predominantly in pachytene spermatocytes and in more highly differentiated germ cells. The substrate specificity of PP2Cbeta-4 determined with artificial substrates differed from those of PP2Cbeta-3 and -5, suggesting that the difference in the structure of PP2Cbeta-3, -4 and -5 reflect their unique physiological functions in testicular germ cells.