Disparate roles for the regulatory A subunit isoforms in Arabidopsis protein phosphatase 2A

The heterotrimeric protein phosphatase 2A (PP2A) complex comprises a catalytic subunit and regulatory A and B subunits that modulate enzyme activity and mediate interactions with other proteins. We report here the results of a systematic analysis of the Arabidopsis (Arabidopsis thaliana) regulatory A subunit gene family, which includes the ROOTS CURL IN NAPHTHYLPHTHALAMIC ACID1 (RCN1), PP2AA2, and PP2AA3 genes. All three A subunit isoforms accumulate in the organs of seedlings and adult plants, suggesting extensive overlap in expression domains. We have isolated pp2aa2 and pp2aa3 mutants and found that their phenotypes are largely normal and do not resemble that of rcn1. Whereas rcn1 pp2aa2 and rcn1 pp2aa3 double mutants exhibit striking abnormalities in all stages of development, the pp2aa2 pp2aa3 double mutant shows only modest defects. Together, these data suggest that RCN1 performs a cardinal role in regulation of phosphatase activity and that PP2AA2 and PP2AA3 functions are unmasked only when RCN1 is absent.     

ATR kinase activity regulates the intranuclear translocation of ATR and RPA following ionizing radiation

Upon damage of DNA in eukaryotic cells, several repair and checkpoint proteins undergo a dramatic intranuclear relocalization, translocating to nuclear foci thought to represent sites of DNA damage and repair. Examples of such proteins include the checkpoint kinase ATR (ATM and Rad3-related) as well as replication protein A (RPA), a single-stranded DNA binding protein required in DNA replication and repair. Here, we used a microscopy-based approach to investigate whether the damage-induced translocation of RPA is an active process regulated by ATR. Our data show that in undamaged cells, ATR and RPA are uniformly distributed in the nucleus or localized to promyelocytic leukemia protein (PML) nuclear bodies. In cells treated with ionizing radiation, both ATR and RPA translocate to punctate, abundant nuclear foci where they continue to colocalize. Surprisingly, an ATR mutant that lacks kinase activity fails to relocalize in response to DNA damage. Furthermore, this kinase-inactive mutant blocks the translocation of RPA in a cell cycle-dependent manner. These observations demonstrate that the kinase activity of ATR is essential for the irradiation-induced release of ATR and RPA from PML bodies and translocation of ATR and RPA to potential sites of DNA damage.     

Activation of heterotrimeric G proteins by a high energy phosphate transfer via nucleoside diphosphate kinase (NDPK) B and Gbeta subunits. Specific activation of Gsalpha by an NDPK B.Gbetagamma complex in H10 cells

Formation of GTP by nucleoside diphosphate kinase (NDPK) can contribute to G protein activation in vitro. To study the effect of NDPK on G protein activity in living cells, the NDPK isoforms A and B were stably expressed in H10 cells, a cell line derived from neonatal rat cardiomyocytes. Overexpression of either NDPK isoform had no effect on cellular GTP and ATP levels, basal cAMP levels, basal adenylyl cyclase activity, and the expression of G(s)alpha and G(i)alpha proteins. However, co-expression of G(s)alpha led to an increase in cAMP synthesis that was largely enhanced by the expression of NDPK B, but not NDPK A, and that was confirmed by direct measurement of adenylyl cyclase activity. Cells expressing an inactive NDPK B mutant (H118N) exhibited a decreased cAMP formation in response to G(s)alpha. Co-immunoprecipitation studies demonstrated a complex formation of the NDPK with Gbetagamma dimers. The overexpression of NDPK B, but not its inactive mutant or NDPK A, increased the phosphorylation of Gbeta subunits. In summary, our data demonstrate a specific NDPK B-mediated activation of a G protein in intact cells, which is apparently caused by formation of NDPK B.Gbetagamma complexes and which appears to contribute to the receptor-independent activation of heterotrimeric G proteins.     

Topoisomerase III can serve as the cellular decatenase in Escherichia coli

topB, encoding topoisomerase III, was identified as a high copy suppressor of the temperature-sensitive parC1215 allele, encoding one of the subunits of topoisomerase IV. Overexpression of topoisomerase III at the nonpermissive temperature was shown subsequently to restore timely chromosome decatenation and suppress lethality in strains carrying either temperature-sensitive parE or parC alleles. By developing an assay in vitro for precatenane unlinking, we demonstrated directly that both topoisomerase III and topoisomerase IV were efficient at this task, whereas DNA gyrase was very inefficient at precatenane removal. These observations suggest that precatenane unlinking is sufficient to sustain decatenation of replicating daughter chromosomes in the cell.     

Residue Gln4863 within a predicted transmembrane sequence of the Ca2+ release channel (ryanodine receptor) is critical for ryanodine interaction

Despite the pivotal role of ryanodine in ryanodine receptor (RyR) research, the molecular basis of ryanodine-RyR interaction remains largely undefined. We investigated the role of the proposed transmembrane helix TM10 in ryanodine interaction and channel function. Each amino acid residue within the TM10 sequence, 4844IIFDITFFFFVIVILLAIIQGLII4867, of the mouse RyR2 was mutated to either alanine or glycine. Mutants were expressed in human embryonic kidney 293 cells, and their properties were assessed. Mutations D4847A, F4850A, F4851A, L4858A, L4859A, and I4866A severely curtailed the release of intracellular Ca2+ in human embryonic kidney 293 cells in response to extracellular caffeine and diminished [3H]ryanodine binding to cell lysates. Mutations F4846A, T4849A, I4855A, V4856A, and Q4863A eliminated or markedly reduced [3H]ryanodine binding, but cells expressing these mutants responded to extracellular caffeine by releasing stored Ca2+. Interestingly these two groups of mutants, each with similar properties, are largely located on opposite sides of the predicted TM10 helix. Single channel analyses revealed that mutation Q4863A dramatically altered the kinetics and apparent affinity of ryanodine interaction with single RyR2 channels and abolished the effect of ryanodol, an analogue of ryanodine, whereas the single channel conductance of the Q4863A mutant and its responses to caffeine, ATP, and Mg2+ were comparable to those of the wild type channels. Furthermore the effect of ryanodine on single Q4863A mutant channels was influenced by the transmembrane holding potential. Together these results suggest that the TM10 sequence and in particular the Q4863 residue constitute an important determinant of ryanodine interaction.     

Metabolic distribution of radioactivity in Sprague-Dawley rats and B6C3F1 mice exposed to 1,3-[2,3-14C]-butadiene by whole body exposure

The uptake of 1,3-[2,3-(14)C]-butadiene and its disposition, measured as radioactivity in urine, faeces, exhaled volatiles and CO(2) during and following 6 h whole body exposure to 20 ppm butadiene has been investigated in male Sprague-Dawley rats and B6C3F1 mice. Whilst there were similarities between the two species, the uptake and metabolic distribution of butadiene were somewhat different for rats and mice. The major differences observed were in the urinary excretion of radioactivity and in the exhalation of 14C-CO(2). After 42 h from the start of exposure, 51.1% of radioactivity was eliminated in rat urine compared with 39.5% for mouse urine. 34.9% of the recovered radioactivity was exhaled by rats as 14C-CO(2), compared with 48.7% by mice. Excretion of radioactivity in faeces was similar for both species (3.8% for rats and 3.4% for mice). The tissue concentrations of 14C-butadiene equivalents measured in liver, testes, lung and blood of exposed mice were 0.493, 0460, 0.457, and 1.626 nmol/g tissue, respectively. The values for the corresponding rat tissues were 0.869, 0.329, 0.457, and 1.626 nmol butadiene equivalents/g tissue, respectively. For rats, 6.2% of recovered radioactivity (0.288 nmol butadiene equivalents/g tissue) was retained in carcasses whereas for mice the amount was 3.6% (0.334 nmol butadiene equivalents/g tissue). There were also some significant differences between the metabolic conversion of 1,3-[2,3-(14)C]-butadiene and excretion by mice following the 20 ppm whole body exposure compared to previously reported data for nose-only exposure to 200 ppm butadiene [Richardson et al., Toxicol. Sci. 49 (1999) 186]. The main difference between the high- and low-exposure studies was in the exhalation of 14C-CO(2). At the 200 ppm exposure, 40% of the radioactivity was exhaled as 14C-CO(2) by rats whereas 6% was measured by this route for mice. The proportional conversion of butadiene to CO(2) by mice was significantly greater at the low exposure concentration compared with that reported for the higher concentration. This shift was not observed for rats. The difference between species could be caused by a saturation of metabolism in mice between 20 and 200 ppm for the pathways leading to CO(2). Restraint or error in collection of CO(2) in the 200 ppm study could also be factors.     

The kic1 kinase of schizosaccharomyces pombe is a CLK/STY orthologue that regulates cell-cell separation

The CLK/STY kinases are a family of dual-specificity protein kinases implicated in the regulation of cellular growth and differentiation. Some of the kinases in the family are shown to phosphorylate serine-arginine-rich splicing factors and to regulate pre-mRNA splicing. However, the actual cellular mechanism that regulates cell growth, differentiation, and development by CLK/STY remains unclear. Here we show that a functionally conserved CLK/STY kinase exists in Schizosaccharomyces pombe, and this orthologue, called Kic1, regulates the cell surface and septum formation as well as a late step in cytokinesis. The Kic1 protein is modified in vivo, likely by phosphorylation, suggesting that it can be involved in a control cascade. In addition, kic1(+) together with dsk1(+), which encodes a related SR-specific protein kinase, constitutes a critical in vivo function for cell growth. The results provide the first in vivo evidence for the functional conservation of the CLK/STY family through evolution from fission yeast to mammals. Furthermore, since cell division and cell-cell interaction are fundamental for the differentiation and development of an organism, the novel cellular role of kic1(+) revealed from this study offers a clue to the understanding of its counterparts in higher eukaryotes.     

The Mechanosensory Lateral Line System of the Hypogean form of Astyanax Fasciatus

The mechanosensory lateral line is a distributed, hair-cell based system which detects the water flow regime at the surface of the fish. Superficial neuromasts densely scattered over the surface of some cave fish detect the pattern of flow over the surface of the body and are important in rheotactic behaviors and perhaps in the localization of small vibrating sources. Canal neuromasts are very likely also involved in the detection of small planktonic prey, but seem also to play an essential role in replacing vision as the major sense by which blind cave-fish perceive their surroundings. The flow-field that exists around a gliding fish is perturbed by objects in the immediate vicinity, these perturbations are detected by the lateral line system. In this way the fish can build up a picture of its environment, a process that has been called active hydrodynamic imaging. None of the lateral line behaviors exhibited by blind cave fish are necessarily exclusive to these species, but there is some evidence that their lateral line capabilities are enhanced with respect to their sighted relatives.     

Phenology and demography of two species of Botrychium (Ophioglossaceae)

Temporal and demographic aspects of the growth of Botrychium gallicomontanum and B. mormo were studied for 2 yr. A total of 219 B. gallicomontanum and 412 B. mormo plants were monitored in a prairie and maple-basswood forest, respectively. Growth events were divided into four stages: leaf emergence, leaf separation, spore release, and senescence. Botrychium gallicomontanum emerged in April, peaked during the first week of June, and declined rapidly. The largest plants were found in late June and early July with a mean peak trophophore size of 4.0 ± 1.8 cm. Botrychium mormo emerged in June, and the population size peaked in early July. The largest plants occurred late in August with a mean peak trophophore size of 3.0 ± 1.1 cm. The mean season span, or period of emergence aboveground annually, for B. gallicomontanum and B. mormo was 7.7 ± 2.4 and 11.9 ± 3.5 wk, respectively. Late-emerging plants produced spores in shorter periods. The separation stage was prolonged in B. gallicomontanum plants, whereas B. mormo plants had a much longer separation stage. Phenological differences are related to different habitat parameters of grassland and forest. Understanding the phenology of these rare species will help us more accurately predict the impact of management practices.