Bistability from double phosphorylation in signal transduction. Kinetic and structural requirements

Previous studies have suggested that positive feedback loops and ultrasensitivity are prerequisites for bistability in covalent modification cascades. However, it was recently shown that bistability and hysteresis can also arise solely from multisite phosphorylation. Here we analytically demonstrate that double phosphorylation of a protein (or other covalent modification) generates bistability only if: (a) the two phosphorylation (or the two dephosphorylation) reactions are catalyzed by the same enzyme; (b) the kinetics operate at least partly in the zero-order region; and (c) the ratio of the catalytic constants of the phosphorylation and dephosphorylation steps in the first modification cycle is less than this ratio in the second cycle. We also show that multisite phosphorylation enlarges the region of kinetic parameter values in which bistability appears, but does not generate multistability. In addition, we conclude that a cascade of phosphorylation/dephosphorylation cycles generates multiple steady states in the absence of feedback or feedforward loops. Our results show that bistable behavior in covalent modification cascades relies not only on the structure and regulatory pattern of feedback/feedforward loops, but also on the kinetic characteristics of their component proteins.     

Viability determination of Helicobacter pylori using propidium monoazide quantitative PCR

Background: While Helicobacter pylori exists in a bacillary form in both the natural habitat and the human host, detrimental environmental circumstances have been observed to lead to the conversion of H. pylori from the bacillary to the coccoid form. However, the viability or nonviability of coccoid forms remains to be established in H. pylori. The aim of this study was to determine whether the quantitative PCR combined with propidium monoazide could be an alternative and good technique to determine H. pylori viability in environmental samples and, to contribute to understanding of the role of the H. pylori forms. Materials and Methods: Viability, morphological distribution, and the number of live H. pylori cells were determined using a propidium monoazide‐based quantitative PCR method, at various time points. Results: Under adverse environmental conditions was observed the conversion of H. pylori from the bacillary to the coccoid form, and the decrease in amplification signal, in samples that were treated with propidium monoazide, over the time. Conclusions: Incorporation of propidium monoazide indicates that there is an increase in H. pylori cells with the damaged membrane over the study, leading to the manifestation of cellular degeneration and death. Consequently, quantitative PCR combined with propidium monoazide contributes to our understanding of the role of H. pylori cells, under adverse environmental conditions.     

Phosphorylation of Hsl1 by Hog1 leads to a G2 arrest essential for cell survival at high osmolarity

Control of cell cycle progression by stress-activated protein kinases (SAPKs) is essential for cell adaptation to extracellular stimuli. Exposure of yeast to osmostress leads to activation of the Hog1 SAPK, which controls cell cycle at G1 by the targeting of Sic1. Here, we show that survival to osmostress also requires regulation of G2 progression. Activated Hog1 interacts and directly phosphorylates a residue within the Hsl7-docking site of the Hsl1 checkpoint kinase, which results in delocalization of Hsl7 from the septin ring and leads to Swe1 accumulation. Upon Hog1 activation, cells containing a nonphosphorylatable Hsl1 by Hog1 are unable to promote Hsl7 delocalization, fail to arrest at G2 and become sensitive to osmostress. Together, we present a novel mechanism that regulates the Hsl1-Hsl7 complex to integrate stress signals to mediate cell cycle arrest and, demonstrate that a single MAPK coordinately modulates different cell cycle checkpoints to improve cell survival upon stress.     

Proteome of the bacterium Mycoplasma penetrans

A proteome map of Mycoplasma penetrans has been constructed using two-dimensional gel electrophoresis in combination with mass spectrometry (MS). Mycoplasma penetrans infects the urogenital and respiratory tracts of humans. A total of 207 spots were characterized with MS and, in comparing the experimental data with the DNA sequence-derived predictions, it was possible to assign these 207 spots to 153 genes. The Pro-Q Diamond phosphoprotein dye technology was used for the fluorescent detection of 26 phosphoproteins in the 4-7 pH range.     

Linguistic and maternal genetic diversity are not correlated in Native Mexicans

Mesoamerica, defined as the broad linguistic and cultural area from middle southern Mexico to Costa Rica, might have played a pivotal role during the colonization of the American continent. The Mesoamerican isthmus has constituted an important geographic barrier that has severely restricted gene flow between North and South America in pre-historical times. Although the Native American component has been already described in admixed Mexican populations, few studies have been carried out in native Mexican populations. In this study, we present mitochondrial DNA (mtDNA) sequence data for the first hypervariable region (HVR-I) in 477 unrelated individuals belonging to 11 different native populations from Mexico. Almost all of the Native Mexican mtDNAs could be classified into the four pan-Amerindian haplogroups (A2, B2, C1, and D1); only two of them could be allocated to the rare Native American lineage D4h3. Their haplogroup phylogenies are clearly star-like, as expected from relatively young populations that have experienced diverse episodes of genetic drift (e.g., extensive isolation, genetic drift, and founder effects) and posterior population expansions. In agreement with this observation, Native Mexican populations show a high degree of heterogeneity in their patterns of haplogroup frequencies. Haplogroup X2a was absent in our samples, supporting previous observations where this clade was only detected in the American northernmost areas. The search for identical sequences in the American continent shows that, although Native Mexican populations seem to show a closer relationship to North American populations, they cannot be related to a single geographical region within the continent. Finally, we did not find significant population structure in the maternal lineages when considering the four main and distinct linguistic groups represented in our Mexican samples (Oto-Manguean, Uto-Aztecan, Tarascan, and Mayan), suggesting that genetic divergence predates linguistic diversification in Mexico.     

Topological control of the spin coupling in dinuclear copper(II) complexes with meta- and para-phenylenediamine bridging ligands

A novel series of copper(II) complexes of formula [Cu(tren)(mpda)](ClO₄)₂ · 1/2H₂O (1), [Cu₂(tren)₂(mpda)](ClO₄)₄ · 2H₂O (2), and [Cu₂(tren)₂(ppda)](ClO₄)₄ · 2H₂O (3) containing the tetradentate tris(2-aminoethyl)amine (tren) terminal ligand and the potentially bridging 1,n-phenylenediamine [n = 3 (mpda) and 4 (ppda)] ligand have been prepared and spectroscopically characterized. X-ray diffraction on single crystals of 1 and 3 show the presence of mono- (1) and dinuclear (3) copper(II) units where the mpda (1) and ppda (3) ligands adopt terminal monodentate (1) and bridging bis(monodentate) (3) coordination modes toward [Cu(tren)]²⁺ cations with an overall non-planar, orthogonal disposition of the phenylene group and the N-Cu-N threefold axis of the trigonal bipyramid of each copper(II) ion [values of the Cu-N-C-C torsion angle (?) in the range of 50.8(3)-79.2(2) (1) and 80.9(2)-86.5(2)° (3)]. Variable-temperature magnetic susceptibility measurements on the dinuclear complexes 2 and 3 show the occurrence of moderate ferromagnetic (J = +8.3 cm⁻¹, 2) and strong antiferromagnetic (J = −51.4 cm⁻¹, 3) couplings between the two copper(II) ions across the meta- and para-phenylenediamine bridges, leading to S = 1 (2) and S = 0 (3) ground spin states [H = −JS₁ · S₂ with S₁ = S₂ = S_Cu = 1/2]. Density functional theory (DFT) calculations on the triplet (2) and broken-symmetry (BS) singlet (3) ground spin states, support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction through the predominantly π-type orbital pathway of the 1,n-phenylenediamine bridge. Finally, a new magneto-structural correlation between the magnitude of the magnetic coupling (J) and the Cu-N-C-C torsion angle (?) has been found which reveals the role of σ- versus π-type orbital pathways in the modulation of the magnetic coupling for m- and p-phenylenediamine-bridged dicopper(II) complexes.     

Biophysical properties of Saccharomyces cerevisiae and their relationship with HOG pathway activation

Parameterized models of biophysical and mechanical cell properties are important for predictive mathematical modeling of cellular processes. The concepts of turgor, cell wall elasticity, osmotically active volume, and intracellular osmolarity have been investigated for decades, but a consistent rigorous parameterization of these concepts is lacking. Here, we subjected several data sets of minimum volume measurements in yeast obtained after hyper-osmotic shock to a thermodynamic modeling framework. We estimated parameters for several relevant biophysical cell properties and tested alternative hypotheses about these concepts using a model discrimination approach. In accordance with previous reports, we estimated an average initial turgor of 0.6 ± 0.2 MPa and found that turgor becomes negligible at a relative volume of 93.3 ± 6.3% corresponding to an osmotic shock of 0.4 ± 0.2 Osm/l. At high stress levels (4 Osm/l), plasmolysis may occur. We found that the volumetric elastic modulus, a measure of cell wall elasticity, is 14.3 ± 10.4 MPa. Our model discrimination analysis suggests that other thermodynamic quantities affecting the intracellular water potential, for example the matrix potential, can be neglected under physiological conditions. The parameterized turgor models showed that activation of the osmosensing high osmolarity glycerol (HOG) signaling pathway correlates with turgor loss in a 1:1 relationship. This finding suggests that mechanical properties of the membrane trigger HOG pathway activation, which can be represented and quantitatively modeled by turgor.