Showing papers by "Stefan Hohmann published in 2014"

A novel chloroplast localized Rab GTPase protein CPRabA5e is involved in stress, development, thylakoid biogenesis and vesicle transport in Arabidopsis

Abstract: A novel Rab GTPase protein in Arabidopsis thaliana, CPRabA5e (CP = chloroplast localized) is located in chloroplasts and has a role in transport. Transient expression of CPRabA5e:EGFP fusion protein in tobacco (Nicotiana tabacum) leaves, and immunoblotting using Arabidopsis showed localization of CPRabA5e in chloroplasts (stroma and thylakoids). Ypt31/32 in the yeast Saccharomyces cerevisiae are involved in regulating vesicle transport, and CPRabA5e a close homolog of Ypt31/32, restores the growth of the ypt31Δ ypt32 ts mutant at 37 °C in yeast complementation. Knockout mutants of CPRabA5e displayed delayed seed germination and growth arrest during oxidative stress. Ultrastructural studies revealed that after preincubation at 4 °C mutant chloroplasts contained larger plastoglobules, lower grana, and more vesicles close to the envelopes compared to wild type, and vesicle formation being enhanced under oxidative stress. This indicated altered thylakoid development and organization of the mutants. A yeast-two-hybrid screen with CPRabA5e as bait revealed 13 interacting partner proteins, mainly located in thylakoids and plastoglobules. These proteins are known or predicted to be involved in development, stress responses, and photosynthesis related processes, consistent with the stress phenotypes observed. The results observed suggest a role of CPRabA5e in transport to and from thylakoids, similar to cytosolic Rab proteins involved in vesicle transport.

Glucose de-repression by yeast AMP-activated protein kinase SNF1 is controlled via at least two independent steps

Abstract: The AMP-activated protein kinase, AMPK, controls energy homeostasis in eukaryotic cells but little is known about the mechanisms governing the dynamics of its activation/deactivation The yeast AMPK, SNF1, is activated in response to glucose depletion and mediates glucose de-repression by inactivating the transcriptional repressor Mig1 Here we show that overexpression of the Snf1-activating kinase Sak1 results, in the presence of glucose, in constitutive Snf1 activation without alleviating glucose repression Co-overexpression of the regulatory subunit Reg1 of the Glc-Reg1 phosphatase complex partly restores glucose regulation of Snf1 We generated a set of 24 kinetic mathematical models based on dynamic data of Snf1 pathway activation and deactivation The models that reproduced our experimental observations best featured (a) glucose regulation of both Snf1 phosphorylation and dephosphorylation, (b) determination of the Mig1 phosphorylation status in the absence of glucose by Snf1 activity only and (c) a regulatory step directing active Snf1 to Mig1 under glucose limitation Hence it appears that glucose de-repression via Snf1-Mig1 is regulated by glucose via at least two independent steps: the control of activation of the Snf1 kinase and directing active Snf1 to inactivating its target Mig1

Bridging the gaps in systems biology.

Abstract: Systems biology aims at creating mathematical models, i.e., computational reconstructions of biological systems and processes that will result in a new level of understanding-the elucidation of the basic and presumably conserved "design" and "engineering" principles of biomolecular systems. Thus, systems biology will move biology from a phenomenological to a predictive science. Mathematical modeling of biological networks and processes has already greatly improved our understanding of many cellular processes. However, given the massive amount of qualitative and quantitative data currently produced and number of burning questions in health care and biotechnology needed to be solved is still in its early phases. The field requires novel approaches for abstraction, for modeling bioprocesses that follow different biochemical and biophysical rules, and for combining different modules into larger models that still allow realistic simulation with the computational power available today. We have identified and discussed currently most prominent problems in systems biology: (1) how to bridge different scales of modeling abstraction, (2) how to bridge the gap between topological and mechanistic modeling, and (3) how to bridge the wet and dry laboratory gap. The future success of systems biology largely depends on bridging the recognized gaps.

Integrated view on a eukaryotic osmoregulation system

Abstract: Osmoregulation encompasses active homeostatic processes that ensure proper cell volume, shape and turgor as well as an intercellular milieu optimal for the diverse biochemical processes. Recent studies demonstrate that yeast cells operate within a tight window of cellular water concentrations that still allows rapid diffusion of biomolecules while already moderate cell compression following hyper-osmotic stress leads to macromolecular crowding and a slow-down of cellular processes. Yeast cells accumulate glycerol as compatible osmolyte under hyper-osmotic stress to regain cell volume and turgor and release glycerol following a hypo-osmotic shock. The high osmolarity glycerol (HOG) response pathway controls glycerol accumulation at various levels, where each mechanism contributes to the temporal and quantitative pattern of volume recovery: inhibition of glycerol efflux, direct activation of the first enzyme in glycerol biosynthesis, stimulation of glycolytic flux as well as upregulation of expression of genes encoding enzymes in glycerol biosynthesis and an active glycerol uptake system. The HOG mitogen-activated protein kinase (MAPK) pathway communicates with the other yeast MAPK pathways to control cell morphogenesis. Cross-talk between the MAPK pathways has recently been used to re-wire osmostress-controlled expression of glycerol biosynthesis genes from Hog1 to Kss1-Fus3. The results of this study further illustrate the key importance of glycerol accumulation under osmostress and allow studying Hog1-dependent and independent processes as well as redundancy and robustness of the MAPK system.