Showing papers by "Stefan Hohmann published in 2009"

Crystal structure of a yeast aquaporin at 1.15 angstrom reveals a novel gating mechanism.

Abstract: Aquaporins are transmembrane proteins that facilitate the flow of water through cellular membranes An unusual characteristic of yeast aquaporins is that they frequently contain an extended N terminus of unknown function Here we present the X-ray structure of the yeast aquaporin Aqy1 from Pichia pastoris at 115 A resolution Our crystal structure reveals that the water channel is closed by the N terminus, which arranges as a tightly wound helical bundle, with Tyr31 forming H-bond interactions to a water molecule within the pore and thereby occluding the channel entrance Nevertheless, functional assays show that Aqy1 has appreciable water transport activity that aids survival during rapid freezing of P pastoris These findings establish that Aqy1 is a gated water channel Mutational studies in combination with molecular dynamics simulations imply that gating may be regulated by a combination of phosphorylation and mechanosensitivity

Robustness and fragility in the yeast high osmolarity glycerol (HOG) signal‐transduction pathway

Abstract: Cellular signalling networks integrate environmental stimuli with the information on cellular status These networks must be robust against stochastic fluctuations in stimuli as well as in the amounts of signalling components Here, we challenge the yeast HOG signal-transduction pathway with systematic perturbations in components' expression levels under various external conditions in search for nodes of fragility We observe a substantially higher frequency of fragile nodes in this signal-transduction pathway than that has been observed for other cellular processes These fragilities disperse without any clear pattern over biochemical functions or location in pathway topology and they are largely independent of pathway activation by external stimuli However, the strongest toxicities are caused by pathway hyperactivation In silico analysis highlights the impact of model structure on in silico robustness, and suggests complex formation and scaffolding as important contributors to the observed fragility patterns Thus, in vivo robustness data can be used to discriminate and improve mathematical models

Short cut to 1,2,3-triazole-based p38 MAP kinase inhibitors via [3+2]-cycloaddition chemistry

Abstract: A series of 4,5-substituted 1,2,3-triazoles was synthesised viaCu(I)-catalysed azide–alkyne 1,3-dipolar [2+3]-cycloaddition reactions followed by a Suzuki coupling. The 1,2,3-triazoles were evaluated as inhibitors of the p38α MAP kinase, showing IC50 values in the high nanomolar range.

Expression of the yeast aquaporin Aqy2 affects cell surface properties under the control of osmoregulatory and morphogenic signalling pathways

Abstract: Aquaporins mediate rapid and selective water transport across biological membranes. The yeast Saccharomyces cerevisiae possesses two aquaporins, Aqy1 and Aqy2. Here, we show that Aqy2 is involved in controlling cell surface properties and that its expression is controlled by osmoregulatory and morphogenic signalling pathways. Deletion of AQY2 results in diminished fluffy colony morphology while overexpression of AQY2 causes strong agar invasion and adherence to plastic surfaces. Hyper-osmotic stress inhibits morphological developments including the above characteristics as well as AQY2 expression through the osmoregulatory Hog1 mitogen-activated protein kinase. Moreover, two pathways known to control morphological developments are involved in regulation of AQY2 expression: the protein kinase A pathway derepresses AQY2 expression through the Sfl1 repressor, and the filamentous growth Kss1 mitogen-activated protein kinase pathway represses AQY2 expression in a Kss1 activity-independent manner. The AQY2 expression pattern resembles in many ways that of MUC1/FLO11, which encodes a cell surface glycoprotein required for morphological developments. Our observations suggest a potential link between aquaporins and cell surface properties, and relate to the proposed role of mammalian aquaporins in tumour cell migration and invasion.

Robustness and fragility in the high osmolarity glycerol (HOG) pathway in S. cerevisiae

Abstract: Cellular signalling networks integrate environmental stimuli with information on cellular status. These networks must be robust against stochastic fluctuations in external stimuli as well as in the amounts of signalling components. Here [1], we challenge the yeast HOG signal transduction pathway with systematic perturbations in components’ expression levels implemented by a “genetic tug-of-war” methodology under various external conditions in search of nodes of fragilities. We observe a substantially higher frequency of fragile nodes in this signal transduction pathway than has been observed for other cellular processes. These fragilities disperse without any clear pattern over biochemical functions or location in pathway topology, with the most sensitive nodes being the proteins PBS2 and SSK1. They are also largely independent of pathway activation by external stimuli. However, the strongest toxicities are caused by pathway hyperactivation. We studied the influence of seven regulatory motifs around these HOG pathway components in silico through ODE models. Based on the SLN1 and the MAPK modules of a mathematical model of osmoregulation in budding yeast by Klipp et al. [2] we included new motifs and fitted the affected parameters to time courses of dually phosphorylated Hog1p generated by the original model under stress and stress-free conditions. The regulations taken into account by our analysis include Pbs2p scaffolding, Ssk1p and Pbs2p autoactivation, and the formation of a stable dimer between Ssk2p and Ssk1p. A subsequent sensitivity analysis identified Pbs2's role as a scaffold protein and Ssk1p-Ssk2p dimerization as the important contributors to the observed robustness pattern in silico. Thus, in vivo robustness data can be used to discriminate and improve mathematical models.