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What are we interested in?

  • Desalination and distillation are among the most resource intensive industrial processes whereby an enormous effort is taken to develop filter membranes with increased energy efficiency. Aquaporins (AQPs) are principally seen as potential building blocks of such next generation filter membranes. Due to their superior performance in respect to their immense permeability and optimal selectivity they are used as templates for the design of artificial water channels on the one hand. On the other hand, their direct use is in dispute, because of their predicted low stability, short lifetime, incompatibility with solvents, low incorporation rate and low effective cross-sectional area. We are confident that it is possible to circumvent these objections by engineering optimized AQP variants suitable for water purification applications.
  • More than 50% of the world’s population is infected with Helicobactor pylori, a pathogenic bacterium responsible for numerous gastroduodenal disorders such as chronic gastritis, peptic ulcer disease and gastric cancer. Standard treatment comprises the use of a proton-pump-inhibitor in combination with two antibiotics but struggles with increasing therapy failures. A different therapeutic toehold might pose H. pylori’s life insurance, a small proton-gated inner membrane channel, HpUreI. It ensures survival in the acidic gastric juice, by means of urea transport from the periplasm to the cytoplasm, where urea is hydrolyzed by urease. The hydrolysis products, ammonia and carbon dioxide, in turn presumably buffer the cytoplasm to a neutral pH and the periplasm to pH 6.1. We investigate (i) the pH dependent gating mechanism of this hourglass-shaped channel and (ii) how HpUreI allows the selective passage of urea and water while blocking the passage of protons or hydronium ions.

In the framework of the above-mentioned projects we work on …

… molecular determinants of solute and solvent permeability through membrane channels.

Our current model proteins include several water facilitators called aquaporins (AQPs) from plants, bacteria and mammals as well as the proton gated urea channel UreI from Helicobacter Pylori. We study water, urea, ammonia/ammonium, proton and sugar alcohol permeation through these channels to pinpoint the selectivity filter for these substances and elucidate the molecular determinants of permeation.

... causes and consequences of protein oligomerization.

Membrane protein oligomerization may have profound functional consequences: for example, the potassium-selective pore of voltage-gated potassium channels forms only upon tetramerization. Most often, however, both the functional significance of oligomerization and the driving forces behind oligomer formation are unclear. In general, oligomerization might lead to structural and/or proteolytic stability, functional diversity, regulatory mechanisms and formation of binding cavities. Our model proteins, AQP and UreI, form tetrameric and hexameric assemblies, respectively, even though the functional unit resides within each monomer. We genetically modify the protein interfaces to change the oligomeric state of the protein to (i) quantify the functional consequences of protein oligomerization and (ii) estimate the interaction strength as well as the influence of single amino acids on protein oligomer stability.

… methodological advancements for quantitative in vitro protein characterization.

Accurate methods to quantify permeability values of solutes and solvents through lipid bilayers and membrane proteins are a prerequisite for our research. Therefore, we take a major effort to develop and refine methods to determine the unitary permeability values of water, protons, weak acids or bases and neutral substances as urea or sugar alcohols. Our measurements are based on light scattering or fluorescence in a stopped-flow configuration, counting the number of fluorescently labeled reconstituted channels with fluorescence correlation spectroscopy (FCS) and estimating the vesicle diameter and vesicle size distribution with photon correlation spectroscopy (PCS).