What Happens When You Think You Think?
Membrane Physics and Fusion

Lipid membranes have rich physical properties and undergo a variety of conformational changes in the course of cellular processes. As part of a collaborative research center SFB 803, and in collaboration with other groups, we study these remodeling processes and how they are regulated by membrane proteins.
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Ion channels play an essential role in cellular control and signalling processes. Ion selectivity and permeation rates are key quantities measured in experiments. We show that these quantities can be directly measured in molecular dynamics simulations using the "Computational Electrophysiology" setup.
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Most plasmalemmal proteins organize in submicrometer-sized clusters whose architecture and dynamics are still enigmatic. With Syntaxin 1 as an example, we applied a combination of far-field optical nanoscopy, biochemistry, fluorescence recovery after photobleaching (FRAP) analysis, and simulations explaining clustering by self-organization.
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Activation of syntaxin is key to synaptic membrane fusion, triggering the release of neuotransmitters into the synaptic gap. Although it has been suspected that structural transitions are crucial for the activation, their nature and dynamics were unknown. In a close collaboration with other groups we have monitored these dynamics.
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For exocytosis and for the selective transport of macromolecules between the various organelles of eukaryotic cells the merging of a transport vesicle membrane with a target membrane is an essential step.
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Electrostatic interactions govern structural and dynamical properties of membranes and can vary considerably with the composition of the aqueous buffer. We studied the influence of sodium chloride on a pure POPC lipid bilayer molecular dynamics simulations.
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MD simulations revealed a strong electrostatic field in aquaporin water channels, which not only strongly polarizes passing water molecules, but also contributes to the barrier against protons. This electrostatic effect therefore aids proton exclusion by aquaporins, prerequisite for maintaining vital bioenergetic proton gradients across membranes.
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Why are aquaporins strictly selective for water, whereas the closely related aquaglyceroporins also permeate glycerol and other small solutes? MD simulations revealed a remarkable correlation between hydrophobicity and permeability for aquaporins, with only water and ammonia showing efficient permeation.
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