Our research activities lie at the interface of the three scientific core disciplines biology, chemistry, and physics with a particular focus on the biophysical chemistry of membrane proteins and membrane-mimetic systems. Membrane proteins are involved in numerous biological processes, play key roles in cellular communication and transport, represent the majority of drug targets, and increasingly find use in biotechnology. However, membrane proteins are utterly challenging research objects for biophysical, structural, and functional investigations as well as for engineering purposes, as they usually need to be extracted from their complex cellular membrane environment to become tractable in vitro. After extraction, membrane proteins depend on membrane mimics, which should reproduce the most important features of their natural membrane environment in order to retain their native structures and functions. Our efforts are devoted to the three major lines of research described below.
Polymer-encapsulated lipid-bilayer nanodiscs for membrane biophysics
Detergents are traditionally used for extracting and solubilising membrane proteins. In the highly dynamic environment of a detergent micelle, however, many membrane proteins tend to denature irreversibly. Some styrene/maleic acid (SMA) copolymers enable a fundamentally new approach for investigating membrane proteins, as they obviate the use of conventional detergents. These polymers can extract proteins and surrounding lipids directly from cellular membranes to form nanosized discs, where the polymer wraps around a lipid-bilayer patch.
We have recently discovered that a copolymer named diisobutylene/maleic acid (DIBMA) is equally capable of accommodating membrane proteins and lipids in native nanodiscs, thus rendering them amenable to biophysical, structural, and functional scrutiny. The major advantage of this new polymer lies in the fact that it is compatible with optical spectroscopy in the ultraviolet range, does not disturb the order, dynamics, and hydration of the extracted membrane fragment, and tolerates elevated concentrations of metal ions often required for membrane-protein activity.
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Self-assembling fluorinated amphiphiles as mild membrane mimics
A different approach towards engineering mild membrane mimics relies on fluorinated amphiphiles. Owing to the weak affinity of fluorocarbons for hydrocarbons and to the larger volume of the former, such fluorosurfactants are less destabilising because they do not compete with native protein/protein and protein/lipid interactions. For the same reason, however, fluorosurfactants are thought to be unable to extract proteins directly from biological membranes.
We have found that the poor miscibility of fluorocarbons and hydrocarbons at the macroscale need not apply at the nanoscale. We now exploit this discovery to develop fluorosurfactants that display favourable physicochemical properties such as small and well-defined micelles, partition into, translocate across, and solubilise membranes in a rapid, thermodynamically controlled manner, extract proteins directly from cellular membranes, and provide these proteins with a stabilising membrane-mimetic environment that preserves their native structures and functions.
Structural dynamics and interactions of membrane proteins
The third line of research focusses on the interactions of proteins with small-molecule ligands, lipid membranes, and other proteins. To this end, we combine the new membrane mimics developed in the above-mentioned projects with spectroscopic, scattering, chromatographic, calorimetric, and other methods as well as model building to elucidate the structures, dynamics, and functions of membrane proteins.
We currently focus our efforts on G-protein-coupled receptors (GPCRs), ligand-gated ion channels, and pore-forming proteins, all of which are of great physiological and therapeutic relevance. Moreover, we continuously develop and improve methods and protocols for rendering these proteins accessible to in vitro investigations in a native-like yet nanoscale lipid-bilayer environment.
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