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Spontaneous Formation of Detergent Micelles around the Outer Membrane Protein OmpX

Department of Biochemistry, University of Zürich, Zürich, Switzerland

Correspondence: Address reprint requests to R. A. Böckmann or A. Caflisch, Dept. of Biochemistry, University of Zürich Winterthurerstrasse 190, CH-8057 Zürich, Switzerland. E-mail: rainer.boeckmann{at}bioinf.uni-sb.de, or caflisch{at}bioc.unizh.ch.

The structure and flexibility of the outer membrane protein X (OmpX) in a water-detergent solution and in pure water are investigated by molecular dynamics simulations on the 100-ns timescale and compared with NMR data. The simulations allow for an unbiased determination of the structure of detergent micelles and the protein-detergent mixed micelle. The short-chain lipid dihexanoylphosphatidylcholine, as a detergent, aggregates into pure micelles of 18 molecules, or alternatively, it binds to the protein surface. The detergent binds in the form of a monolayer ring around the hydrophobic ß-barrel of OmpX rather than in a micellar-like oblate; 40 dihexanoylphosphatidylcholine lipids are sufficient for an effective suppression of water from the surface of the ß-barrel region. The phospholipids bind also on the extracellular, protruding ß-sheet. Here, polar interactions between charged amino acids and phosphatidylcholine headgroups act as condensation seed for detergent micelle formation. The polar protein surface remains accessible to water molecules. In total, 90–100 detergent molecules associate within the protein-detergent mixed micelle, in agreement with experimental estimates. The simulation results indicate that OmpX is not a water pore and support the proposed role of the protruding ß-sheet as a "fishing rod".

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