Detergent structure in tetragonal crystals of OmpF porin

Background: The high-resolution structures of five porins have been solved by X-ray crystallography including the trigonal crystal form of the trimeric OmpF porin from Escherichia coli. In an accompanying article, the structure of the tetragonal form of OmpF porin is presented. In contrast to the trigonal crystal form, the protein surfaces normally in contact with lipids in the membrane are exposed and interact with amphiphiles in the tetragonal crystal. Thus, the tetragonal form can be used to investigate protein–detergent interactions. Results Using single-crystal neutron diffraction studies and two different detergents (one of them deuterated in its hydrophobic moiety), details of the amphiphile–protein interactions are revealed. Detergent molecules bind to the so-called hydrophobic zone that surrounds the OmpF porin trimer and which is exposed to lipid in the native environment. The aromatic rings on both sides of the hydrophobic zone coincide with the boundary between non-polar and polar moieties of the detergents. Conclusion In the tetragonal crystal form of OmpF porin, the membrane-exposed area is accessible from the aqueous solution. It is coated by a film of detergent molecules, which presumably mimics the interactions of the protein with lipids in the biological membrane. In the trigonal form, protein–protein interactions predominate in the hydrophobic zone. These may reflect the tight interactions between trimers that are observed in the biological membrane.