DETERGENT LOCALIZATION IN MEMBRANE PROTEIN CRYSTALS USING NEUTRON DIFFRACTION P. A. Timmins, Institut Laue-Langevin, BP156, 38042 Grenoble Cedex 9, France. E. Pebay-Peyroula, Institut de Biologie Structurale et Université Joseph Fourier, 41 avenue des Martyrs, 38027 Grenoble Cedex, France. S. Penel, Institut Laue-Langevin, BP156, 38042 Grenoble Cedex 9, France.

The organization of detergent in single crystals of membrane proteins can only be visualized by neutron diffraction. Due to the similarity of electron density between detergent and water these two disordered phases are indistinguishable by X-ray diffraction. By soaking crystals in mixtures of H₂O and D₂O its is possible to enhance the relative contrasts for neutrons of any of the components in the crystals. The linearity of the complex structure factor as a function of contrast allows phase information to be derived as well as efficient scaling of data containing different amounts of D₂O. Contrast may also be varied by substituting components of a macromolecular complex with their perdeuterated analogues. This technique has recently been used to study the detergent organization in a number of different proteins including three different crystal forms of two different porins. In addition to varying the contrast through the deuterium content of the water, detergents deuterated in their acyl chains have been used to investigate in more detail the interactions between detergent and protein. The detergent in crystals of the porin from Rhodobacter capsulatus forms what appears to be a layer structure but which in fact consists of rather flat micelles interacting through their head groups. In the case of the Ompf porin from E. Coli the protein does not on its own form a continuous three-dimensional lattice. The interactions between proteins are in fact mediated in part by detergent-detergent interactions.

Pebay-Peyroula, E., Garavito, R.M., Rosenbusch, J.P., Zulauf, M. and Timmins, P.A. (1995) Detergent structure in tetragonal crystals of Ompf porin. Structure 3, 1051 - 1059.

Timmins, P.A., Pebay-Peyroula, E. & Welte, W. (1994) Detergent Organisation in Solution and in Crystals of Membrane Proteins. Biophys. Chem. 53 (1-2), 27 - 36.