1. The Biomembrane Structure Unit, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K., Leiden Institute of Chemistry, Gorlaeus Laboratories, Einsteinweg 55, 2333 CC Leiden, The Netherlands, and Nijmegen Center for Molecular Life Sciences, Department of Biochemistry, UMC-160, University of Nijmegen Medical School, 6500 HB Nijmegen, The Netherlands