1. Laboratory of Structure and Function of Biological Membranes (SFMB), Free University of Brussels, B-1050 Brussels, Belgium, Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nl-9751 NN Haren, The Netherlands, Department of Pharmacology, University of Cambridge, Cambridge CB2 1QJ, United Kingdom, and Department of Biological Chemistry, University of Mons-Hainaut, B-7000 Mons, Belgium