1. Department of Pharmaceutical Biology, Philipps-University of Marburg, Deutschhausstrasse 17A, D-35032 Marburg, Germany, Max Planck Institute for Biophysical Chemistry, Department of Membrane Biophysics, Am Fassberg 11, D-37077 Göttingen, Germany, and Institute of Physiological Chemistry, Rheinische Friedrich-Wilhelms-University, Nussallee 11, D-53115 Bonn, Germany