1. Molecular and Cellular Neuropharmacology Group, Max Planck Institute for Experimental Medicine, Hermann-Rein-Strasse 3, D-37075 Göttingen, Germany, Department of NMR Based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen, Germany, The Clayton Foundation Laboratories for Peptide Biology, The Salk Institute, La Jolla, California 92037, and Department of Biology, University of Utah, Salt Lake City, Utah 84112