1. Ion Channel Structure Research Group, Max Planck Institute for Medical Research, Jahnstrasse 29, 69120 Heidelberg, Germany, European Molecular Biology Laboratory, Hamburg Outstation, EMBL c/o DESY, Notkestrasse 85, 22603 Hamburg, Germany, Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, 117333 Moscow, Russia, and Viikki Biocenter, Department of Biosciences, Division of Biochemistry, P.O.B. 56, FIN-00014 University of Helsinki, Finland