1. Institute of Biochemistry and Center for Molecular Biosciences, University of Innsbruck, A-6020 Innsbruck, Austria
2. Present address: Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, CA 92037, USA
3. Max F. Perutz Laboratories, Department of Structural and Computational Biology, University of Vienna, A-1030 Vienna, Austria