1. IBI-2, Structural Biology, Research Center Jülich, 52425 Jülich, Germany, FB Biologie, Philipps-Universität, Karl-von-Frisch-Strasse, 35032 Marburg, Germany, Université Paris VI, Casier 156, 4 Place Jussieu, 75252 Paris Cedex 05, France, Pennsylvania State University, 25 Yearsley Mill Road, Media, Pennsylvania 19063, and Biophysical Chemistry, Universität Bielefeld, Universitätsstrasse 25, 33615 Bielefeld, Germany