1. Max Planck Unit for Structural Molecular Biology, Notkestrasse 85, D-22607 Hamburg, Germany, Maurice E. Müller Institute for Structural Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland, and Department of Pathology and Neuroscience, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10416