1. Division of Structural Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland, Dipartimento di Scienze Biochimiche, “A. Rossi Fanelli” Università La Sapienza, P.le Aldo Moro 5, I-00185 Rome, Italy, and School of Biosciences, University of Wales, Cardiff CF1 3US, United Kingdom