1. European Molecular Biology Laboratory, 38042 Grenoble, France
2. Department of Biochemistry, University of Colorado at Boulder, Boulder, CO 80309, USA
3. Department of Pharmacology, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA
4. Hubrecht Institute, Utrecht 3584 CT, the Netherlands
5. Leicester Institute of Structural and Chemical Biology, Department of Molecular and Cell Biology, University of Leicester, Lancaster Road, Leicester LE1 7RH, UK
6. Howard Hughes Medical Institute, Chevy Chase, MD 20815-6789, USA
7. Department of Pharmacology and Program in Structural Biology and Biochemistry, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA