1. Howard Hughes Medical Institute, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093–0647, USA.
2. Department of Pharmacology, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093–0647, USA.
3. Institut de Biologie Structurale 41, rue Jules Horowitz, 38027 Grenoble CEDEX 1, France.
4. Departments of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093–0647, USA.