1. Department of Pharmacological Sciences and Center for Structural Biology, Stony Brook University, Stony Brook, New York 11794-5115, Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, and Rudolf Virchow Center for Experimental Biomedicine, Institute for Structural Biology, University of Würzburg, Versbacher Str. 9, 97078 Würzburg, Germany