1. Department of Pharmacological Sciences, State University of New York at Stony Brook, Stony Brook, New York 11794-5115, Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709, and Rudolf Virchow Center for Experimental Biomedicine, Institute for Structural Biology, University of Würzburg, Versbacher Str. 9, 97078 Würzburg, Germany