1. Department of Biochemistry, University of WisconsinMadison, Madison, Wisconsin 53726-4087, Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664, Department of Physiology & Biophysics, Albert Einstein College of Medicine, Bronx, New York 10461-1602, and Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853