1. Department of Biochemistry and Chemistry and Rosenstiel Basic Medical Sciences Research Center MS029, Brandeis University, Waltham, Massachusetts 02454-9110, Department of Chemistry, Department of Biochemistry, Molecular Biology, and Cell Biology and the Center for Drug Discovery and Chemical Biology, Northwestern University, Evanston, Illinois 60208-3113, and Department of Pharmaceutical Sciences, University of Maryland, Baltimore, Maryland 21201