1. UCLA-DOE Laboratory of Structural Biology and Molecular Medicine, Box 951570, University of California, Los Angeles, Los Angeles, California 90095-1570, Department of Molecular Biology, Research Institute of Scripps Clinic, La Jolla, California 92037, Department of Chemistry and Biochemistry, Box 951569, University of California, Los Angeles, Los Angeles, California, 90095-1569, and Department of Chemistry and Biochemistry, Loyola Marymount University, Los Angeles, California 90045