1. Center for Structural Biology and Department of Chemistry, Vanderbilt University, 5142 Medical Research Building III, 465 21st Avenue South, Nashville, Tennessee 37232-8725; Department of Biological Structure, University of Washington, Seattle, Box 357420, Seattle, Washington 98195-7420; Department of Biochemistry, University of Washington, Seattle, Box 357350, Seattle, Washington 98195-7350; and Biomolecular Structure Center, University of Washington, Seattle, Seattle, Washington 98195-7742