1. Ruhr-Universität Bochum, Institut für Physiologische Chemie, Abt. für Biochemie Supramolekularer Systeme, 44801 Bochum, Germany, Department of Structural Biology, SmithKline Beecham Pharmaceuticals, King of Prussia, Pennsylvania, Department of Molecular Biology, DuPont Pharmaceutical Company, Wilmington, Delaware 19880-0361, Howard Hughes Medical Institute and University of California, San Diego, Department of Chemistry and Biochemistry, 9500 Gilman Drive, La Jolla, California 92093-0654