1. Department of Biochemistry and Molecular Biology, Oregon Graduate Institute of Science and Technology, P.O. Box 91000, Portland, Oregon 97291-1000, Department of Molecular Biology and Biotechnology, University of Sheffield, P.O. Box 594, Firth Court, Western Bank, Sheffield S10 2TN, U.K., and The Institute of Crystallography, Russian Academy of Sciences, Leninsky pr.59, Moscow 117333, Russia