1. The Krebs Institute & The Department of Molecular Biology and Biotechnology, The University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, U.K., Structural Biology Group, European Synchrotron Radiation Facility, 6 rue Jules Horowitz, F-38043 Grenoble, France, Institute of Enzymology, Biological Research Center, Hungarian Academy of Sciences, P.O. Box 7, H-1518 Budapest, Hungary, Manchester Interdisciplinary Biocentre, University of Manchester, Manchester M1 7DN, U.K.