1. Howard Hughes Medical Institute, UCLA-DOE Institute of Genomics and Proteomics, Los Angeles, California 90095-1570, Centre for Crystallographic Studies, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 KBH, Denmark, and European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble Cedex, France