1. Contribution from the European Molecular Biology Laboratory, Hamburg Outstation, 22603 Hamburg, Germany, Institut de Recerca Biomèdica, Parc Científic de Barcelona, Josep Samitier 1-5, 08028 Barcelona, Spain, Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, 117333 Moscow, Russia, and Institut de Biologie Structurale Jean-Pierre Ebel, CEA-CNRS-UJF, 41 Rue Jules Horowitz, 38027 Grenoble, France