1. Section de Recherche, Institut Curie, UMR 176 CNRS, Bât. 110-112, Centre Universitaire, 91405 Orsay Cedex, France, Station Biologique, CNRS, B.P. 74, 29682 Roscoff Cedex, France, and Department of Biochemistry, Laboratory of Molecular Biophysics and Oxford Centre for Molecular Sciences, Rex Richards Building, South Parks Road, Oxford OX1 3QU, U.K.