1. UPR 39. CNRS, Bat. 24-Biosystèmes Membranaires, 91 198 Gif-sur-Yvette, France
2. Botanisches Institut der Universität Bonn, Kirschallee 1, 5300 Bonn 1, Germany
3. Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, U.K.