1. Groupe de recherche en transport membranaire, Université de Montréal, P.O. Box 6128, Centre Ville Station, Montreal, Quebec, H3C 3J7, Canada, Biotechnology Research Institute, National Research Council, 6100 Royalmount Avenue, Montreal, Quebec, H4P 2R2, Canada, and IGEPAM, CIRAD, Montpellier, France