1. Institut de Pharmacologie et de Biologie Structurale, Centre National de la Recherche Scientifique, 205 route de Narbonne, 31077 Toulouse Cedex, France, Institute of Veterinary Biochemistry and Molecular Biology IVB-MB, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland, and Istituto di Genetica Molecolare IGM-CNR, National Research Council, via Abbiategrasso 207, 27100 Pavia, Italy