1. Istituto di Genetica Molecolare IGM-CNR, Consiglio Nazionale delle Ricerche, via Abbiategrasso 207, I-27100 Pavia, Italy, Institute of Veterinary Biochemistry and Molecular Biology IVB-MB, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland, and Institut de Pharmacology et de Biologie Structurale, Centre National de la Recherche Scientifique, 205 Route de Narbonne, 310077 Toulouse, Cedex France