1. Lehrstuhl für Biotechnologie, Biocenter of the University of Würzburg, Am Hubland, D-97074 Würzburg, Federal Republic of Germany, Institut de Pharmacologie et Biologie Structurale, 205 Route de Narbonne, F-31077 Toulouse, France, INSERM U-570, CHU Necker-Enfants Malades, F-75730 Paris Cedex 15, France, and School of Engineering and Science, International University of Bremen, D-28727 Bremen, Federal Republic of Germany