1. Universität des Saarlandes, FR 8.8 Biochemie, Postfach 151150, D-66041 Saarbrücken, Germany, Structural Biology Section, Kansai Advanced Research Center, Communications Research Laboratory, 588−2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe 651-2492, Japan, and Max-Delbrück-Centrum für Molekulare Medizin, Robert-Rössle-Strasse 10, D-13122 Berlin-Buch, Germany