1. Max-Delbrück-Center for Molecular Medicine, Robert-Rössle-Strasse 10, D-13125 Berlin, Germany, Robert Koch-Institute, Nordufer 20, D-13353 Berlin, Germany, Institute of Protein Research, Russian Academy of Sciences, Pushchino, Moscow Region 142292, Russia, and J.W. Goethe University Frankfurt, Institute for Biophysical Chemistry, D-60439 Frankfurt, Germany