1. Institut für Biochemie und Biotechnologie, Martin-Luther-Universität Halle-Wittenberg, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany, Laboratory for Structural Biology at the National Space Science and Technology Center, University of Alabama in Huntsville, Huntsville, Alabama 35899, and Department of Biochemistry, School of Medicine and Biomedical Sciences, State University of New York, Buffalo, New York 14214