1. Institute of Biochemistry, Faculty of Medicine, University of Giessen, Friedrichstrasse 24, D-35392 Giessen, Germany, Center of Molecular Neurobiology, University of Hamburg, Martinistrasse 52, D-20246 Hamburg, Germany, and Division of Molecular, Cellular, & Biomedical Sciences, University of New Hampshire, Gregg Hall, 32 Colovos Road, Durham, New Hampshire 03824