1. CNR Center of Molecular Biology and Department of Biochemical Sciences, University La Sapienza, 00185 Rome, Italy
2. Riken, Laboratory of Biophysical Chemistry, Hirosawa 2-1, Wako City, Saitama Prefecture, 351-01, Japan
3. Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York 10461