1. Institute of Biochemistry, Justus Liebig University of Giessen, D-35392 Giessen, Germany;
2. State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Yue-Yang Road 320, Shanghai 200031, China; and
3. Bioanalytical Mass Spectrometry Group, Max Planck Institute for Biophysical Chemistry, D-37077 Göttingen, Germany