[1] Johannes Gutenberg Univ Mainz, Inst Pharm & Biochem, D-55128 Mainz, Germany
[2] Goethe Univ, Ctr Biomol Magnet Resonance BMRZ, D-60438 Frankfurt, Germany
[3] Goethe Univ, Inst Mol Biosci, D-60438 Frankfurt, Germany
[4] Harvard Univ, Dept Mol & Cellular Biol, Cambridge, MA 02138 USA