Acknowledgements
We would like to thank M. Heil, B. Hoffmann and R. Merkel for the biophysical experiments, L. Eichinger for discussion, B. Gassen for expert technical assistance and R. Müller for protein purification. Mass spectrometric analysis was performed in the central service facilities of the ZMMK. This worked is supported by a grant from the ZMMK (Center of Molecular Medicine Cologne).