1. European Molecular Biology Laboratory, Structural Biology and Computational Biology Programme Meyerhofstrasse 1 D‐69117 Heidelberg Germany
2. Present address: Astra‐Zeneca, Molecular Biology–Target Development S‐431 83 Mölndal Sweden
3. Biochemistry Center Heidelberg (BZH) Im Neuenheimer Feld 328 D‐69120 Heidelberg Germany
4. Department of Biology, New York University, 10009 Main Building 100 Washington Square East New York NY 10003 USA