1. LCCP, Institut de Biologie Structurale J.P. Ebel, CEA‐CNRS 41 rue Jules Horowitz F38027 Grenoble France
2. Present address: Max‐Planck Institut Notkestrasse 85 22603 Hamburg Germany
3. Present address: CBM‐CNRS, rue Charles Sadron 45071 Orléans Cedex France
4. Departamento de Bioquímica y Biología Molecular y Celular, Universidad de Zaragoza 50009 Zaragoza Spain