1. Gatty Marine Laboratory, School of Biology, University of St Andrews, Fife KY16 8LB, Scotland, U.K.
2. Centre for Biomolecular Sciences, School of Biology, University of St Andrews, Fife KY16 9QT, Scotland, U.K.,
3. Centre de Biochimie Structurale, UMR 5048 CNRS, 554 INSERM, 29 rue de Navacelles, 34090 Montpellier, France