1. Departamento de Bioquímica y Biología Molecular y Celular, Facultad de Ciencias and Institute of Biocomputation and Physics of Complex Systems (BIFI), Universidad de Zaragoza, 50009-Zaragoza, Spain, and Department of Biochemistry, Conway Institute for Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland