1. Instituto de Biología Molecular y Celular de Rosario (IBR-CONICET), Universidad Nacional de Rosario, S2002LRK Rosario, Argentina, Institut Pasteur de Montevideo (IPM) and Instituto de Investigaciones Biológicas Clemente Estable (IIBCE), 11400 Montevideo, Uruguay, and DFG Research Center for the Molecular Physiology of the Brain (CMPB) and Department of NMR-Based Structural Biology, Max Planck Institute for Biophysical Chemistry (MPIbpc), D-37077 Göttingen, Germany