1. The Department of Molecular Biosciences, The Howard Hughes Medical Institute, The University of Texas at Austin, Austin, TX 78712, USA, 2Institute for Cellular and Molecular Biology, The University of Texas at Austin, Austin, TX 78712, USA, 3Université de Toulouse, UPS, LBCMCP, 31062 Toulouse, France and 4CNRS, LBCMCP, F-31062 Toulouse, France