1. CIHR Membrane Protein Research Group, Department of Biochemistry, University of Alberta, 474 Medical Sciences Building, Edmonton, Alberta T6G 2H7, Canada, Department of Biochemistry, University of British Columbia, 2146 Health Sciences Mall, Vancouver, British Columbia V6T 1Z3, Canada, Department of Life Sciences, King's College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NN, U.K., and Laboratoire de Chimie Bactérienne, CNRS, 31 chemin Joseph Aiguier, 13402 Marseille Cedex 9, France