1. Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104
2. Howard Hughes Medical Institute, California Institute of Technology, Pasadena, CA 91125
3. Department of Microbiology, University of Illinois at Urbana–Champaign, Urbana, IL 61801
4. Department of Membrane Biochemistry, Institute of Chemistry and Biochemistry, Freie Universität Berlin, D-14195 Berlin, Germany