1. Department of Genetics, Cell Biology, and Development, University of Minnesota Medical School, Minneapolis, MN 55455
2. Biology Department and Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, MA 02454
3. Vienna Biocenter Core Facilities, 1030 Vienna, Austria
4. Departments of Cell Biology and Biophysics, University of Texas Southwestern Medical Center, Dallas, TX 75390