1. Institute for Biophysical Dynamics, University of Chicago, Chicago, Illinois 60637;
2. James Franck Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637;
3. Department of Chemical and Biological Engineering and Department of Genetics, Development, and Cell Biology, Iowa State University, Ames, Iowa 50011
4. Cell Biology and Physiology Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892;