1. Department of Molecular and Integrative Physiology, University of Michigan, 2800 Plymouth Rd., Ann Arbor, Michigan 48109, USA
2. Department of Cell Biology and Center for Cell Analysis and Modeling, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, Connecticut 06030-6406, USA and Department of Physics and Molecular and Cellular Biology, University of Arizona, Tucson, Arizona 85721, USA