1. *UCSD Division of Biological Sciences, Section of Cell and Developmental Biology, University of California, San Diego, La Jolla, CA 92093-0347;
2. National Center for Microscopy and Imaging Research, Department of Neurosciences, University of California, San Diego, La Jolla, CA 92093-0608;
3. Banting and Best Department of Medical Research, University of Toronto, Toronto, Ontario, Canada M5G 1L6; and
4. Laboratory of Cell Biochemistry and Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, United States Department of Health and Human Services, Bethesda, MD 20892