1. Laboratory of Cell Biochemistry and Biology, Laboratory of Biochemical Pharmacology, NIDDK, and Laboratory of Neurobiology, NINDS, National Institutes of Health, Bethesda, Maryland 20892, Department of Pathology, Anatomy, & Cell Biology, Jefferson Medical College, 1020 Locust Street, Philadelphia, Pennsylvania 19107, and Developmental Biology Program, Oklahoma Medical Research Foundation, Oklahoma City, Oklahoma 73104