1. Laboratory of Cell Biology and Genetics, NIDDK, NIH, Bethesda, Maryland 20892, Laboratory of Bioorganic Chemistry, NIDDK, NIH, Bethesda, Maryland 20892, Departments of Physiology and Medicine, UAB, Birmingham, Alabama 35294, and Departments of Anatomy and Cell Biology, Uniformed Services University, School of Medicine, Bethesda, Maryland 20814