1. Departments of Biochemistry and
2. Genetics, Stanford University School of Medicine, Stanford, CA 94305-5428;
3. Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-5430;
4. Lawrence Berkeley National Labs and Department of Molecular and Cellular Biology, University of California Berkeley, Berkeley, CA 94720; and