1. Membrane Biology Program and Renal Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, 77 Avenue Louis Pasteur, Boston, MA 02115, U.S.A.
2. Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, PO Box 670576, Cincinnati, OH 45267-0576, U.S.A.
3. European Molecular Biology Laboratory, Gene Expression Programme, Meyerhofstrasse 1, D-69117 Heidelberg, Federal Republic of Germany