1. Department of Pediatrics, Duke University Medical Center, Durham, North Carolina 27710, Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229, Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, Department of Physiology, University of California, Los Angeles, California 90095, and Department of Molecular and Cellular Pharmacology, University of Miami, Miami, Florida 33124