1. From the Department of Molecular Physiology and Biological Physics (X.F.F., K.H.D., D.N.D., B.R.D.), University of Virginia, Charlottesville, Va; the Department of Neurobiology (D.L.P.) and Department of Cell Biology (D.A.G.), Harvard Medical School, Boston, Mass; and the Department of Physiology (A.M.S.), University of Arizona, Tucson, Ariz.