1. Muscle and Motility Group, Boston Biomedical Research Institute, Watertown, Massachusetts 02472, Neurology Service, Massachusetts General Hospital, Boston, Massachusetts, 02114, Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, Department of Neurology, Harvard Medical School, Boston, Massachusetts 02115, and Department of Biochemistry, Tufts University School of Medicine, Boston, Massachusetts 02111