1. Muscle Research Group, Boston Biomedical Research Institute, Watertown, Massachusetts 02472, Neurology Service, Massachusetts General Hospital, Boston, Massachusetts 02114, Departments of Biological Chemistry and Molecular Pharmacology and of Neurology, Harvard Medical School, Boston, Massachusetts 02115, Department of Biochemistry, Tufts University School of Medicine, Boston, Massachusetts 02111, and Department of Biochemistry, University of Massachusetts School of Medicine, Shrewsbury, Massachusetts 01545