1. Muscle Research Group, Boston Biomedical Research Institute, 20 Staniford Street, Boston, Massachusetts 02114, Neurology Service, Massachusetts General Hospital, Boston, Massachusetts 02114, Department of Biological Chemistry and Molecular Pharmacology, and Department of Neurology, Harvard Medical School, Boston, Massachusetts 02115, and Department of Biochemistry, Tufts University School of Medicine, 136 Harrison Avenue, Boston, Massachusetts 02111