1. Division of Experimental Medicine, Brigham and Women's Hospital, Program in Biological and Biomedical Sciences, Harvard Medical School, 221 Longwood Avenue, LMRC 301, Boston, Massachusetts 02115, Department of Biophysics, Kyoto University School of Science, Kyoto 606-01, Japan, Muscle Research Group, Boston Biomedical Research Institute, 20 Staniford Street, Boston, Massachusetts 02114, and Max Planck Institute for Cell Biology, Ladenburg bei Heidelberg, Germany