1. Muscle Cell Biology Group, Medical Research Council Clinical Sciences Centre, Imperial College School of Medicine, Hammersmith Hospital, London, W12 ONN United Kingdom
2. Centre National de la Recherche Scientifique Unité de Recherche Associée 1947, Département de Biologie Moléculaire, Institut Pasteur, 75724 Paris Cedex 15, France
3. Physiologisches Institut II der Universitat Bonn, Neurophysiologie, D-53111, Bonn, Germany