Size and localization of dystrophin molecule: immunoelectron microscopic and freeze etching studies of muscle plasma membranes of murine skeletal myofibers

Abstract

The ultrastructure and mode of existence of the dystrophin molecule and its relations to actin filaments were examined in murine skeletal myofibers. Electron microscopy of freeze-etched replicas of gold-labelled dystrophin molecules in quick-freeze, deep-etch, rotary-shadow preparations revealed rod-like structures 108.2 +/- 16.3 nm long and 3.1 +/- 1.5 nm thick. Some dystrophin molecules appeared to link their ends to form anastomosing networks; others were separate from each other. The dystrophin molecules were parallel or nearly parallel to the inner surface of the muscle plasma membrane. Double immuno-labelling transmission electron microscopy using N- and C-terminal dystrophin antibodies showed that the group mean distances of the N- and C-terminal signals from the muscle plasma membrane were 52.7 +/- 8.1 nm and 45.9 +/- 11.3 nm, respectively, which were not significantly different. Histograms of the distribution of the N- and C-terminal distances from the muscle plasma membrane had similar patterns with peaks 10-20 nm from the membrane. This was consistent with the findings of the mode of existence of dystrophin molecules seen in freeze-etched replicas. Finally, the dystrophin molecules were linked with the most peripheral sarcoplasmic actin like filaments, end to side as well as end to end.