SIZE AND LOCALIZATION OF DYSTROPHIN MOLECULE - IMMUNOELECTRON MICROSCOPIC AND FREEZE-ETCHING STUDIES OF MUSCLE PLASMA-MEMBRANES OF MURINE SKELETAL MYOFIBERS

The ultrastructure and mode of existence of the dystrophin molecule an d its relations to actin filaments were examined in murine skeletal my ofibers. Electron microscopy of freeze-etched replicas of gold-labelle d dystrophin molecules in quick-freeze, deep-etch, rotary-shadow prepa rations 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. T he dystrophin molecules were parallel or nearly parallel to the inner surface of the muscle plasma membrane. Double immune-labelling transmi ssion electron microscopy using N- and C-terminal dystrophin antibodie s showed that the group mean distances of the N- and C-terminal signal s 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 muscl e plasma membrane had similar patterns with peaks 10 similar to 20 nm from the membrane. This was consistent with the findings of the mode o f existence of dystrophin molecules seen in freeze-etched replicas. Fi nally, the dystrophin molecules were linked with the most peripheral s arcoplasmic actin like filaments, end to side as well as end to end.