THE MEMBRANE SKELETON OF ACETYLCHOLINE-RECEPTOR DOMAINS IN RAT MYOTUBES CONTAINS ANTIPARALLEL HOMODIMERS OF BETA-SPECTRIN IN FILAMENTS QUANTITATIVELY RESEMBLING THOSE OF ERYTHROCYTES

I used immunogold labeling and quick-freeze, deep-etch, rotary replica tion to characterize the membrane skeleton at regions with high concen trations of acetylcholine receptor domains in receptor clusters of cul tured rat muscle cells, This membrane skeleton consists of a network o f filaments closely applied to the cytoplasmic membrane surface. The f ilaments are specifically decorated by immunogold labeling with a mono clonal antibody, VIIF7, that recognizes an isoform of beta-spectrin co localizing with acetylcholine receptors, The filaments are 32 +/- 11 n m in length and three to four filaments (average 3.1-3.3) join at each intersection to form the network. These parameters are nearly identic al to those reported previously for the membrane skeleton of erythrocy tes. Depending on the amount of platinum coating, filament diameters r ange from 9 to 11 nm in diameter, and are 1.4 nm larger on average tha n spectrin filaments of erythrocytes replicated at the same time. Fila ments are decorated with gold particles close to one end, consistent w ith the location of the epitope recognized by the monoclonal antibody. Computer modeling shows that all filament intersections in the membra ne skeletal network are equally capable of being labeled by the monocl onal antibody. This pattern of labeling is consistent with a network c ontaining antiparallel homodimers of beta-spectrin.