Native skeletal muscle dihydropyridine receptor exists as a supramoleculartriad complex

One of the central elements of excitation-contraction coupling, the voltage -sensing dihydropyridine receptor, is believed to exist as a high-molecular -mass complex in the triad junction. Although freeze-fracture electron micr oscopical analysis suggests a tetrad complex, no direct biochemical evidenc e exists demonstrating the actual size of the native membrane complex. Usin g a combination of various two-dimensional gel electrophoresis techniques, we show here that the principal alpha (1)-subunit of the dihydropyridine re ceptor and its auxiliary alpha (2)-subunit form a triad complex of approxim ately 2800 kDa under native conditions. Established Ca2+-ATPase tetramers a nd calsequestrin monomers were employed for the internal standardization of the gel systems used. Thus, the large voltage-sensing complex appears to b e tightly associated, since it does not disintegrate during subcellular fra ctionation and native electrophoresis procedures. Our findings support the cell biological hypothesis that native dihydropyridine receptor units form a tetrad structure within the transverse tubules.