DIHYDROPYRIDINE RECEPTOR-ALPHA SUBUNITS IN NORMAL AND DYSGENIC MUSCLE INVITRO - EXPRESSION OF ALPHA-1 IS REQUIRED FOR PROPER TARGETING AND DISTRIBUTION OF ALPHA-2

We have studied the subcellular distribution of the alpha-1 and alpha-2 subunits of the skeletal muscle dihydropyridine (DHP) receptor with immunofluorescence labeling of normal and dysgenic (mdg) muscle in culture. In normal myotubes both alpha-subunits were localized in clusters associated with the T-tubule membranes of longitudinally as well as transversely oriented T-tubules. The DHP receptor-rich domains may represent the sites where triad junctions with the sarcoplasmic reticulum are being formed. In cultures from dysgenic muscle the alpha-1 subunit was undetectable and the distribution patterns of the alpha-2 subunit were abnormal. The alpha-2 subunit did not form clusters nor was it discretely localized in the T-tubule system. Instead, alpha-2 was found diffusely distributed in parts of the T-system, in structures in the perinuclear region and in the plasma membrane. These results suggest that an interaction between the two alpha-subunits is required for the normal distribution of the alpha-2 subunit in the T-tubule membranes. Spontaneous fusion of normal non-muscle cells with dysgenic myotubes resulted in A regional expression of the alpha-1 polypeptide near the foreign nuclei, thus defining the nuclear domain of a T-tubule membrane protein in multi-nucleated muscle cells. Furthermore, the normal intracellular distribution of the alpha-2 polypeptide was restored in domains containing a foreign "rescue" nucleus; this supports the idea that direct interactions between the DHP receptor alpha-1 and alpha-2 subunits are involved in the organization of the junctional T-tubule membranes.