Dihydropyridine receptor a subunits in normal and dysgenic muscle in vitro: Expression of α1 is required for proper targeting and distribution of α2

Abstract

We have studied the subcellular distribution of the α1 and α2 subunits of the skeletal muscle dihydropyridine (DHP) receptor with immunofluorescence labeling of normal and dysgenic (mdg) muscle in culture. In normal myotubes both a 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 α1 subunit was undetectable and the distribution patterns of the α2 subunit were abnormal. The α2 subunit did not form clusters nor was it discretely localized in the T-tubule system. Instead, α2 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 α subunits is required for the normal distribution of the α2 subunit in the T-tubule membranes. Spontaneous fusion of normal non-muscle cells with dysgenic myotubes resulted in a regional expression of the α1 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 α2 polypeptide was restored in domains containing a foreign "rescue" nucleus; this supports the idea that direct interactions between the DHP receptor α1 and α2 subunits are involved in the organization of the junctional T-tubule membranes.