The triad targeting signal of the skeletal muscle calcium channel is localized in the COOH terminus of the α1S subunit

The specific localization of L-type Ca2+ channels in skeletal muscle triads is critical for their normal function in excitation-contraction (EC) coupling. Reconstitution of dysgenic myotubes with the skeletal muscle Ca2+ channel als subunit restores Ca2+ currents, EC coupling. and the normal localization of alpha (1S) in the triads. In contrast, expression of the neuronal alpha (1A) subunit gives rise to robust Ca2+ currents but not to triad localization. To identify regions in the primary structure of als involved in the targeting of the Ca2+ channel into the triads, chimeras of alpha (1S) and alpha (1A) were constructed, expressed in dysgenic myotubes, and their subcellular distribution was analyzed with double immunofluorescence labeling of the alpha (1S)/alpha (1A) chimeras and the ryanodine receptor. Whereas chimeras containing the COOH terminus of cu,, were not incorporated into triads, chimeras containing the COOH terminus of alpha (1S) were correctly targeted. Mapping of the COOH terminus revealed a triad-targeting signal contained in the 55 amino-acid sequence (1607-1661) proximal to the putative clipping site of alpha (1S). Transferring this triad targeting signal to alpha (1A) was sufficient for targeting and clustering the neuronal isoform into skeletal muscle triads and caused a marked restoration of Ca2+-dependent EC coupling.