The Triad Targeting Signal of the Skeletal Muscle Calcium Channel Is Localized in the Cooh Terminus of the α(1S) Subunit

The specific localization of L-type Ca(2+) channels in skeletal muscle triads is critical for their normal function in excitation–contraction (EC) coupling. Reconstitution of dysgenic myotubes with the skeletal muscle Ca(2+) channel α(1S) subunit restores Ca(2+) currents, EC coupling, and the normal localization of α(1S) in the triads. In contrast, expression of the neuronal α(1A) subunit gives rise to robust Ca(2+) currents but not to triad localization. To identify regions in the primary structure of α(1S) involved in the targeting of the Ca(2+) channel into the triads, chimeras of α(1S) and α(1A) were constructed, expressed in dysgenic myotubes, and their subcellular distribution was analyzed with double immunofluorescence labeling of the α(1S)/α(1A) chimeras and the ryanodine receptor. Whereas chimeras containing the COOH terminus of α(1A) were not incorporated into triads, chimeras containing the COOH terminus of α(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 α(1S). Transferring this triad targeting signal to α(1A) was sufficient for targeting and clustering the neuronal isoform into skeletal muscle triads and caused a marked restoration of Ca(2+)-dependent EC coupling.