Abstract
The adaptor protein STAC3 is essential for skeletal muscle excitation-contraction (EC) coupling and a mutation in the STAC3 gene has been linked to a severe muscle disease, Native American myopathy (NAM). However the function of STAC3, its interaction partner, and the mode of interaction within the EC-coupling complex remained elusive. Here we demonstrate that STAC3 forms a stable interaction with the voltage-sensor of EC-coupling, CaV1.1, and that this interaction depends on a hitherto unidentified protein-protein binding pocket in the C1 domain of STAC3. While the NAM mutation does not affect the stability of the STAC3-CaV1.1 interaction, mutation of two crucial residues in the C1 binding pocket increases the turnover of STAC3 in skeletal muscle triads. Thus, the C1 domain of STAC3 is responsible for its stable incorporation into the CaV1.1 complex, whereas the SH3 domain containing the NAM mutation site may be involved in low-affinity functional interactions in EC-coupling.
Highlights
Excitation-contraction (EC) coupling is a fundamental process of muscle physiology, in which an electrical signal, the action potential, is converted into a mechanical response, muscle contraction
When dysgenic myotubes were reconstituted with CaV1.1, STAC3-GFP formed clusters and these STAC3 clusters were colocalized with CaV1.1, β1a, and the RyR1 (Fig. 1B)
These results are consistent with similar previous observations[5,7] and conclusively demonstrate that heterologously expressed STAC3-GFP incorporates in skeletal muscle triads only in the presence of CaV1.1
Summary
Excitation-contraction (EC) coupling is a fundamental process of muscle physiology, in which an electrical signal, the action potential, is converted into a mechanical response, muscle contraction. The resulting elevation of the cytoplasmic calcium concentration triggers muscle contraction Consistent with their respective functions as voltage-sensor and calcium release channel, the CaV1.1 α1S and β1a subunits[1,2] and the RyR13 were shown to be essential for skeletal muscle EC coupling. Expression of STAC3 is specific to skeletal muscle, whereas STAC1 and STAC2 are expressed in brain and a variety of other tissues, but not in skeletal muscle[4] In both fish and mouse, knockout of STAC3 resulted in paralysis and perinatal death from suffocation, similar to the phenotype of the other EC coupling null models[4,5]. By generating multiple STAC2/STAC3 chimeras and point mutants we identified two residues in the C1 domain of STAC3 which are critical for its stable association with CaV1.1 and CaV1.2 calcium channels in skeletal muscle triads
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