STAC3 binding to CaV1.1 II-III loop is nonessential but critically supports skeletal muscle excitation-contraction coupling

Skeletal muscle excitation-contraction (EC) coupling depends on the direct coupling between CaV1.1 on the sarcolemma and ryanodine receptor (RyR1) on the sarcoplasmic reticulum. A key regulator of this process is STAC3, a protein essential for both the functional expression of CaV1.1 and its conformational coupling with RyR1. Mutations in Stac3 cause STAC3 disorder, a congenital myopathy characterized by muscle weakness. STAC3 interacts with CaV1.1 in 2 key regions: the II-III loop and the proximal C-terminus. While the II-III loop has been previously found to be essential for skeletal muscle EC coupling, here we demonstrated that the interaction between STAC3 and the proximal C-terminus is necessary and sufficient for CaV1.1 functional expression and minimal EC coupling. In contrast, the interaction with the II-III loop is not essential for EC coupling, though it plays a facilitating role in enhancing the process. Supporting this finding, we identified a patient with STAC3 disorder carrying a mutation that deletes the domain of STAC3 involved in the II-III loop interaction. Collectively, our results established that STAC3 binding to CaV1.1 C-terminus is essential for its functional expression, whereas STAC3 interaction with the II-III loop serves to enhance the conformational coupling with RyR1.