Structural Insights Into The Stac Adaptor Protein And Voltage-Gated Calcium Channel Interaction

Skeletal muscle Excitation-Contraction (EC) coupling relies on the proper trafficking of CaV1.1 to regions of the plasma membrane to facilitate its physical interaction with the ryanodine receptor (RyR1). CaV1.1 acts as a voltage sensor for RyR1, allowing the plasma membrane depolarization to be transduced into a rapid torrent of cytosolic calcium that culminates in muscle contraction. Recent studies have shown that failure of skeletal muscle EC coupling occurs in mice and fish null for the protein STAC3 (SH3 and cysteine-rich domain 3) and has led to the conclusion that STAC3 is critical in trafficking and maintaining components of the EC coupling machinery in a functional state. We present isothermal titration calorimetry data showing a direct interaction between STAC3 and CaV1.1 α1s. This association is conserved among the members of the STAC family of adaptor proteins, as STAC1 and STAC2 show similar low-micromolar affinity binding. The STAC3 Native American Myopathy (NAM) mutation significantly reduces the affinity of association. Our binding studies corroborate with our structural findings. We present the first high-resolution (1.2 - 2.6Å) crystallographic insights into the three STAC protein isoforms in apo-form and a complex between STAC2 and our CaV1.1 α1s binding determinant. EC coupling assays and two-electrode voltage clamp electrophysiology was used to assess the functional effect of STAC3 on CaV1.2 α1c.