Sorcin Associates with the Pore-forming Subunit of Voltage-dependent L-type Ca²⁺ Channels*

Abstract

Intracellular Ca²⁺release in muscle is governed by functional communication between the voltage-dependent L-type Ca²⁺ channel and the intracellular Ca²⁺ release channel by processes that are incompletely understood. We previously showed that sorcin binds to cardiac Ca²⁺ release channel/ryanodine receptors and decreases channel open probability in planar lipid bilayers. In addition, we showed that sorcin antibody immunoprecipitates ryanodine receptors from metabolically labeled cardiac myocytes along with a second protein having a molecular weight similar to that of the α₁ subunit of cardiac L-type Ca²⁺ channels. We now demonstrate that sorcin biochemically associates with cardiac and skeletal muscle L-type Ca²⁺ channels specifically within the cytoplasmically oriented C-terminal region of the α₁ subunits, providing evidence that the second protein recovered by sorcin antibody from cardiac myocytes was the 240-kDa L-type Ca²⁺ channel α₁ subunit. Anti-sorcin antibody immunoprecipitated full-length α₁ subunits from cardiac myocytes, C2C12 myotubes, and transfected non-muscle cells expressing α₁ subunits. In contrast, the anti-sorcin antibody did not immunoprecipitate C-terminal truncated forms of α₁ subunits that were detected in myotubes. Recombinant sorcin bound to cardiac and skeletal HIS₆-tagged α₁ C termini immobilized on Ni²⁺ resin. Additionally, anti-sorcin antibody immunoprecipitated C-terminal fragments of the cardiac α₁ subunit exogenously expressed in mammalian cells. The results identified a putative sorcin binding domain within the C terminus of the α₁ subunit. These observations, along with the demonstration that sorcin accumulated substantially during physiological maturation of the excitation-contraction coupling apparatus in developing postnatal rat heart and differentiating C2C12 muscle cells, suggest that sorcin may mediate interchannel communication during excitation-contraction coupling in heart and skeletal muscle.