SORCIN ASSOCIATES WITH THE PORE-FORMING SUBUNIT OF VOLTAGE-DEPENDENT L-TYPE CA2+ CHANNELS

Intracellular Ca2+ release in muscle is governed by functional communi cation between the voltage-dependent L-type Ca2+ channel and the intra cellular Ca2+ release channel by processes that are incompletely under stood, 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 imm unoprecipitates ryanodine receptors from metabolically labeled cardiac myocytes along with a second protein having a molecular weight simila r to that of the alpha(1) 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 cytoplasm ically oriented C-terminal region of the alpha(1) subunits, providing evidence that the second protein recovered by sorcin antibody from car diac myocytes was the 240-kDa L-type Ca2+ channel alpha(1) subunit, An ti-sorcin antibody immunoprecipitated full-length alpha(1) subunits fr om cardiac myocytes, C2C12 myotubes, and transfected non-muscle cells expressing alpha(1) subunits, In contrast, the anti-sorcin antibody di d not immunoprecipitate C-terminal truncated forms of alpha(1) subunit s that were detected in myotubes. Recombinant sorcin bound to cardiac and skeletal HIS6-tagged alpha(1) C termini immobilized on Ni2+ resin, Additionally, anti-sorcin antibody immunoprecipitated C-terminal frag ments of the cardiac alpha(1) subunit exogenously expressed in mammali an cells. The results identified a putative sorcin binding domain with in the C terminus of the alpha(1) subunit, These observations, along w ith the demonstration that sorcin accumulated substantially during phy siological maturation of the excitation-contraction coupling apparatus in developing postnatal rat heart and differentiating C2C12 muscle ce lls, suggest that sorcin may mediate interchannel communication during excitation-contraction coupling in heart and skeletal muscle.