Sorcin associates with the pore-forming subunit of voltage-dependent L-type Ca2+ channels.
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 alpha1 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 alpha1 subunits, providing evidence that the second protein recovered by sorcin antibody from cardiac myocytes was the 240-kDa L-type Ca2+ channel alpha1 subunit. Anti-sorcin antibody immunoprecipitated full-length alpha1 subunits from cardiac myocytes, C2C12 myotubes, and transfected non-muscle cells expressing alpha1 subunits. In contrast, the anti-sorcin antibody did not immunoprecipitate C-terminal truncated forms of alpha1 subunits that were detected in myotubes. Recombinant sorcin bound to cardiac and skeletal HIS6-tagged alpha1 C termini immobilized on Ni2+ resin. Additionally, anti-sorcin antibody immunoprecipitated C-terminal fragments of the cardiac alpha1 subunit exogenously expressed in mammalian cells. The results identified a putative sorcin binding domain within the C terminus of the alpha1 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.
Department of Medicine, Cardiovascular Institute, Mount Sinai School of Medicine, New York, New York 10029, USA. marian_meyers@smtplink.mssm.edu
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MB
Meyers
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Puri
A J
AJ
Chien
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Gao
P H
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Hsu
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Hosey
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Fishman
D000818:Animals; D000906:Antibodies; D019556:COS Cells; D015220:Calcium Channels / Q000378:metabolism*; D020746:Calcium Channels, L-Type; D002135:Calcium-Binding Proteins / Q000276:immunology / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D015640:Ion Channel Gating; D018482:Muscle, Skeletal / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D051381:Rats; D012508:Sarcolemma / Q000378:metabolism; D018411:Spodoptera
D000906:Antibodies; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D002135:Calcium-Binding Proteins; C503113:SRI protein, human