Modulation of Cardiac Ryanodine Receptors by Sorcin*

Sorcin is a widely expressed, 22-kDa Ca²⁺-binding protein initially identified in multidrug-resistant cells. In the heart, sorcin localizes to the dyadic junctions of transverse tubules and sarcoplasmic reticulum and coimmunoprecipitates with the Ca²⁺ release channel/ryanodine receptor (RyR) (Meyers, M. B., Pickel, V. M., Sheu, S.-S., Sharma, V. K., Scotto, K. W., and Fishman, G. I. (1995)J. Biol. Chem.270, 26411–26418). We have investigated a possible functional interaction between sorcin and cardiac RyR using purified recombinant sorcin in [³H]ryanodine binding experiments and single channel recordings of RyR. The open probability of single RyR was decreased significantly by the addition of sorcin to the cytoplasmic side of the channel (IC₅₀ ∼ 480 nm). In addition, sorcin completely inhibited [³H]ryanodine binding with an IC₅₀ ∼ 700 nm. Inhibition occurred over a wide range of [Ca²⁺], and sorcin-modulated RyR remained Ca²⁺-dependent. Furthermore, caffeine-activated RyRs were also inhibited by sorcin at low [Ca²⁺] (pCa 7), suggesting that Ca²⁺ is not an obligatory factor for sorcin inhibition of RyR. Comparisons of these inhibitory effects with those of calmodulin and calpain, proteins structurally related to sorcin, suggested that the interaction of sorcin with cardiac RyR was distinct from and independent of either of these modulatory proteins. Phosphorylation of sorcin with the catalytic subunit of protein kinase A significantly decreased the ability of sorcin to modulate RyR. These results suggest that sorcin may modulate RyR function in a normal cell environment and that the level of modulation is in turn influenced by signaling pathways that increase protein kinase A activity.