Figure 4. Proposed protein interactions in the SR lumen under low and high SR Ca²⁺.

A, when the SR Ca²⁺ levels are low, maximal interaction may occur between HRC and SERCA thereby mediating the rate of SR Ca²⁺ uptake, while interaction between JCN and CSQ reduces Ca²⁺ leak through the RyR. B, when the SR Ca²⁺ content is high, HRC dissociates from SERCA and promotes interaction with triadin, which may simultaneously affect Ca²⁺ uptake and RyR activity. Junctin interaction with CSQ is reduced, possibly facilitating Ca²⁺ leak through the RyR.