Phospholamban and CaTArΔpolyK inhibit SERCA. (A) Localization of phospholamban-GFP (PLN-GFP) and mCh-STIM1 in HeLa cells before (Ctr) and after store depletion (Tg). (B) PLN-GFP transfection followed by loading cells with Calbryte 590. The numbered cells match the traces in C. (C) Ca²⁺ release induced by histamine (His, 100 μM) leads to Ca²⁺ release in cell#1, which does not express PLN-GFP; but not in cell#2, which expresses high levels of PLN-GFP. (D) Bar chart summarizing the inhibition of Ca²⁺ release in PLN-expressing cells. The recordings were performed in the presence of 1 mM lanthanum in the extracellular media to inhibit Ca²⁺ influx and recycling at the PM (n = 38–176; unpaired t test). (E) ER localization of CaTArΔpolyK (green), a CaTAr1 construct lacking the terminal poly-lysine domain that localizes it to ERPMCS. (F) Example of Calbryte intensity images before (Ctr) and after histamine addition (His). (G) Ca²⁺ release traces induced by histamine (His, 100 μM) in the two cells labeled in panel F. (H) Bar chart summarizing the inhibition of Ca²⁺ release in CaTArΔpolyK-expressing cells (n = 20–39; unpaired t test; P < 0.0001).