The regulatory interaction of phospholamban (PLN) with Ca(2+)-ATPase controls 
the uptake of calcium into the sarcoplasmic reticulum, modulating heart muscle 
contractility. A missense mutation in PLN cytoplasmic domain (R9C) triggers 
dilated cardiomyopathy in humans, leading to premature death. Using a 
combination of biochemical and biophysical techniques both in vitro and in live 
cells, we show that the R9C mutation increases the stability of the PLN 
pentameric assembly via disulfide bridge formation, preventing its binding to 
Ca(2+)-ATPase as well as phosphorylation by protein kinase A. These effects are 
enhanced under oxidizing conditions, suggesting that oxidative stress may 
exacerbate the cardiotoxic effects of the PLN(R9C) mutant. These results reveal 
a regulatory role of the PLN pentamer in calcium homeostasis, going beyond the 
previously hypothesized role of passive storage for active monomers.