Membrane contact sites (MCSs) between the sarco/endoplasmic reticulum (SER) and the plasma membrane provide cytosolic nanodomains for local Ca2+ release in electrically excitable cells. We have discovered that ryanodine receptor (RyR2) Ca2+ release channels and phospholamban form a protein supercomplex with the Ca2+-ATPase SERCA2a in cardiomyocyte MSCs. In addition, we identified the protein phosphatase 1 (PP1) subunit PPP1R3 in the RyR2 channel complex, which targets the same phosphatase to phospholamban. Hence, we hypothesize that MCSs provide a flexible platform for proteomic and nanometric co-regulation of Ca2+ release and uptake. As most MCSs are located deep inside cardiomyocytes at tubular membrane invaginations near mitochondria or the nucleus, we will further elucidate the function of protein-protein interactions at both MCS locations, particularly for novel phospholamban and RyR2 interacting proteins. This project will expand insight about the role of MCSs for local Ca2+ control and co-regulation of essential Ca2+ transport proteins and their significance for survival of non-regenerative electrically excitable cell types.