The plasma membrane calcium pump (PMCA) is the sole Ca 2+ extrusion system in many cells that is responsible for maintaining the low cytosolic calcium concentration critical to cell function. The role of this protein is to eject Ca 2+ from the cytosol to the extra cellular space using the energy of ATP. The carboxyl terminus of PMCA contains a high affinity calmodulin-binding sequence that interacts with the catalytic core and inhibits the activity of the pump in resting cells. In activated cells binding of Ca 2+-calmodulin to the calmodulin-binding sequence frees the catalytic core from the auto-inhibition and the pump becomes activated. In addition to the changes between the activated and inhibited states, the conformation of the pump also changes between the high (E1) and low (E2) Ca 2+ affinity states during the reaction cycle. In this study we utilized limited proteolysis and site directed mutagenesis to monitor the regions involved in the auto-inhibitor
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