Mitochondrial free calcium is critically linked to the regulation of bioenergetics and cellular signaling. Free calcium concentrations within the organelle are regulated by two processes: flux across the mitochondrial inner membranes and buffering by phosphate. The key role of phosphate in the buffering of free calcium in mitochondria is well-established. Specifically, during stimulated calcium uptake, calcium is partially buffered by orthophosphate, allowing for elevated calcium

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... while preventing calcium toxicity. However, this buffering system is expected to lead to the irreversible formation of insoluble precipitates, which are not observed in living cells, under physiological conditions. Here, we demonstrate that the regulation of free mitochondrial calcium requires the presence of free inorganic polyphosphate (polyP) within the organelle. Specifically, we found that the verexpression of a mitochondrial targeted enzyme hydrolyzing polyP, leads to the loss of the cellular ability to maintain elevated calcium concentrations within the organelle, following stimulated cytoplasmic signal. We hypothesize that the presence of polyP prevents the formation of calciumphosphate insoluble clusters, allowing for the maintenance of elevated free calcium levels, during stimulated calcium uptake.