Acidocalcisome-like vacuoles constitute a feedback-controlled phosphate buffering system for the cytosol

Cells experience strong variations in the consumption and availability of inorganic phosphate (P_i). Since P_i is an essential macronutrient but excess P_i has negative impacts on nucleotide hydrolysis and metabolism, its concentration must be maintained in a suitable range. Conserved storage organelles, acidocalcisomes, provide this buffering function. We used acidocalcisome-like yeast vacuoles to study how such organelles are set up to perform this task. Our combined in vitro and in vivo analyses revealed that their ATP-driven polyphosphate polymerase VTC converts cytosolic P_i into inorganic polyphosphates (polyP), which it transfers into the vacuole lumen. Luminal polyphosphatases immediately hydrolyse this polyP to establish a growing reservoir of vacuolar P_i. Product inhibition by this P_i pool silences the polyphosphatases, caps P_i accumulation, and favours vacuolar polyP storage. Upon cytosolic P_i scarcity, the declining inositol pyrophosphate levels activate the vacuolar P_i exporter Pho91 to replenish cytosolic P_i. In this way, acidocalcisome-like vacuoles constitute a feedback-regulated buffering system for cytosolic P_i, which the cells can switch between P_i accumulation, P_i release, and high-capacity phosphate storage through polyP.