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.