Mislocalized Cytosolic Polyphosphate Forms Potentially Toxic Droplets, Highlighting the Essentiality of Vacuolar Polyphosphate in Pi Homeostasis

ABSTRACT Inorganic polyphosphate is a phosphate polymer found in nearly all species examined to date. Over decades of research, this biopolymer has been revealed to participate in a broad range of biological phenomena, from phosphate storage to human diseases. Despite these advances, its essential physiological roles that are critical for cellular or organismal survival remain elusive. In the fission yeast Schizosaccharomyces pombe , the vacuolar transporter chaperone (VTC) complex synthesizes polyphosphate within the vacuolar lumen and supports cell viability synergistically with two additional phosphate regulators with the SPX domain, the phosphate sensor: Pqr1, restricting phosphate import, and Xpr1, a phosphate exporter. In this study, we show that the polyphosphate level increases in response to the elevated extracellular Pi concentration or severe phosphate stress upon the loss of both Pqr1 and Xpr1. The VTC complex may alleviate the lethal phosphate stress by polymerizing and sequestering excess phosphates into the vacuoles. Notably, artificial cytoplasmic synthesis of polyphosphate by the bacterial polyphosphate synthase PPK failed to compensate for the essential function of the VTC complex, forming cytoplasmic polyphosphate droplets via liquid–liquid phase separation, which are potentially cytotoxic. These findings highlight the critical importance of spatial regulation of polyphosphate synthesis for its physiological function.