Dictyostelium discoideum is a soil-dwelling unicellular eukaryote that accumulates extracellular
polyphosphate (
polyP). At high cell densities, when the cells are about to overgrow their food supply and starve, the corresponding high extracellular concentrations of
polyP allow the cells to preemptively anticipate
starvation, inhibit proliferation, and prime themselves to begin development. In this report, we show that starved D. discoideum cells accumulate cell surface and extracellular
polyP.
Starvation reduces macropinocytosis, exocytosis, and phagocytosis, and we find that these effects require the
G protein-coupled
polyP receptor (GrlD) and two
enzymes,
Polyphosphate kinase 1 (Ppk1), which is required for synthesizing intracellular
polyP, cell surface
polyP, and some of the extracellular
polyP, and
Inositol hexakisphosphate kinase (I6kA), which is required for cell surface
polyP and
polyP binding to cells, and some of the extracellular
polyP.
PolyP reduces membrane fluidity, and we find that
starvation reduces membrane fluidity, and this effect requires GrlD and Ppk1 but not I6kA. Together, these data suggest that in starved cells, extracellular
polyP decreases membrane fluidity, possibly as a protective measure. In the starved cells, sensing
polyP appears to decrease energy expenditure from ingestion, and decrease exocytosis, to both decrease energy expenditures and retain nutrients.