|Abstract:Five or more orthophosphates bound together by high energy phospho-anhydride bonds are highly ubiquitous inorganic molecules called polyphosphate. Polyphosphate acts as a signaling molecule eliciting a number of responses in eukaryotic cells, but the mechanisms mediating these effects are poorly understood. Proliferating Dictyostelium discoideum cells accumulate extracellular polyphosphate. At extracellular concentrations similar to those observed in stationary phase cells, polyphosphate inhibits proteasome activity and proliferation, and induces aggregation. Here we identify GrlD as a putative G protein-coupled receptor that mediates binding of extracellular polyphosphate to the cell surface. Cells lacking GrlD do not respond to polyphosphate-induced proteasome inhibition, aggregation, or proliferation inhibition. Polyphosphate also elicits differential effects on cell-substratum adhesion and cytoskeletal F-actin levels based on nutrient availability, and these effects were also mediated by GrlD. Starving cells also accumulate extracellular polyphosphate. Starved cells treated with exopolyphosphatase failed to aggregate effectively, suggesting that polyphosphate also acts as a signaling molecule during starvation-induced development of Dictyostelium. Together, these results suggest that a eukaryotic cell uses a G protein-coupled receptor to mediate the sensing and response to extracellular polyphosphate.