| Abstract |
Phosphate (Pi) homeostasis at the cellular level is crucial, requiring coordinated Pi uptake, storage, and export. However, the regulatory mechanisms, particularly those governing Pi export, remain elusive, despite their relevance to human diseases like primary familial brain calcification. While Xpr1, conserved across eukaryotes, is the only known Pi exporter, the existence of additional Pi exporting factors is evident; however, these factors have been poorly characterized. Using the fission yeast Schizosaccharomyces pombe as a model, we have aimed to better understand cellular Pi homeostasis mechanisms. Previously, we showed three Pi regulators with SPX domains to be critical: Pqr1 (Pi uptake restrictor), Xpr1/Spx2, and the VTC complex (polyphosphate synthase). SPX domains bind to inositol pyrophosphate, modulating Pi regulator functions. The double mutant Δpqr1Δxpr1 hyper-accumulates Pi and undergoes cell death under high Pi conditions, indicating the necessity of both Pi uptake restriction and export. Notably, Δpqr1Δxpr1 exhibits residual Pi export activity independent of Xpr1, suggesting the presence of unidentified Pi exporters. To uncover these cryptic Pi exporters and regulators of Pi homeostasis, we conducted suppressor screening for high Pi hypersensitivity in Δpqr1Δxpr1. Among the eight suppressors identified, Shp2, a plasma-membrane protein, showed Pi export-facilitating activity in an Xpr1-independent manner, supporting cell proliferation at high Pi. The present results provide the first evidence for Pi export facilitator other than the established Xpr1, unprecedented in eukaryotes. As Shp2 is orthologous to the budding yeast Tpo1, a spermidine/polyamine transporter, a potential link between Pi homeostasis and polyamine metabolism can be speculated.
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