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.

中文翻译：

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推定的多胺转运蛋白 Shp2 以 Xpr1 非依赖性方式促进磷酸盐输出，并有助于高磷酸盐耐受性。


细胞水平的磷酸盐 （Pi） 稳态至关重要，需要协调 Pi 的摄取、储存和输出。然而，尽管它们与原发性家族性脑钙化等人类疾病相关，但监管机制，尤其是管理 Pi 出口的监管机制，仍然难以捉摸。虽然在真核生物中保守的 Xpr1 是唯一已知的 Pi 输出因子，但其他 Pi 输出因子的存在是显而易见的;然而，这些因素的表征并不明确。使用裂殖酵母 Schizosaccharomyces pombe 作为模型，我们旨在更好地了解细胞 Pi 稳态机制。之前，我们显示了三个具有 SPX 结构域的 Pi 调节因子是关键的：Pqr1（Pi 摄取限制因子）、Xpr1/Spx2 和 VTC 复合物（聚磷酸合酶）。SPX 结构域与肌醇焦磷酸盐结合，调节 Pi 调节功能。双突变体 Δpqr1Δxpr1 在高 Pi 条件下高度积累 Pi 并经历细胞死亡，表明 Pi 摄取限制和输出的必要性。值得注意的是，Δpqr1Δxpr1 表现出独立于 Xpr1 的残余 Pi 输出活性，表明存在未鉴定的 Pi 输出因子。为了揭示这些神秘的 Pi 输出因子和 Pi 稳态调节因子，我们对 Δpqr1Δxpr1 中的高 Pi 超敏反应进行了抑制因子筛选。在鉴定的 8 个抑制因子中，血浆膜蛋白 Shp2 以 Xpr1 非依赖性方式显示出 Pi 输出促进活性，支持高 Pi 下的细胞增殖。目前的结果为除已建立的 Xpr1 之外的 Pi 输出促进剂提供了第一个证据，这在真核生物中是前所未有的。由于 Shp2 与出芽酵母 Tpo1（一种亚精胺/多胺转运蛋白）同源，因此可以推测 Pi 稳态与多胺代谢之间的潜在联系。