Komamura Tochi, Nishimura Tomoki, Ohta Naoki, Takado Masahiro, Matsumoto Tomohiro, Takeda Kojiro
Faculty of Science and Engineering, Department of Biology, Konan University, Kobe, Japan.
Radiation Biology Center, Graduate School of Biostudies, Kyoto University, Kyoto, Japan.
J Biol Chem. 2025 Jan;301(1):108056. doi: 10.1016/j.jbc.2024.108056. Epub 2024 Dec 9.
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.
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，尽管其与原发性家族性脑钙化等人类疾病相关，但调节机制，尤其是那些控制Pi输出的机制，仍然难以捉摸。虽然Xpr1在真核生物中保守，是唯一已知的Pi输出蛋白，但显然存在其他Pi输出因子；然而，这些因子的特征描述很差。我们以裂殖酵母粟酒裂殖酵母为模型，旨在更好地理解细胞Pi稳态机制。此前，我们发现三个具有SPX结构域的Pi调节因子至关重要：Pqr1（Pi摄取限制因子）、Xpr1/Spx2和VTC复合物（多聚磷酸合酶）。SPX结构域与肌醇焦磷酸结合，调节Pi调节因子的功能。双突变体Δpqr1Δxpr1在高Pi条件下会过度积累Pi并发生细胞死亡，这表明Pi摄取限制和输出都很有必要。值得注意的是，Δpqr1Δxpr1表现出独立于Xpr1的残余Pi输出活性，这表明存在未鉴定的Pi输出蛋白。为了揭示这些神秘的Pi输出蛋白和Pi稳态调节因子，我们对Δpqr1Δxpr1中的高Pi超敏反应进行了抑制子筛选。在鉴定出的八个抑制子中，一种质膜蛋白Shp2以独立于Xpr1的方式表现出促进Pi输出的活性，在高Pi条件下支持细胞增殖。目前的结果首次证明了除已确定的Xpr1之外的Pi输出促进因子，这在真核生物中是前所未有的。由于Shp2与芽殖酵母Tpo1（一种亚精胺/多胺转运蛋白）是直系同源物，因此可以推测Pi稳态与多胺代谢之间存在潜在联系。
