Phosphate, a key plant nutrient, is perceived through inositol polyphosphates (InsPs) by SPX domain-containing proteins. SPX1 an inhibit the PHR2 transcription factor to maintain Pi homeostasis. How SPX1 recognizes an InsP molecule and represses transcription activation by PHR2 remains unclear. Here we show that, upon binding InsP₆, SPX1 can disrupt PHR2 dimers and form a 1:1 SPX1-PHR2 complex. The complex structure reveals that SPX1 helix α1 can impose a steric hindrance when interacting with the PHR2 dimer. By stabilizing helix α1, InsP₆ allosterically decouples the PHR2 dimer and stabilizes the SPX1-PHR2 interaction. In doing so, InsP₆ further allows SPX1 to engage with the PHR2 MYB domain and sterically block its interaction with DNA. Taken together, our results suggest that, upon sensing the surrogate signals of phosphate, SPX1 inhibits PHR2 via a dual mechanism that attenuates dimerization and DNA binding activities of PHR2.

磷酸盐是一种关键的植物营养素，通过含有 SPX 结构域的蛋白质通过肌醇多磷酸盐 (InsPs) 感知。SPX1 抑制 PHR2 转录因子以维持 Pi 稳态。SPX1 如何识别 InsP 分子并抑制 PHR2 的转录激活仍不清楚。在这里我们表明，在结合 InsP _{6 后}，SPX1 可以破坏 PHR2 二聚体并形成 1:1 SPX1-PHR2 复合物。复杂的结构表明 SPX1 螺旋 α1 在与 PHR2 二聚体相互作用时可以施加空间位阻。通过稳定螺旋 α1，InsP ₆变构解耦 PHR2 二聚体并稳定 SPX1-PHR2 相互作用。这样做时，InsP ₆进一步允许 SPX1 与 PHR2 MYB 结构域结合并在空间上阻断其与 DNA 的相互作用。总之，我们的结果表明，在感知磷酸盐的替代信号后，SPX1 通过双重机制抑制 PHR2，减弱 PHR2 的二聚化和 DNA 结合活性。