背景回顾：Excess phosphate (Pi) is stored into the vacuole through Pi transporters so that cytoplasmic Pi levels remain stable in plant cells. 

提出假设：We hypothesized that the vacuolar Pi transporters may harbor a Pi-sensing mechanism so that they are activated to deliver Pi into the vacuole only when cytosolic Pi reaches a threshold high level. 

验证假设：We tested this hypothesis using Vacuolar Phosphate Transporter 1 (VPT1), a SPX domain-containing vacuolar Pi transporter. Recent studies defined SPX as a Pi-sensing module that binds inositol polyphosphate signaling molecules (InsPs) produced at high cellular Pi status.

结果：We showed here that Pi deficient conditions or mutation of SPX domain severely impaired the transport activity of VPT1. We further identified an auto-inhibitory domain in VPT1 that suppresses its transport activity. 

结论：Taking together, results from detailed structure-function analyses, we propose that VPT1 is in the autoinhibitory state when Pi status is low and InsPs produced at high cellular Pi status binds SPX domain to switch on VPT1 activity to deliver Pi into the vacuole. 

总结：This thus provides an auto-regulatory mechanism for VPT1-mediated Pi sensing and homeostasis in plant cells.

 摘 要 

植物会将体内多余的磷通过磷转运蛋白储存进液泡，从而使得胞质中磷含量水平保持稳定。作者假设液泡磷转运蛋白可能存在一个磷感知机制，从而确保只有在胞质磷含量达到一定水平后才会激活液泡磷转运蛋白将多余的磷转运进液泡。本文中，作者利用一个含有SPX结构域的液泡磷转运蛋白VPT1测试了该假设。最近的研究发现，SPX是一个磷感知模块，能够结合多磷酸肌醇信号分子InsPs，这些信号分子只有在磷含量较高的细胞中产生。作者发现，缺磷条件或者SPX结构域突变情况下，会导致VPT1的转运活性大大地降低。作者进一步发现，VPT1蛋白上存在一个自抑制域，作用于自身转运活性的抑制。综上，作者认为细胞内磷含量较低时，VPT1处于一个自抑制的状态；但是，在胞内磷含量很高时，会刺激InsPs的产生，从而结合VPT1蛋白的SPX结构域，最终激活了VPT1蛋白的转运活性，将胞质中的磷转运进液泡。因此，本文的研究揭示了一个由VPT1介导的磷感知自调控机制，作用于植物细胞内的磷稳态。

 Sheng Luan