Nature2014-06-24 5:34 PM

多功能的硝酸盐转运机制2 Crystal structure of the plant dual-affinity nitrate transporter NRT1.1

论文摘要 

土壤中的硝酸盐(植物生长的一种主要营养物)水平可以相差巨大。因此植物需要一个多功能机制来从环境中获取硝酸盐。在模型植物拟南芥中,双亲和性转运分子NRT1.1能吸收一系列不同浓度的硝酸盐,根据一个关键的苏氨酸残基的磷酸化状态从低亲和性模式向高亲和性模式切换。本期Nature上发表的两项研究描述了全长度NRT1.1的晶体结构,为了解这种转录后修饰怎样在低亲和性和高亲和性状态之间切换该转运分子提供了信息。

Abstract 

Nitrate is a primary nutrient for plant growth, but its levels in soil can fluctuate by several orders of magnitude. Previous studies have identified Arabidopsis NRT1.1 as a dual-affinity nitrate transporter that can take up nitrate over a wide range of concentrations. The mode of action of NRT1.1 is controlled by phosphorylation of a key residue, Thr 101; however, how this post-translational modification switches the transporter between two affinity states remains unclear. Here we report the crystal structure of unphosphorylated NRT1.1, which reveals an unexpected homodimer in the inward-facing conformation. In this low-affinity state, the Thr 101 phosphorylation site is embedded in a pocket immediately adjacent to the dimer interface, linking the phosphorylation status of the transporter to its oligomeric state. Using a cell-based fluorescence resonance energy transfer assay, we show that functional NRT1.1 dimerizes in the cell membrane and that the phosphomimetic mutation of Thr 101 converts the protein into a monophasic high-affinity transporter by structurally decoupling the dimer. Together with analyses of the substrate transport tunnel, our results establish a phosphorylation-controlled dimerization switch that allows NRT1.1 to uptake nitrate with two distinct affinity modes.

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