Auxin is a plant hormone that is central to plant growth and development from embryogenesis to senescence. Auxin signaling is mediated by auxin response transcription factors (ARFs) and Aux/IAA repressors that regulate the expression of a multitude of auxin response genes. ARF and Aux/IAA proteins assemble into homomeric and heteromeric complexes via their conserved PB1 domains. Here we report the first crystal structure of the PB1 complex between ARF5 and IAA17 of Arabidopsis thaliana, which represents the transcriptionally repressed state at low auxin levels. The PB1 domains assemble in a head-to-tail manner with a backbone arrangement similar to that of the ARF5:ARF5 PB1 complex. The ARF5:IAA17 complex, however, reveals distinct points of contact that promote the ARF5:IAA17 interaction over the ARF5:ARF5 interaction. Specifically, surface charges at the interface form salt-bridges that distinguish the homomeric and heteromeric complexes, revealing common and specific interfaces between transcriptionally repressed and derepressed states. Further, the salt-bridges can be reconfigured to switch the affinity between homomeric and heteromeric complexes in an incremental manner. The complex structure combined with quantitative binding analyses would be essential for deciphering the PB1 interaction code underlying the transcriptional regulation of auxin signaling.

生长素是一种植物激素，对于植物从胚发生到衰老的生长发育至关重要。生长素信号转导是由生长素应答转录因子（ARF）和Aux / IAA阻遏物调节的，该调节因子调节多种生长素应答基因的表达。ARF和Aux / IAA蛋白通过其保守的PB1域组装成同聚和异聚复合物。在这里，我们报告拟南芥ARF5和IAA17之间的PB1复合物的第一个晶体结构。，代表在低生长素水平下的转录抑制状态。PB1域以与ARF5：ARF5 PB1复合体相似的主干结构以头尾方式组装。然而，ARF5：IAA17复合体显示出比ARF5：ARF5相互作用更促进ARF5：IAA17相互作用的不同接触点。具体而言，界面处的表面电荷形成盐桥，可区分同聚和异聚复合物，从而揭示出转录抑制状态和去抑制状态之间的公共和特定接口。此外，可以将盐桥重新配置为以递增方式在同聚和异聚复合物之间切换亲和力。