The T3SS chaperone CesT is recently shown to interact with the post-transcriptional regulator CsrA to modulate post-attachment signaling in enteropathogenic and enterohemorrhagic Escherichia coli. The molecular basis of the CesT/CsrA binding, however, remains elusive. Here, we show that CesT and CsrA both created two ligand binding sites in their homodimers, forming irregular multimeric complexes in solution. Through construction of a recombinant CsrA-dimer (Re-CsrA) that contains a single CesT binding site, the atomic binding features between CesT and CsrA are delineated via the structure of the CesT/Re-CsrA complex. In contrast to a previously reported N-terminally swapped dimer-form, CesT adopts a dimeric architecture with a swapped C-terminal helix for CsrA engagement. In CsrA, CesT binds to a surface patch that extensively overlaps with its mRNA binding site. The binding mode therefore justifies a mechanism of CsrA-modulation by CesT via competitive inhibition of the CsrA/mRNA interactions.

中文翻译：

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全局转录后调控因子CsrA和T3SS伴侣CesT之间结合的分子基础。

最近显示，T3SS伴侣CesT与转录后调节因子CsrA相互作用，可调节肠致病性和肠出血性大肠杆菌中的附着后信号。然而，CesT / CsrA结合的分子基础仍然难以捉摸。在这里，我们显示CesT和CsrA都在其同二聚体中创建了两个配体结合位点，在溶液中形成了不规则的多聚体复合物。通过构建包含单个CesT结合位点的重组CsrA-二聚体（Re-CsrA），可通过CesT / Re-CsrA复合物的结构描绘出CesT和CsrA之间的原子结合特征。与先前报道的N末端交换的二聚体形式相反，CesT采用具有C末端交换的C末端螺旋的二聚体结构。在CsrA中，CesT结合与其mRNA结合位点广泛重叠的表面补丁。因此，结合模式通过竞争抑制CsrA / mRNA相互作用来证明CesT调节CsrA的机制。