Molecular basis of binding between the global post-transcriptional regulator CsrA and the T3SS chaperone CesT.

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.