Phosphorylation-independent interaction between 14-3-3 and exoenzyme S: from structure to pathogenesis

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Abstract
14‐3‐3 proteins are phosphoserine/phosphothreonine‐recognizing adapter proteins that regulate the activity of a vast array of targets. There are also examples of 14‐3‐3 proteins binding their targets via unphosphorylated motifs. Here we present a structural and biological investigation of the phosphorylation‐independent interaction between 14‐3‐3 and exoenzyme S (ExoS), an ADP‐ribosyltransferase toxin of Pseudomonas aeruginosa . ExoS binds to 14‐3‐3 in a novel binding mode mostly relying on hydrophobic contacts. The 1.5 Å crystal structure is supported by cytotoxicity analysis, which reveals that substitution of the corresponding hydrophobic residues significantly weakens the ability of ExoS to modify the endogenous targets RAS/RAP1 and to induce cell death. Furthermore, mutation of key residues within the ExoS binding site for 14‐3‐3 impairs virulence in a mouse pneumonia model. In conclusion, we show that ExoS binds 14‐3‐3 in a novel reversed orientation that is primarily dependent on hydrophobic residues. This interaction is phosphorylation independent and is required for the function of ExoS.