Role of the N-terminal domain of the chaperone ClpX in the recognition and degradation of lambda phage protein O

The ClpXP ATPase - protease complex is a key element of the protein quality control machinery in the cell. ClpX consists of a zinc-binding domain (ZBD) that forms dimers and a AAA⁺ domain that arranges into a hexamer in an ATP-dependent manner. Here, we report the binding site of the ClpX substrate λ phage protein O (λO) on ZBD₂ in ClpX using NMR and mutagenesis analysis. λO protein was found to interact with a hydrophobic patch on the larger surface of ZBD₂. The affinity of λO toward ZBD₂ was investigated using a quantitative optical biosensor method of dual polarization interferometry. The data suggest overlapping binding sites of λO and the ClpX cofactor SspB on the ZBD₂. Interestingly, a single key mutation in ZBD was found to enhance the ClpXP-dependent degradation of λO.