The HslVU complex is a bacterial two-component ATP-dependent protease, consisting of HslU chaperone and HslV peptidase. Investigation of protein-protein interactions using SPR in Escherichia coli HslVU and the protein substrates demonstrates that HslU and HslV have moderate affinity (Kd = 1 μM) for each other. However, the affinity of HslU for HslV fivefold increased (Kd ∼ 0.2 μM) after binding with the MBP-SulA protein indicating the formation of a "ternary complex" of HslV-HslU-MBP-SulA. The molecular interaction studies also revealed that HslU strongly binds to MBP-SulA with 10-9 M affinity but does not associate with nonstructured casein. Conversely, HslV does not interact with the MBP-SulA whereas it strongly binds with casein (Kd = 0.2 μM) requiring an intact active site of HslV. These findings provide evidence for "substrate-induced" stable HslVU complex formation. Presumably, the binding of HslU to MBP-SulA stimulates a conformational change in HslU to a high-affinity form for HslV.
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