Association and dissociation kinetics of colicin E3 and immunity protein 3: Convergence of theory and experiment

The rapid binding of cytotoxic colicin E3 by its cognate immunity protein Im3 is essential in safeguarding the producing cell. The X‐ray structure of the E3/Im3 complex shows that the Im3 molecule interfaces with both the C‐terminal ribonuclease (RNase) domain and the N‐terminal translocation domain of E3. The association and dissociation rates of the RNase domain and Im3 show drastically different sensitivities to ionic strength, as previously rationalized for electrostatically enhanced diffusion‐limited protein–protein associations. Relative to binding to the RNase domain, binding to full‐length E3 shows a comparable association rate but a significantly lower dissociation rate. This outcome is just what was anticipated by a theory for the binding of two linked domains to a protein. The E3/Im3 system thus provides a powerful paradigm for the interplay of theory and experiment.

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