Thermodynamic dissection of the Ezrin FERM/CERMAD interface

ERM (Ezrin-Radixin-Moesin) proteins are key cross-linkers of the plasma membrane and the actin cytoskeleton. They are regulated by the intramolecular association of the N-terminal FERM (band-four point one, Ezrin, Radixin, Moesin) and C-terminal CERMAD (ERM association domain) domains (N/C interaction), which masks the binding surfaces of the domains for other molecules. The N/C interface is characterized by the highly distributed binding of CERMAD through a beta-strand and four alpha-helices to a globular FERM. Though it is a target for multiple regulatory signals, little is known about the dynamics/thermodynamics governing this interface. Recent implications of Ezrin in cancer metastasis have increased the necessity to understand this regulatory switch. In this study, we report residue-specific stabilities of Ezrin CERMAD at the Ezrin N/C interface obtained using hydrogen-deuterium exchange NMR. These stabilities vary across secondary structural elements and identify F583 and L586 as key anchor residues for the most stable element, alphaD. Macroscopic N/C binding energetics, obtained using isothermal titration calorimetry (ITC) reveals a high affinity (Kd =176 nM) enthalpy-driven binding (DeltaH = -26 kcal/mol, TDeltaS = -17 kcal/mol) at 25 degrees C at pH 7 in MES and phosphate buffers. A 10-fold increase in affinity was observed for measurements in acetate buffer, suggesting that an acetate-like molecule might promote the repressed form of the complex, possibly through interaction with the F2 subdomain of FERM, which resembles the acyl-CoA binding protein. In summary, our results have illustrated the dynamic nature of this regulatory interface and provide a foundation for investigating the role of regulatory signals on the stability of this interface.