On the Mechanism of Ezrin Activation
Ezrin is a peripheral membrane protein that contributes to the organization and stability of cellular membrane structures by reversibly linking the plasma membrane to actin filaments. The formation of this membrane-actin linkage has been experimentally shown to require ezrin N-terminal (FERM) domain binding to PI(4,5)P2 phospholipid-enriched membrane sites and the phosphorylation of the ezrin C-terminal domain (CTD) at residue T567. Collectively, membrane association and T567 phosphorylation are believed to promote separation of the FERM and CTD domains; however, the underlying molecular mechanism remains less clear. In this study, we investigate the mechanistic steps of ezrin activation and the thermodynamic free energy landscape of FERM-CTD dissociation using enhanced sampling molecular dynamics (MD). We find that upon ezrin attachment to a lipid membrane, PI(4,5)P2 molecules outcompete other phospholipids at the surface of the FERM F1 and F3 subdomains. This interaction triggers a major conformational rearrangement within the FERM domain that destabilizes the FERM F2-CTD interface and initiates dissociation between the FERM and CTD. By employing well-tempered metadynamics (WTMetaD) with a contact-map collective variable, we determine that the principal barrier to FERM-CTD dissociation comes from F3-CTD interactions and that this dissociation can happen spontaneously with a moderate free energy barrier. We also show that the FERM-CTD reassociation after ezrin T567 phosphorylation is impeded due to reduced dissociation energy barrier. The free energy profile of dissociation between FERM and the CTD-replacing EBP50 protein is similar to that of the FERM-CTD system with nonphosphorylated T567, which agrees well with an in vivo experimental observation that EBP50 competes with the CTD for F2-F3 binding after CTD is dissociated. Together, our results help establish a revised view on the ezrin activation mechanism where FERM binding to PI(4,5)P2 enables spontaneous dissociation of the nonphosphorylated CTD. SIGNIFICANCEEzrin and related ERM proteins control how cells link their plasma membrane to the actin cytoskeleton, a process fundamental to cell shape, signaling and motility. Despite decades of study, the molecular basis of ezrin activation - how it transitions from a self-inhibited to an active membrane-bound state - has remained unresolved. Using atomistic and enhanced sampling molecular dynamics together with biochemical validation, we show that binding of the FERM domain to PI(4,5)P2-enriched membranes alone is sufficient to trigger spontaneous dissociation of the nonphosphorylated C-terminal domain. Phosphorylation of T567 subsequently stabilizes the open conformation and prevents domain reassociation, enabling actin engagement and binding of FERM partners such as EBP50. Collectively, these findings advance a more integrated view of ezrin activation, highlighting how membrane interactions, conformational flexibility and phosphorylation act in concert to regulate membrane-cytoskeleton coupling.