Open Conformation of Ezrin Bound to Phosphatidylinositol 4,5-Bisphosphate and to F-actin Revealed by Neutron Scattering*

Background: The structure of activated ezrin is not known. Results: We have determined the conformation of activated ezrin upon binding to PIP2 and to F-actin. Conclusion: Activated ezrin forms more extensive contacts with F-actin than generally depicted. Significance: This study provides new insight into the mechanisms by which ezrin assembles signaling complexes at the membrane-cytoskeleton interface. Ezrin is a member of the ezrin-radixin-moesin family (ERM) of adapter proteins that are localized at the interface between the cell membrane and the cortical actin cytoskeleton, and they regulate a variety of cellular functions. The structure representing a dormant and closed conformation of an ERM protein has previously been determined by x-ray crystallography. Here, using contrast variation small angle neutron scattering, we reveal the structural changes of the full-length ezrin upon binding to the signaling lipid phosphatidylinositol 4,5-bisphosphate (PIP2) and to F-actin. Ezrin binding to F-actin requires the simultaneous binding of ezrin to PIP2. Once bound to F-actin, the opened ezrin forms more extensive contacts with F-actin than generally depicted, suggesting a possible role of ezrin in regulating the interfacial structure and dynamics between the cell membrane and the underlying actin cytoskeleton. In addition, using gel filtration, we find that the conformational opening of ezrin in response to PIP2 binding is cooperative, but the cooperativity is disrupted by a phospho-mimic mutation S249D in the 4.1-ezrin/radixin/moesin (FERM) domain of ezrin. Using surface plasmon resonance, we show that the S249D mutation weakens the binding affinity and changes the kinetics of 4.1-ERM to PIP2 binding. The study provides the first structural view of the activated ezrin bound to PIP2 and to F-actin.