FERM domains recruit ample PI(4,5)Ps to form extensive protein-membrane attachments.

The four-point-one ezrin-radixin-moesin homology (FERM) protein domain is a multifunctional protein-lipid binding site, constituting an integral part of numerous membrane-associated proteins. Its interaction with the lipid phosphatidylinositol-4,5-bisphosphate (PIP), located at the inner leaflet of eukaryotic plasma membranes, is important for localization, anchorage, and activation of FERM-containing proteins. FERM-PIP complexes structurally determined so far exclusively feature a 1:1 binding stoichiometry of protein and lipid, with a few basic FERM residues neutralizing the -4 charge of the bound PIP. Whether this picture from static crystal structures also applies to the dynamic interaction of FERM domains on PIP membranes is unknown. We here quantified the stoichiometry of FERM-PIP binding in a lipid bilayer using atomistic molecular dynamics simulations and experiments on solid supported membranes for the FERM domains of focal adhesion kinase and ezrin. In contrast to the structural data, we find much higher average stoichiometries of FERM-PIP binding, amounting to 1:3 or 1:4 ratios, respectively. In simulations, the full set of basic residues at the membrane interface, 7 and 15 residues for focal adhesion kinase and ezrin, respectively, engages in PIP interactions. In addition, Na ions enter the FERM-membrane binding interface, compensating negative PIP charges in case of high charge surpluses from bound PIP. We propose the multivalent binding of FERM domains to PIP in lipid bilayers to significantly enhance the stability of FERM-membrane binding and to render the FERM-membrane linkage highly adjustable.