FERM domains recruit ample PI(4,5)P2s 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 (PIP2), located at the inner leaflet of eukaryotic plasma membranes, is important for local-ization, anchorage, and activation of FERM-containing proteins. FERM-PIP2 complexes structurally determined so far exclu-sively feature a 1:1 binding stoichiometry of protein and lipid, with a few basic FERM residues neutralizing the-4 charge of the bound PIP2. Whether this picture from static crystal structures also applies to the dynamic interaction of FERM domains on PIP2 membranes is unknown. We here quantified the stoichiometry of FERM-PIP2 binding in a lipid bilayer using atomistic molecular dynamics simulations and experiments on solid supported membranes for the FERM domains of focal adhesion ki-nase and ezrin. In contrast to the structural data, we find much higher average stoichiometries of FERM-PIP2 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 PIP2 interactions. In addition, Na ions enter the FERM-membrane binding interface, compensating negative PIP2 charges in case of high charge surpluses from bound PIP2. We propose the multi-valent binding of FERM domains to PIP2 in lipid bilayers to significantly enhance the stability of FERM-membrane binding and to render the FERM-membrane linkage highly adjustable.

Automated summary

The four-point-one ezrin-radixin-moesin homology (FERM) protein domain is a multifunctional protein-lipid binding site, important for localization and activation of membrane-associated proteins. In contrast to static crystal structures, atomistic molecular dynamics simulations and experiments reveal higher stoichiometry of FERM-PIP2 binding, with ratios of 1:3 or 1:4, indicating a multi-valent binding of FERM domains to PIP2 in lipid bilayers.