The cell cortex is a dynamic and heterogeneous structure that governs cell surface properties and cell-cell interactions. ERM proteins are a group of highly conserved plasma membrane-cytoskeleton coupling proteins and major architects of the cell cortex. As such, ERM proteins have been implicated in numerous fundamental cellular processes such as cell adhesion; cortical morphogenesis; bacterial and viral infection; cancer invasion and metastasis.
Three distinct ERM proteins (ezrin, radixin and moesin) are highly conserved throughout vertebrate evolution along with a single variant of the related tumour suppressor protein NF2, or merlin. ERM proteins consist of three domains: i) a globular N-terminal FERM domain, which contains the PIP2 lipid binding sites; ii) a flexible α-helical coiled coil region; and iii) a C-terminal domain, which contains an F-actin binding domain and conserved phosphorylation site. Recent studies have uncovered surprisingly dynamic and complex molecular activities of the ERM proteins. ERM proteins and merlin can associate via the FERM and C-terminal domains through both intra- and inter-molecular interactions. However, these interactions mask both the membrane and actin binding sites. Hence, ERM activation must involve an unfolding event to allow the protein to bind to the plasma membrane either directly, or indirectly through linker proteins. However, the factors controlling ERM activation and the mode of ERM-membrane binding are unclear.
We are working towards understanding the ERM-membrane architecture and, in turn, the factors controlling ERM-membrane coupling. Using several recombinant ezrin and merlin constructs (wild-type, phosphomimetic and FERM domain) we have employed SEC-MALS, membrane co-sedimentation and single-molecule fluorescence to investigate ezrin-erzin, ezrin-merlin and ezrin-lipid interactions. Through understanding the various transitions between different ERM structures and their interaction with membranes, we will provide new mechanistic insight into how ERM proteins integrate and transmit signalling information to build and maintain the cell cortex.