8.A.25 The Ezrin/Radixin/Moesin (Ezrin) Family

Ezrin (cytovillin, villin-2, band 41 homologue, p81; 586 aas) binds to the PDZ domains in EBP50 (NHERF; 8.A.24) which organizes a number of receptors and channels. It contains a FERM domain (residues 200-290) that binds to PDZ domains in many proteins. The binding of ezrin to EBP50 induces major changes in CFTR (3.A.1.202.1) (Li et al., 2005). It provides connections to the cytoskeleton near the plasma membrane. It is a soluble protein that is primarily localized in the cell, for example, to the apical membrane of parietal cells in animals, dependent on other proteins. It is phosphorylated by tyrosine kinases. There are hundreds of homologues in the family including the neurofibromatosis-2 protein, myosin-like proteins and the tegumental antigen. It shows sequence similarity to domains in tyrosine protein-P phosphatases.  There are four 4.1 proteins: 4.1R, 4.1N, 4.1G and 4.1B.  These proteins promote organization of many channel proteins in the cytoplasmic membrane (Baines et al. 2009).

Proteins of the 4.1 family, homologous to ezrin throughout most of their lengths, are characteristic of eumetazoan organisms (Baines et al. 2014). Invertebrates contain single 4.1 genes and the Drosophila model suggests that 4.1 is essential for animal life. Vertebrates have four paralogues, known as 4.1R, 4.1N, 4.1G and 4.1B, which are additionally duplicated in the ray- finned fish. Protein 4.1R was the first to be discovered: it is a major mammalian erythrocyte cytoskeletal protein, essential to the mechanochemical properties of red cell membranes because it promotes the interaction between spectrin and actin in the membrane cytoskeleton. 4.1R also binds certain phospholipids and is required for the stable cell surface accumulation of a number of erythrocyte transmembrane proteins that span multiple functional classes; these include cell adhesion molecules, transporters and a chemokine receptor. The vertebrate 4.1 proteins are expressed in most tissues, and they are required for the correct cell surface accumulation of a very wide variety of membrane proteins including G-Protein coupled receptors, voltage-gated and ligand-gated channels, as well as the classes identified in erythrocytes. Indeed, such large numbers of protein interactions have been mapped for mammalian 4.1 proteins, most especially 4.1R, that it appears that they can act as hubs for membrane protein organization. The range of critical interactions of 4.1 proteins is reflected in disease relationships that include hereditary anaemias, tumour suppression, control of heartbeat and nervous system function. The 4.1 proteins are defined by their domain structure: apart from the spectrin/actin-binding domain they have FERM and FERM-adjacent domains and a unique C-terminal domain. Both the FERM and C-terminal domains can bind transmembrane proteins. The spectrum of interactions of the 4.1 proteins overlaps with that of another membrane-cytoskeleton linker, ankyrin (Baines et al. 2014).

The Ezrin/Radixin/Moesin (ERM) family of proteins act as cross-linkers between the plasma membrane and the actin cytoskeleton (Strandberg et al. 2024). This mechanism plays an essential role in processes related to membrane remodeling and organization, such as cell polarization, morphogenesis and adhesion, as well as in membrane protein trafficking and signaling pathways. For several human aquaporin (AQP) isoforms, an interaction between the ezrin band Four-point-one, Ezrin, Radixin, Moesin (FERM)-domain and the AQP C-terminus has been demonstrated, and this is believed to be important for AQP localization in the plasma membrane. Strandberg et al. 2024 investigated the structural basis for the interaction between ezrin and two human AQPs: AQP2 and AQP5. Full-length AQP2 and AQP5 as well as peptides corresponding to their C-termini interact with the ezrin FERM-domain with affinities in the low micromolar range. Modelling of the AQP2 and AQP5 FERM complexes using ColabFold reveals a common mode of binding in which the proximal and distal parts of the AQP C-termini bind simultaneously to distinct binding sites of FERM. While the interaction at each site closely resembles other FERM-complexes, the concurrent interaction with both sites has only been observed in the complex between moesin and its C-terminus which causes auto-inhibition. The proposed interaction between AQP2/AQP5 and FERM thus represents a novel binding mode for extrinsic ERM-interacting partners (Strandberg et al. 2024).