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).
References:
Ezrin of 586 aas and 0 TMSs. It is probably involved in connections of major cytoskeletal structures to the plasma membrane. In epithelial cells, it is required for the formation of microvilli and membrane ruffles on the apical pole. Along with PLEKHG6, it is required for normal macropinocytosis. A head-to-tail association, of the N-terminal and C-terminal halves, results in a closed conformation (inactive form) which is incapable of actin or membrane-binding (Matsui et al. 1998).
Animals
Ezrin of Mus musculus (P26040)
The 4.1R (Band 4.1) protein of 864 aas. Interacts with many membrane transporters, adhesins and receptors as well as cytoskelectal proteins to help organize the cell envelope (Baines et al. 2014). For example, it interacts with the PMCA1b ATPase (TC# 3.A.3.2.25) (Diaz de Barboza et al. 2015). It affects photodynamic therapy for B16 melanoma by regulating the transport of 5-aminolevulinic acid (Li et al. 2021). This protein forms a complex with TMEM24 (TC# 8.A.78.1.1) and C2CD2 (TC# 8.A.78.1.2) (Johnson et al. 2024).
Animals
The 4.1R protein of Homo sapiens
Merlin, Mer, EMR2, of 635 aas and 1 TMS. Mer acts synergistically along with Ex and Kibra to regulate the Hippo signaling pathway (Yu et al. 2010). Merlin and Expanded (Q07436; 1,427 aas) function synergistically to control proliferation by regulating the abundance, localization, and turnover of cell-surface receptors (Maitra et al. 2006).
Merlin of Drosophila melanogaster (Fruit fly)
Moesin, Moe, EMR1 of 578 aas and 1 TMS. Moesin is involved in connections of major cytoskeletal structures to the plasma membrane (McCartney and Fehon 1996). It plays a role in nuclear mRNA export together with the mRNA export factor PCID2 (Q9VTL1; 395 aas) and other messenger ribonucleoprotein (mRNP) particles (Kristó et al. 2017).
Moesin of Drosophila melanogaster (Fruit fly)
FERM domain-containing protein 7, FERMD7, of 714 aas and 0 TMSs. It plays a role in neurite development, maybe through the activation of the GTPase RAC1, and it may also function in the control of eye movement and gaze stability. FRMD7 mutations disrupt the interaction with GABRA2 and may result in the Infantile Nystagmus Syndrome (Jiang et al. 2020).
FERMD7 of Homo sapiens
Talin-1, TL-1, of 2541 aas with 1 N-terminal TMS possibly + one or two more later in the protein.
Talin-1 of Homo sapiens
Talin-2, TLN2, of 2542 aas and one N-terminal TMS, possibly with more TMSs in the rest of the protein. As a major component of focal adhesion plaques that links integrin to the actin cytoskeleton, it may play a role in cell adhesion. It recruits PIP5K1C to focal adhesion plaques and strongly activates its kinase activity.
Talin-2 of Homo sapiens
Fermitin-1, Fermt1 (or Kindlerin or Kindlin (KIND or URP1), of 677 aas with one N-terminal TMS + two possible TMSs within the first 120 aas, and another possible TMS at the C-terminus. It is involved in cell adhesion and contributes to integrin activation. When coexpressed with talin (see 8.A.25.1.6/7), it potentiates activation of ITGA2B and is required for normal keratinocyte proliferation. It is required for normal polarization of basal keratinocytes in skin and for normal cell shape. It is required for normal adhesion of keratinocytes to fibronectin and laminin, and for normal keratinocyte migration to wound sites. It may also mediate TGF-beta 1 signaling in tumor progression (Kloeker et al. 2004). Talin and kindlin cooperate to control the density of integrin clusters (Pernier et al. 2023).
Fermitin-1 of Homo sapiens