Ezrin/radixin/moesin proteins and Rho GTPase signalling in leucocytes

Abstract

The ezrin/radixin/moesin (ERM) family of actin-binding proteins act both as linkers between the actin cytoskeleton and plasma membrane proteins and as signal transducers in responses involving cytoskeletal remodelling. The Rho family of GTPases also regulate cytoskeletal organisation, and several molecular pathways linking ERM proteins and Rho GTPases have been described. This review discusses recent findings on ERM protein function in leucocytes and how these may be integrated with Rho GTPase signalling.

Figure 1

Schematic representation of ezrin/radixin/moesin (ERM) protein activation in leucocytes. In their inactive conformation, the N-terminal FERM (band Four point one Ezrin Radixin Moesin) domain of ERM proteins (red) binds to the C-terminal actin-binding domain (green) via either an intramolecular or an intermolecular interaction. The α-helical domain is coloured in orange. Phosphatidylinositol 4,5 bis-phosphate (PIP₂) binding to the FERM domain and phosphorylation of the critical C-terminal threonine (amino acid position indicated for each ERM protein) induce and stabilize the unfolded active conformation of ERM proteins, allowing the FERM domain to bind to transmembrane receptors and the actin-binding domain to interact with actin filaments (F-actin). Activation of ERM proteins can be stimulated by Rho. Conversely, dephosphorylation of the C-terminal threonine may be regulated by Rac. Yellow star indicates phosphorylation of a critical C-terminal residue. ICAM-1–3, intercellular adhesion molecule types 1–3.

Figure 2

Reciprocal regulation of Rho GTPases and ezrin/radixin/moesin (ERM) proteins. ERM proteins are able to act both upstream (red arrows) and downstream (green arrows) of Rho GTPases. Blue arrows represent inhibitory pathways. Kinases indicated in grey are not directly linked to Rho GTPases. Note that only ezrin is phosphorylated by a tyrosine kinase (Tyr-K) at position Y353, and that it specifically interacts with the regulatory subunit of phosphatidylinositide 3-kinase (PI3-K). PI3-K is coloured red because it has the potential to activate Rac, but this is not indicated in the diagram. Calpain specifically cleaves ezrin and not moesin in lymphocytes. The calpain cleavage site is currently unknown. Phosphorylation of residues Y145, T235 (both in the FERM domain), Y353 (in the α-helical domain) and T558 (moesin numbering – in the C-terminal actin-binding domain) is indicated with a blue ‘P’. Dashed lines indicate pathways that are known to exist, but not known to regulate ERM proteins. Cdk5, cyclin-dependent kinase 5; FERM, band Four point one Ezrin Radixin Moesin; MBS, myosin-binding subunit; PKC, protein kinase C, Myp, myosin phosphatase; PPase, phosphatase.

Figure 3

Regulation of Rho GTPases. In unstimulated cells, Rho GTPases are maintained in an inactive state in the cytoplasm by binding to RhoGDI (purple) via their isoprenylated C-termini (shown in red). Dissociation of RhoGDI from Rho may occur through phosphorylation or interaction with activated and unfolded ezrin/radixin/moesin (ERM) proteins, allowing the isoprenyl group to incorporate into membranes. Membrane recruitment and interaction with RhoGEFs (e.g. Dbl) induces the exchange of GDP for GTP on Rho GTPases. When bound to GTP, Rho GTPases interact with and activate downstream effectors (e.g. ROCK). RhoGAPs (green) inactivate Rho GTPases by increasing their intrinsic GTPase activity, resulting in accelerated hydrolysis of GTP to GDP. RhoGDIs are able to extract GDP-bound Rho GTPases from membranes. PKCα, protein kinase C α.