Activation of Ras Requires the ERM-Dependent Link of Actin to the Plasma Membrane

Abstract

BackgroundReceptor tyrosine kinases (RTKs) participate in a multitude of signaling pathways, some of them via the small G-protein Ras. An important component in the activation of Ras is Son of sevenless (SOS), which catalyzes the nucleotide exchange on Ras.Principal FindingsWe can now demonstrate that the activation of Ras requires, in addition, the essential participation of ezrin, radixin and/or moesin (ERM), a family of actin-binding proteins, and of actin. Disrupting either the interaction of the ERM proteins with co-receptors, down-regulation of ERM proteins by siRNA, expression of dominant-negative mutants of the ERM proteins or disruption of F-actin, abolishes growth factor-induced Ras activation. Ezrin/actin catalyzes the formation of a multiprotein complex consisting of RTK, co-receptor, Grb2, SOS and Ras. We also identify binding sites for both Ras and SOS on ezrin; mutations of these binding sites destroy the interactions and inhibit Ras activation. Finally, we show that the formation of the ezrin-dependent complex is necessary to enhance the catalytic activity of SOS and thereby Ras activation.ConclusionsTaking these findings together, we propose that the ERM proteins are novel scaffolds at the level of SOS activity control, which is relevant for both normal Ras function and dysfunction known to occur in several human cancers.

ERM proteins in cancer progression

“…3). It is becoming increasingly apparent that all three ERM proteins can modulate the activity of the Ras superfamily of small GTPases, including the Ras and Rho subgroups, by interacting with their guanine exchange factors (GEFs) and guanine nucleotide dissociation inhibitors (GDIs), which are vital for their activation (Sperka et al, 2011;Takahashi et al, 1997;Valderrama et al, 2012). Ras is a molecular switch that is important for RTK-dependent activation of the MAP kinase pathway, such as through EGFR and HGFR (Easty et al, 2011;Orian-Rousseau et al, 2007;Ridley et al, 1995).…”

“…ERM proteins have been proposed to be responsible for a new step in Ras activation in that they are recruited to F-actin and co-receptors, specifically to receptors involved in cell adhesion, such as b1 integrin and CD44, at the plasma membrane in response to activation of RTKs. In doing so, ERM proteins can form a complex that acts as a scaffold and binds both GDP-bound Ras and the N-terminal region of the GEF son of sevenless homolog (SOS) (Sperka et al, 2011). Ezrin has specific binding domains for CD44, Ras and the autoinhibitory Dblhomology-pleckstrin-homology (DH-PH) domain of SOS but only after ezrin is in a complex with F-actin and its co-receptor can it bind to the DH-PH domain of SOS and expose the allosteric binding site that is specific for GDP binding of Ras (Sperka et al, 2011).…”

Integrative analysis of miRNA and mRNA expression profiles in pheochromocytoma and paraganglioma identifies genotype-specific markers and potentially regulated pathways

“…This is highly relevant given that EZR together with radixin and/or moesin form ERM complexes that connect actin to other membrane proteins (Sperka et al 2011). Sperka et al (2011) showed that activation of RAS requires the essential participation of ERM complexes and actin and that disrupting either the interaction of the ERM proteins with co-receptors or by downregulation of ERM proteins abolished growth factor-induced RAS activation.…”

“…This is highly relevant given that EZR together with radixin and/or moesin form ERM complexes that connect actin to other membrane proteins (Sperka et al 2011). Sperka et al (2011) showed that activation of RAS requires the essential participation of ERM complexes and actin and that disrupting either the interaction of the ERM proteins with co-receptors or by downregulation of ERM proteins abolished growth factor-induced RAS activation. The NGFinduced neuronal differentiation initiates a signal cascade requiring activation of RAS signaling, and interfering interference with the activity or function of RAS has been shown to impede or abolish NGF-induced differentiation (Klesse et al 1999, Sperka et al 2011.…”

“…Sperka et al (2011) showed that activation of RAS requires the essential participation of ERM complexes and actin and that disrupting either the interaction of the ERM proteins with co-receptors or by downregulation of ERM proteins abolished growth factor-induced RAS activation. The NGFinduced neuronal differentiation initiates a signal cascade requiring activation of RAS signaling, and interfering interference with the activity or function of RAS has been shown to impede or abolish NGF-induced differentiation (Klesse et al 1999, Sperka et al 2011. Taken altogether, disruption of RAS signaling by downregulation of EZR by miR-183 and/or miR-96 could in part explain our results showing that these miRNAs impede NGF-induced differentiation in PC12 cells.…”