Ras Requires ERMs Figure 7. Model of ezrin-mediated activation of SOS. Growth factor (GF) induction promotes ezrin activation, which localizes to co-receptors/ adhesion receptors, while SOS couples to the activated RTK. Ezrin in conjunction with F-actin binds to the DH-PH domains of SOS, creating a stable anchorage to the membrane and assisting the release of SOS autoinhibition. With the unmasking of the distal/allosteric site (unmasked site now illustrated by yellow star; masked conformation in grey), ezrin binding to Ras-GDP increases the ability of Ras-GDP to engage this allosteric site, promoting the ability of SOS to catalyze nucleotide exchange (GDP illustrated by two and GTP by three red balls). doi:10.1371/journal.pone.0027511.g007 Ras and SOS via separate domains of ezrin, but it interacts with the autoinhibitory DH-PH domains of SOS possibly contributing to the release of SOS autoinhibition and/or presenting Ras to the allosteric regulatory site of SOS (a model illustrating the proposed mechanism of SOS activation is shown in Figure 7). While this study presents evidence that ezrin is required for the activation of SOS; we should however, consider that other partners are required for the full activation of SOS. A likely candidate is the plasma membrane due to its proximity. Indeed, our in vitro GEF assay with isolated SOS contained lipids in addition to ezrin. Lipid interaction has previously been shown to release SOS autoinhibition. For example, binding of phosphoino- sitides to the PH domain [1] as well as binding of phosphatidic acid to the histone-like domain of SOS are important modulators of its autoinhibition [2,25,26]. Taking these findings together, the in vivo targeting of ezrin to the plasma membrane and its direct interaction with SOS are both essential features for the full activation of SOS (see proposed model, Figure 7). It is noteworthy to mention that these principles of ezrin-mediated activation of SOS may have a more widespread significance. As several other GEFs also require release from autoinhibition, ezrin might also promote the activation of these GEFs. Indeed, binding of GEFs other than SOS, such as Dbl and Dock180, to ERM proteins has been reported by several laboratories [27,28,29]. However, these groups did not address the mechanism of action. Another feature of ezrin that is required for the activation of Ras is the binding of F-actin. This is demonstrated by the dominant-negative effect of ezrin R579A (which eliminates the binding of F-actin whilst retaining the association with membrane proteins) and by the similar effect of an actin polymerization inhibitor. The most straightforward explanation of these findings is that F-actin is part of a stabilizing complex, which specifically focuses the ERM proteins to RTKs at the plasma membrane. However, our in vitro data showing that ezrin can only interact with SOS in the presence of F-actin imply more than a scaffold function: ezrin might adopt a specific SOS-activating conforma- tion only when associated with F-actin. While our findings suggest a role for the ezrin-actin link in signaling, another interesting but different function of actin in signaling relates to the activation of the transcription factor SRF. The abundance of free G-actin inhibits SRF activity by retaining one of its co-activators, MAL, in the cytoplasm [30]. Signaling pathways inhibiting F-actin depolymerization or promoting filament assembly co-operate in preventing the activation of the SRF/MAL complex – this complex addresses a subset of SRF target genes, e.g. the actin promoter itself [31]. Another subset requiring the interaction of SRF and the co-activator TCF is addressed by Erk [32] and is likely to be dependent on the signal-promoting effect of cortical F- actin presented here. The coupling of co-receptors to ezrin/F-actin could be regarded as a platform to co-ordinate and restrict the encounter of SOS with Ras, thereby fine-tuning growth factor signals. The control of Ras activity is critical and can falter by mutation at several levels, e.g. oncogenic mutations of Ras [33] or an inactivating mutation of the gene NF1 encoding neurofibromin [34]. In addition, dysregulation of RTKs is often the cause of the development and maintenance of cancer [35]. It is of no surprise that the components of the RTK-dependent signaling steps identified PLoS ONE | www.plosone.org 9 November 2011 | Volume 6 | Issue 11 | e27511