Regulation of RalA GTPase by phosphatidylinositol 3-kinase as visualized by FRET probes

Small GTPases, which are binary switches regulating various signal transduction cascades, function not only to relay signals but also to integrate them from multiple signalling branches. For example, RalA activity is regulated by at least three signalling cascades involving Ras, Rac or PI3K (phosphoinositide 3-kinase). To untangle such complicated regulatory mechanisms, we have been developing probes for GTPases, kinases and phosphatidylinositols based on the principle of FRET (fluorescence resonance energy transfer). We demonstrated previously that, upon EGF (epidermal growth factor) stimulation, Ras activity increases diffusely in the plasma membrane, whereas RalA activity increases predominantly in lamellipodial protrusions. Here, we show that the level of PtdIns(3,4,5)P₃ is increased diffusely in the plasma membrane, whereas, in the central region, the level of PtdIns(3,4)P₂ is increased more in the nascent lamellipodia than in the plasma membrane. The distribution and time course of Akt activation are similar to those of increased PtdIns(3,4)P₂ levels. These observations suggest that the increase in PtdIns(3,4)P₂ and the subsequent activation of Akt may be responsible for the localized activation of RalA. Thus the signals from Ras and PI3K converge at the level of Ral GEFs (guanine nucleotide-exchange factors), and this convergence restricts the area of RalA activation.