At the plasma membrane, biochemical reactions involving phosphatidylinositol phosphate (PIP) lipids and small GTPases serve essential roles in signal transduction immediately downstream of receptor activation. Interconnected positive and negative feedback loops link these distinct classes of molecules allowing cells to rapidly establish steep and precisely oriented intracellular activity gradients that are a hallmark of cell polarity. Although genetic approaches have identified numerous molecules that potentially regulate feedback between GTPase and PIP lipids in cells, it is unclear how different feedback loops modulate the spatiotemporal dynamics of these signaling reactions. Here, we reconstitute communication and feedback between Ras GTPase and phosphatidylinositol 3-kinase gamma (PI3Kγ) mediated production of PI(3,4,5)P 3 on supported membranes. We employ light-induced membrane recruitment of guanine nucleotide exchange factors (GEFs) to rapidly and reversibly shift steady-state conditions and monitor how Ras(GTP) and PI(3,4,5)P 3 evolve in time and space. Alone, the Ras-PI3Kγ module exhibits weak and transient activation in the presence of global inhibitors. However, the introduction of GEF-mediated positive feedback drives local amplification of Ras(GTP) and PI(3,4,5)P 3 , resulting in a wave of activity that propagates across membrane surfaces. Spatial coupling between Ras(GTP) and PI(3,4,5)P 3 depends on their respective diffusion coefficients and the feedback circuit architecture. This work demonstrates the importance of positive feedback in enabling small GTPases and PIP lipid modifying enzymes to overcome global inhibitors to amplify membrane signaling reactions.
Hansen et al. (Sun,) studied this question.