Ras activation leads to the formation of specialized microdomains--called nanoclusters--tethered to the inner leaflet of the plasma membrane; indeed, the formation of these transient nanoclusters appears to be critical to activation of the Ras-Raf-MEK (mitogen-activated or extracellular signal-regulated protein kinase kinase)-ERK (extracellular signal-regulated kinase) signaling pathway [the mitogen-activated protein kinase (MAPK) pathway]. Tian et al. combined immunogold electron microscopic analysis of the plasma membrane distribution of labeled forms of Raf1 and constitutively active K-RasG12V with fluorescence lifetime imaging microscopy-fluorescence resonance energy transfer (FLIM-FRET) analysis to show that Raf was specifically recruited to Ras nanoclusters, rather than to freely diffusing Ras monomers. Moreover, experiments with a constitutively active Raf mutant that could not undergo Ras-dependent recruitment to the plasma membrane indicated that Raf-dependent activation of MEK failed to occur in the cytosol and thus occurred at Ras nanoclusters as well. The authors incorporated this information into a spatiotemporal model of epidermal growth factor (EGF) activation of MAPK signaling. In silico analysis combined with experimental results led them to the conclusion that, whereas Ras nanocluster formation was a linear function of EGF concentration, each nanocluster functioned as an on-off "nanoswitch" in activating MAPK signaling. In turn, the existence of these nanoswitches enabled cellular ERK activation to faithfully track EGF input. Thus, the authors conclude that Ras signaling depends on the existence of nanoclusters and that organization of signaling molecules into microdomains may be required for high-fidelity signal transduction. Kenworthy discusses the implications.
T. Tian, A. Harding, K. Inder, S. Plowman, R. G. Parton, J. F. Hancock, Plasma membrane nanoswitches generate high-fidelity Ras signal transduction. Nat. Cell Biol. 9, 905-914 (2007). [PubMed]
A. K. Kenworthy, Nanoclusters digitize Ras signalling. Nat. Cell Biol. 9, 875-877 (2007). [PubMed]