Nanoclusters of Ras–mitogen activated protein kinase (MAPK) signaling molecules in the plasma membrane convert binary Erk activation into an overall graded response to stimuli, affording exquisite sensitivity to upstream pathway activation.
Far from being a uniform structure, the plasma membrane is a variegated lipid bilayer partitioned into discrete subdomains. The specific localization of signaling molecules to these subdomains can govern signaling specificity as well as the spatiotemporal control of signal propagation. The Ras small GTPase is known to exist in nanoclusters at the plasma membrane that are similar to lipid rafts, although the biological role of Ras targeting has not been extensively studied. In Nature Cell Biology, Tianhai Tian et al. have developed a computational model of the Ras MAPK signaling cascade based on in vitro and in vivo data of plasma membrane dynamics, and found that the precise submembrane localization of Ras and MAPK kinases is required for high-fidelity signal transduction.
Ras receives extracellular signals from membrane receptors and recruits Raf to the plasma membrane, thereby activating it. Raf then phosphorylates Mek, which in turn phosphorylates the Erk/MAPK kinases that relay information to multiple cytoplasmic and nuclear effector targets. Previous studies have shown that Ras activation promotes the formation of signaling component nanoclusters at discrete subdomains in the plasma membrane. Nanoclusters are composed of approximately seven Ras–GTP monomers, scaffolds such as kinase suppressor of Ras (KSR) and galectin, and the downstream kinases Raf, Mek and Erk. Using live-cell fluorescence imaging techniques, the authors confirmed that GTP-bound K-ras and Raf1 co-localized at the plasma membrane in discrete nanoclusters, and that membrane-localized, but not cytoplasmic Raf could promote phosphorylation of Erk. Erk phosphorylation was constrained spatially and temporally, as Ras-GTP promoted Erk phosphorylation only when localized to nanoclusters, and only during the lifetime of the nanocluster.
Data generated from these in vivo imaging studies were combined with a computational model of pathway activation to determine the kinetics of signal transmission. Membrane targeting of Raf caused binary, all-or-nothing activation of Erk. Nevertheless, EGF-mediated stimulation of the entire MAPK pathway generated a graded response — that is, Erk phosphorylation rose in direct correlation with the concentration of EGF. How are these observations reconciled? The authors determined that the binary Raf output was converted to a graded overall response through coordinated control of the generation and function of the nanoclusters. Because the generation of nanoclusters was graded, Erk phosphorylation followed EGF concentration in a linear manner, even though Raf signals in an all-or-nothing response.
The conclusions presented by Tian et al. provide an elegant mechanism for measuring graded pathway activation kinetics. Ultimately, the presence of nanoclusters of MAPK signaling molecules in the plasma membrane ensures that Erk activation and effector response are exquisitely sensitive to the strength of the upstream activating signal. Since many other signaling molecules appear to exist in nanoclusters, the conclusions presented in this study are likely to be relevant to multiple signaling pathways.
Emily J. Chenette Signaling Gateway
Reference
Tianhai Tian, Angus Harding, Kerry Inder, Sarah Plowman, Robert G. Parton and John F. Hancock. Plasma membrane nanoswitches generate high-fidelity Ras signal transduction. Nature Cell Biology9, 905-914 (2007) | Article | PubMed |
