Research Article

Ras activation by SOS: Allosteric regulation by altered fluctuation dynamics

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Science  04 Jul 2014:
Vol. 345, Issue 6192, pp. 50-54
DOI: 10.1126/science.1250373

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Outliers dominate signaling at cell membrane

SOS enzymes act at cell membranes to activate Ras, a regulatory protein often overactive in cancer cells. Iversen et al. devised a system where they could observe the activity of individual enzymes at work. The single SOS molecules occupied stable states that varied greatly in their catalytic activity. Regulation appeared to occur by altering the time spent in active states. The overall activity of SOS was determined by just a few molecules that achieved the highest catalytic activity. The methods described should allow further detailed kinetic analysis of this and other signaling events that occur at the cell membrane — properties that it is not possible to discern from bulk biochemical measurements.

Science, this issue p. 50


Activation of the small guanosine triphosphatase H-Ras by the exchange factor Son of Sevenless (SOS) is an important hub for signal transduction. Multiple layers of regulation, through protein and membrane interactions, govern activity of SOS. We characterized the specific activity of individual SOS molecules catalyzing nucleotide exchange in H-Ras. Single-molecule kinetic traces revealed that SOS samples a broad distribution of turnover rates through stochastic fluctuations between distinct, long-lived (more than 100 seconds), functional states. The expected allosteric activation of SOS by Ras–guanosine triphosphate (GTP) was conspicuously absent in the mean rate. However, fluctuations into highly active states were modulated by Ras-GTP. This reveals a mechanism in which functional output may be determined by the dynamical spectrum of rates sampled by a small number of enzymes, rather than the ensemble average.

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