Fig. 4. Molecular assembly enhances Ras activation by SOS.

(A and B) LAT assemblies are tuned by the addition of Grb2 and the PR domain of SOS, which enables multivalent cross-linking but has no intrinsic catalytic activity. Images represent LAT-Alexa Flour 555 after the addition of Grb2 and the PR domain. (C to E) Snapshot images of RBD after 15 min of reaction time. The histograms show the RBD intensities per corral of the images. The dashed lines are guidelines for the maximum extent of intrinsic nucleotide turnover. The percentages show the number of corrals with turnover above the level of the dashed line. Scale bar, 10 μm. pdf, probability density function. (F) Calculation of the probability of SOS activation as a function of Grb2-mediated mean dwell time (left) and the increase in SOS activity as a function of the change in mean dwell time (right). The change in mean dwell time is defined as $\mathrm{\Delta }\langle \tau \rangle ={\langle \tau \rangle }_{\text{activated}}-{\langle \tau \rangle }_{\text{basal}}$.