Figure 5. Step size distribution analysis reveals that the PR domain does not interfere with lipid interactions with N-terminal domains.

The step size distribution was acquired from multiple single molecule diffusion trajectories for (a) SOS^Cat, (b) SOS^CatPR, (c) SOS^HDPC, (d) SOS^FL on the Ras-modified lipid membrane containing either 3 mol% of 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) or PIP₂. All data are adequately fit to a two-species diffusion model. The extracted coefficients were shown in the insets. D₁ and D₂ correspond to diffusion coefficients of the fast and slow mobility species (unit: μm² s⁻¹). α corresponds to the fraction of the fast species. The N-terminal domains slow SOS mobility on the PIP₂ bilayer, presumably due to specific interaction with the Pleckstrin-homology (PH) domain (c,d)³³. Lipid composition (in mol%): egg-PC/MCC-DOPE/DOPS or PIP₂=94/3/3. Surface density of Ras: ∼1,200 μm⁻².