FIGURE 3.

Spatial coupling of GEF and PI3K binding to the same receptor scaffold enhances PI3K recruitment. Various statistical metrics were calculated as a function of the Ras/PI3K complex lifetime, τ_SP; other relevant parameters were set to their base-case values (Table 1), and N_Ras = 1000. (a) Fractional occupancy of the receptor binding sites for PI3K, T_P. For comparison, a second set of simulations was acquired using a bulk activation model, where receptor-bound GEF is catalytically silent, and its average rate of Ras activation is replaced by spontaneous conversion of Ras-GDP to Ras-GTP in the bulk membrane. The solid line is the analytical solution for the bulk activation model (Data S2). (b) Fraction of receptor/PI3K associations mediated by Ras, M_P. (c) Coincidence ratio for simultaneous GEF and PI3K binding to the receptor, C_EP (Eq. 4).