FIGURE 9.

Common scenarios for nucleotide exchange regimes in Ras/Sos system. The allosteric site is schematically shown at the bottom of Sos and catalytic site is schematically shown at the top. For each scenario, the state of the system and qualitative description of binding are shown on the right, including the expected exchange rate of the nucleotide that was initially bound to incoming Ras. Scenario A corresponds to initial low level activation of Sos when no Ras·GTP is present yet with GDP → GTP exchange driven by the excess of GTP over GDP in the cytosol. Once enough Ras·GTP molecules are produced, they lead to positive feedback activation of Sos (scenario B) where the affinity at the catalytic site is high for Ras·GDP and low for Ras·GTP, leading to efficient, native turnover and expected maximum GDP → GTP exchange rate. Both nucleotides (+GDP and +GTP) are present. In scenarios C–E, typical for assays where only one type of nucleotide is present in excess, a release of labeled nucleotide (marked with *) initially bound to the catalytic site is expected to be slowed down (relative to the native rate for scenario B) due to either stalling of Sos recycling (scenario C) or the presence of weak binding bottleneck steps at the allosteric (D) or catalytic site (E).