Final answer:
GEF regulates small GTPase activity, like that of RAS G-protein, by exchanging GDP for GTP. Inhibition of GTPase activity keeps RAS in an active state, leading to unregulated cell signaling and potential cancer development. It severely affects downstream cellular events by avoiding the natural 'turning off' of cell growth signals.
Step-by-step explanation:
GEF (Guanine Nucleotide Exchange Factors) regulates small GTPase activity by facilitating the exchange of GDP for GTP on the GTPase protein. An example is the RAS G-protein, which becomes activated when GTP is bound. When the GTPase activity of a G-protein like RAS is inhibited, as it occurs in certain cancers, RAS cannot hydrolyze GTP back to GDP, and therefore it remains active indefinitely. This persistent activation can lead to continuous cell signaling, resulting in uncontrolled cell growth, or cancer.
Cell signaling pathways often involve such proteins and their regulators, like Rb (Retinoblastoma protein), which monitors cell size and regulates the cell cycle transition from G₁ to S phase by controlling transcription factors like E2F. Normally, Rb binds to E2F, preventing it from activating genes required for the G₁/S transition unless the cell is ready. However, in cancerous conditions where the checks and balances are disrupted, such as when GTPase activity is impeded, these pathways can lead to unchecked cellular events, facilitating cancer progression. In summary, the inhibition of GTPase activity on RAS has profound effects on downstream cellular events that can contribute to cancer development.