Final answer:
The GTPase activity of the G-protein's alpha subunit regulates G-protein-coupled receptor signaling by hydrolyzing GTP to GDP, thus terminating the signal. When this activity is inhibited, as in some cancers, it can lead to uncontrolled cell growth due to continuous active signaling.
Step-by-step explanation:
GTPase Activity and Its Role in Cell Signaling
The GTPase activity of the alpha subunit is crucial for the proper function of G-protein-coupled receptors (GPCRs) in cell signaling. When a signaling molecule binds a GPCR, the G protein's alpha subunit exchanges guanosine diphosphate (GDP) for guanosine triphosphate (GTP).
This exchange triggers the dissociation of the alpha subunit from the beta and gamma subunits - the GTP-bound alpha subunit can then activate other proteins within the cell.
However, the hydrolysis of GTP back to GDP by the GTPase activity of the alpha subunit is a regulatory mechanism that terminates the signal. The hydrolyzed alpha subunit reassociates with the beta and gamma subunits, rendering the G protein inactive again, ready for a new signaling cycle.
In the context of certain cancers, if the GTPase activity of the RAS G-protein is inhibited, the RAS protein remains activated for extended periods because it cannot hydrolyze GTP to GDP. This results in continuous cell signaling, which can lead to uncontrolled cell growth and thus contribute to cancerous developments.