Final answer:
The alpha subunit of a G-protein activates signaling pathways by exchanging GDP for GTP, dissociating from beta and gamma subunits, and then triggering cellular responses such as activating secondary messengers or opening ion channels. The signal ends when the alpha subunit hydrolyzes GTP to GDP and reassociates with the other subunits.
Step-by-step explanation:
The alpha subunit of a G-protein plays a crucial role in cell signaling pathways.
When a signaling molecule binds to a G-protein-coupled receptor, the alpha subunit exchanges GDP for GTP, activating the G-protein.
This leads to the dissociation of the alpha subunit from the beta and gamma subunits.
Once activated, the alpha subunit can either directly open ion channels in the membrane or generate secondary messengers like cAMP or calcium ions, which further propagate the signal within the cell.
For instance, alpha subunit activation of adenylate cyclase can lead to cAMP production, which then activates a cascade of proteins affecting gene transcription.
Alternatively, alpha subunit activation of phospholipase C can elevate intracellular calcium levels and alter cell functions.
The signal is terminated when the alpha subunit hydrolyzes GTP back to GDP, reassociating with the beta and gamma subunits and returning to its inactive state.
In summary, the alpha subunit therefore goes on to trigger various cell responses, which can involve activating secondary messengers like cAMP or calcium, or directly interacting with ion channels and other cellular proteins.