When the alpha subunit is bound to GTP, what happens to it?

When the alpha subunit of a G-protein is bound to GTP, it undergoes a conformational change that allows it to dissociate from the beta and gamma subunits. This dissociation is crucial for the activation of various intracellular signaling pathways.

In this active state, the GTP-bound alpha subunit can interact with various effector proteins such as enzymes or ion channels, thereby propagating the signal initiated by a ligand binding to a receptor. This dynamic change is fundamental to the function of G-proteins in signal transduction, allowing for a rapid response to extracellular signals.

The process highlights the importance of GTP binding in regulating G-protein activity, as the inactive state of the alpha subunit, when bound to GDP, keeps it associated with the beta and gamma subunits, preventing it from interacting with downstream signaling partners. Thus, the correct option illustrates the key role of GTP-binding in the functional activation of G-proteins.