G protein
A G protein is a molecular relay runner that carries a signal received at the cell surface deeper into the cell. When a GPCR catches a messenger from outside, it hands the baton to a G protein waiting just inside the membrane, and that G protein then sprints off to trigger the next step. Its name comes from the small guanine nucleotides it binds and uses to keep time.
A G protein switches between an off and an on state controlled by which guanine nucleotide it holds: bound to GDP it is resting, and when an activated GPCR prompts it to swap in GTP it turns on. The active G protein then separates into parts that switch on or off downstream enzymes and channels, changing levels of internal messengers. It naturally switches itself off after a short time, resetting the system.
Although drugs rarely target G proteins directly, understanding them is essential because they sit one step downstream of the most heavily drugged receptor family. The honest caveat is that a single GPCR can couple to different G proteins, so a drug acting on the receptor may bias the cell toward one G-protein pathway over another, with real consequences for benefit and side effect.
When adrenaline activates a beta-adrenergic GPCR, the coupled G protein switches on an enzyme that raises the internal messenger cyclic AMP, speeding the heart.
A G protein passes the receptor's signal to the cell's internal machinery.
The large G proteins coupled to GPCRs are distinct from small G proteins like Ras, although both use the same GDP/GTP switching trick.