heterotrimeric G protein
/ het-er-o-try-MER-ic /
A heterotrimeric G protein is the molecular switch that sits just inside the membrane, waiting for a GPCR to flip it. Take the name apart and it explains itself: hetero means different, tri means three, meric means parts — it is a team of three different protein subunits, traditionally called alpha, beta, and gamma. The alpha subunit is the one that holds the actual switch; beta and gamma stay clamped together as a pair. The whole trio idles at the membrane until a signal comes along.
The switch is a small molecule sitting in a pocket on the alpha subunit — either GDP (off) or GTP (on), the cell's two related guanine nucleotides. When a ligand-bound GPCR nudges the resting G protein, it forces the alpha subunit to drop its GDP and grab a GTP instead. That swap is the moment of switching on. The newly armed alpha-GTP lets go of its beta-gamma partner, and now both pieces are free to go activate downstream targets — alpha-GTP might switch on adenylyl cyclase to make cAMP, while beta-gamma might open an ion channel. Crucially, the switch turns itself off: the alpha subunit slowly chops the GTP back down to GDP (it is a slow built-in timer), goes inactive, and re-joins beta-gamma to reset, ready for the next signal. This built-in timer is what makes the switch momentary rather than permanent.
Heterotrimeric G proteins matter because they are the workhorse intermediaries for the largest receptor family in biology — every GPCR signal passes through one. They are also a textbook example of the GTP-binding switch design that the cell reuses everywhere (the small GTPases like Ras work on the same on-with-GTP, off-with-GDP principle). And they sit at the heart of disease and toxins: cholera toxin causes its deadly diarrhoea precisely by jamming a G protein's alpha subunit in the on position, so it can never chop its GTP and cAMP pours out uncontrolled in the gut.
Cholera toxin chemically locks the G-protein alpha subunit in gut cells so it can never switch off. cAMP stays sky-high, the cells dump salt and water into the intestine, and the result is the massive watery diarrhoea of cholera — a deadly illustration of a switch that cannot reset.
Cholera toxin jams the G-protein switch on — showing that turning a signal off is just as vital as turning it on.
Do not confuse heterotrimeric G proteins (three subunits, activated by GPCRs) with the small monomeric GTPases like Ras (a single protein) — they share the GTP/GDP switch trick but are different molecules used in different parts of signaling.