GPCR

A G-protein-coupled receptor is a relay switch rather than a door. The signal molecule binds on the outside of the cell, the receptor changes shape, and on the inside it activates a separate go-between protein — a G protein — which then carries the message onward. Nothing physically passes through the membrane; the message is handed off and amplified.

Structurally, every GPCR threads through the membrane seven times, which is why it is also called a seven-transmembrane receptor. When activated, it acts as a guanine-nucleotide exchange factor, prompting its G protein to swap GDP for GTP and split into subunits that switch effector enzymes and ion channels on or off, often via second messengers such as cyclic AMP or calcium.

GPCRs are the single largest receptor family and the target of roughly a third of all marketed drugs, spanning adrenergic, muscarinic, opioid, histamine, dopamine and many other systems. A modern caveat is that GPCRs can also signal through beta-arrestin independently of G proteins, so some drugs are now designed to bias signalling toward the more therapeutically useful pathway.