G protein-coupled receptor
A G protein-coupled receptor, or GPCR, is like a turnstile linked to a relay runner. The receptor itself sits in the membrane; when a hormone arrives, the receptor nudges a partner protein — the G protein — which then sprints off to carry the message deeper into the cell.
Structurally, a GPCR threads back and forth through the membrane seven times, which is why it is called a seven-transmembrane receptor. Hormone binding on the outside changes its shape so that, on the inside, it activates a coupled G protein. The G protein splits into pieces that switch enzymes on or off — most famously activating adenylyl cyclase to make cyclic AMP, or activating phospholipase C to make IP3 and diacylglycerol.
GPCRs are the largest receptor family in the body and handle a huge share of hormone signaling, including adrenaline, glucagon, TSH, ACTH, and many releasing hormones. Their reliance on intermediate proteins gives many points for amplification and control — but also many points where signaling can be dialed down, as happens in desensitization.
When TSH binds the TSH receptor on a thyroid cell, the coupled Gs protein activates adenylyl cyclase, raising cyclic AMP and driving thyroid hormone production.
A classic GPCR cascade driving an endocrine response.
Different G proteins steer the same kind of receptor toward opposite outcomes: Gs raises cyclic AMP, Gi lowers it, and Gq drives the IP3/diacylglycerol pathway.