What feature is common among G-protein-coupled receptors?

G-protein-coupled receptors (GPCRs) are a large family of membrane receptors that play a key role in signal transduction. The correct answer is that they are activated by GTP binding to the G protein.

When a ligand, such as a hormone or neurotransmitter, binds to the GPCR, it causes a conformational change in the receptor that allows it to interact with an associated G protein. This interaction leads to the exchange of GDP for GTP on the α subunit of the G protein. The binding of GTP activates the G protein, which then dissociates into the active α subunit and the βγ dimer, each of which can then go on to interact with downstream effector proteins. This process is crucial for propagating the signal initiated by the ligand-receptor interaction and can lead to various cellular responses.

The activation mechanism involving GTP is fundamental to GPCR signaling pathways, and it's important to understand that the G protein is inactive when bound to GDP and becomes active only when GTP binds. This is a key element in the role of GPCRs in numerous physiological processes.