Final answer:
The claim that receptors coupled to a heterotrimeric G protein only act as activators is false; GPCR signaling can either activate or inhibit downstream pathways, depending on the specific receptor and signal involved.
Step-by-step explanation:
The statement that receptors coupled to a heterotrimeric G protein may only act as an activator is false. Receptors linked to heterotrimeric G-proteins can either activate or inhibit downstream signaling pathways.
Heterotrimeric G-proteins are made of α, β, and γ subunits. When a ligand binds to a G-protein-coupled receptor (GPCR), it causes a conformational change allowing GDP to be exchanged with GTP on the α subunit.
The Gβγ complex dissociates from the α subunit, and either the α subunit or the Gβγ pair can then initiate or inhibit a cellular response by interacting with other proteins or effector systems. The signal is terminated when GTP on the α subunit is hydrolyzed back to GDP, leading to reassociation of the G-protein subunits and ending the signal.
Therefore, GPCR signaling is versatile and can lead to a variety of cellular responses, including activation or inhibition depending on the specific GPCR and the pathway involved.