Discussion
When a signal molecule binds to a G protein-linked receptor the G Protein :
Answer: A
When the signal binds, the G protein gets activated.
Binding of the ligand changes the shape of the receptor, which then acts as a guanine nucleotide exchange factor: the alpha subunit of the trimeric G protein releases GDP and binds GTP. The GTP-bound alpha subunit separates from the beta-gamma pair, and both go on to regulate effectors such as adenylyl cyclase or phospholipase C. The signal ends when the alpha subunit hydrolyses its GTP.
The G protein does not bind calcium or calmodulin; calcium and calmodulin act further downstream, after phospholipase C has produced IP3. Since activation is correct, 'None of the above' is wrong.
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