G protein-coupled receptors: (A) are intracellular membrane proteins that help to regulate movement within the cell., (B) are plasma membrane proteins that couple the extracellular binding of primary signaling molecules to activation of small G proteins., (C) are plasma membrane proteins that couple the extracellular binding of primary signaling molecules to the activation of heterotrimeric G proteins., (D) are intracellular proteins that couple the binding of primary messenger molecules with transcription.
Answer is B. Upon ligand binding, a conformational change activates a resting heterotrimeric G protein associated with the cytoplasmic leaf of the plasma membrane. Activation of a single receptor can result in 1, 10, or more active heterotrimeric G proteins, providing amplification as well as transduction of the first messenger.