<prompt> G protein-coupled receptorsare intracellular membrane proteins that help to regulate movement within the cell.are plasma membrane proteins that couple the extracellular binding of primary signaling molecules to activation of small G proteins.are plasma membrane proteins that couple the extracellular binding of primary signaling molecules to the activation of heterotrimeric G proteins.are intracellular proteins that couple the binding of primary messenger molecules with transcription.  <response>: 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. 