Final answer:
Receptor kinases directly phosphorylate intracellular proteins, while G protein-coupled receptors activate downstream signaling components through G proteins.
Step-by-step explanation:
A key difference between receptor kinases and G protein-coupled receptors is their mechanism of signal transduction. Receptor kinases are themselves protein kinases and directly phosphorylate tyrosine residues on intracellular proteins upon ligand binding. On the other hand, G protein-coupled receptors activate G proteins, which in turn activate other downstream signaling components, such as ion channels or enzymes, to transmit the signal.