Final answer:
The taste receptors for salty and sour tastes do not use a G-protein-coupled receptor; they are activated by the direct influx of sodium and hydrogen ions respectively, which leads to the depolarization of the taste neurons.
Step-by-step explanation:
The taste receptor that does not use a G-protein-coupled receptor for activation is involved in the perception of salty and sour tastes. These tastes are triggered by the cations Na+ and H+, respectively. Unlike sweet, bitter, and umami taste receptors which bind to food molecules through a G-protein coupled receptor, salty and sour tastes result from the direct influx of ions: sodium ions (Na+) for salty tastes, which enter the taste neurons directly, and hydrogen ions (H+) for sour tastes, which belong to the thermoreceptor protein family and alter ion channels in the taste neurons, leading to depolarization without the need for a G-protein coupled receptor.