Intracellular targeting and trafficking of thrombin receptors. A novel mechanism for resensitization of a G protein-coupled receptor.
The receptor for the protease thrombin is a member of the G protein-coupled receptor family, but is activated by a unique proteolytic mechanism. The irreversibility of this proteolytic mechanism and the fact that the ligand is tethered to its receptor raise special questions about inactivation of cleaved receptors and recovery of thrombin responsiveness. We compared the intracellular trafficking of the thrombin receptor to that of the beta 2-adrenergic receptor in transfected Rat1 fibroblasts. In unstimulated cells almost all beta 2 receptors were located on the plasma membrane; by contrast, part of a cell's thrombin receptors were found in an intracellular membrane compartment which co-localized with Golgi markers. Stimulation by agonist caused internalization and subsequent recycling of the beta 2-adrenergic receptor, but most activated thrombin receptors were internalized and targeted to lysosomes. The intracellular pool of thrombin receptors found in unstimulated cells was protected from activation by thrombin, but was translocated to the plasma membrane upon activation of cell surface thrombin receptors. Replenishment of plasma membrane thrombin receptors correlated with recovery of thrombin responsiveness. These observations reveal a novel trafficking mechanism for resensitizing the thrombin receptor as opposed to the internalization/recycling pathway of other G protein-coupled receptors.