INTERNALIZATION AND RECYCLING OF ACTIVATED THROMBIN RECEPTORS

Shortly after activation by either thrombin or the tethered ligand dom ain peptide SFLLRN, thrombin receptors undergo homologous desensitizat ion, temporarily losing their ability to respond to both agonists. We have examined the role of receptor internalization and recycling in th is process using receptor-directed antibodies as probes. The results s how within 1 min of activation >85% of the approximately 200,000 throm bin receptors on megakaryoblastic human erythroleukemia (HEL) and CHRF -288 cells are sequestered into endosomes via coated pits, after which the majority are transferred to lysosomes. This process does not requ ire proteolysis of the receptor and occurs with sufficient speed to pl ay a major role in the regulation of thrombin receptor function. Altho ugh most of the internalized receptors are ultimately degraded, approx imately 25% return to the cell surface. These recycled receptors are i n a state in which they can respond to SFLLRN but not thrombin; nor do they self-activate despite the apparent continued presence of the tet hered ligand. In contrast to other G protein-coupled receptors, which are internalized and then recycled in an activatable state, recovery o f the thrombin response occurs only after the expression on the cell s urface of adequate numbers of newly synthesized receptors.