Role of the Rab11-Associated Intracellular Pool of Receptors Formed by Constitutive Endocytosis of the β Isoform of the Thromboxane A2 Receptor (TPβ)†

Intracellular trafficking pathways of G protein-coupled receptors (GPCRs), following their agonist-induced endocytosis and their consequences on receptor function, are the subject of intense research efforts. However, less is known regarding their constitutive endocytosis. We previously demonstrated that the β isoform of the thromboxane A2 receptor (TPβ) undergoes constitutive and agonist-induced endocytosis. Constitutive endocytosis of GPCRs can lead to the formation of an intracellular pool of receptors from which they can recycle back to the cell surface. In the present report, we show with the help of two TPβ mutants (TPβ-Y339A and TPβ-I343A) specifically deficient in constitutive endocytosis that this intracellular pool of receptors serves to maintain agonist sensitivity over prolonged receptor stimulation in HEK293 cells. Second messenger generation by the TPβ-Y339A and TPβ-I343A mutants was drastically reduced compared to the wild-type receptor as suggested by dose−response and time-course experime...