TI - A serine cluster prevents recycling of the V2 vasopressin receptor . AB - Receptor recycling plays a critical role in the regulation of cellular responsiveness to environmental stimuli . Agonist -promoted phosphorylation of G protein -coupled receptors has been related to their desensitization , internalization , and sequestration . Dephosphorylation of internalized G protein -coupled receptors by cytoplasmic phosphatases has been shown to be pH -dependent , and it has been postulated to be necessary for receptors to recycle to the cell surface . The internalized V2 vasopressin receptor ( V2R ) expressed in HEK 293 cells is an exception to this hypothesis because it does not recycle to the plasma membrane for hours after removal of the ligand . Because this receptor is phosphorylated only by G protein-coupled receptor kinases ( GRKs ) , the relationship between recycling and GRK -mediated phosphorylation was examined . A nonphosphorylated V2R , truncated upstream of the GRK phosphorylation sites , rapidly returned to the cell surface after removal of vasopressin . Less-drastic truncations of V2R revealed the presence of multiple phosphorylation sites and suggested a key role for a serine cluster present at the C terminus . Replacement of any one of Ser-362 , Ser-363 , or Ser-364 with Ala allowed quantitative recycling of full-length V2R without affecting the extent of internalization . Examination of the stability of phosphate groups incorporated into the recycling S363A mutant V2Rs revealed that the recycling receptor was dephosphorylated after hormone withdrawal , whereas the wild-type V2R was not , providing molecular evidence for the hypothesis that GRK sites must be dephosphorylated prior to receptor recycling . These experiments uncovered a role for GRK phosphorylation in intracellular sorting and revealed a GRK -dependent anchoring domain that blocks V2R recycling .