Phosphorylation of serine 256 is required for cAMP-dependent regulatory exocytosis of the aquaporin-2 water channel

The aquaporin-2 (AQP2) vasopressin water channel is translocated to the apical membrane upon vasopressin stimulation. Phosphorylation of serine 256 of AQP2 by cAMP-dependent protein kinase has been shown, but its relation to vasopressin-regulated translocation has not been elucidated. To address this question, wild type (WT) AQP2 and a mutant with alanine in place of serine 256 of AQP2 (S256A) were expressed in LLC-PK1 cells by electroporation. Measurements by a stopped-flow lightscattering method revealed that the osmotic water permeability (P-f) of LLC-PK1 cells transfected with WT was 69.6 +/- 6.5 mu m/s (24.8 +/- 2.2 mu m/s for mock-transfected), and stimulation by 500 mu M 8-(4-chlorophenylthio)-cAMP increased the P-f by 85 +/- 12%. When S256A AQP2 was transfected, the cAMP-dependent increase in the P-f was only 8 +/- 5%. After cAMP stimulation, the increase in surface expression of AQP2 determined by surface biotin labeling was 4 +/- 10%, significantly less than that for WT (88 +/- 5%). In addition, an in vivo [P-32]orthophosphate labeling assay demonstrated significant phosphorylation of WT AQP2 and only minimal phosphorylation of S256A AQP2 in LLC-PK1 cells. Our results indicated that serine 256 of AQP2 is necessary for regulatory exocytosis and that cAMP-responsive redistribution of AQP2 may be regulated by phosphorylation of AQP2.