Role of multiple phosphorylation sites in the COOH-terminal tail of aquaporin-2 for water transport: evidence against channel gating

Abstract

Moeller HB, MacAulay N, Knepper MA, Fenton RA. Role of multiple phosphorylation sites in the COOH-terminal tail of aquaporin-2 for water transport: evidence against channel gating. Am J Physiol Renal Physiol 296: F649 -F657, 2009. First published January 14, 2009 doi:10.1152/ajprenal.90682.2008.-Arginine vasopressin (AVP)-regulated phosphorylation of the water channel aquaporin-2 (AQP2) at serine 256 (S256) is essential for its accumulation in the apical plasma membrane of collecting duct principal cells. In this study, we examined the role of additional AVP-regulated phosphorylation sites in the COOH-terminal tail of AQP2 on protein function. When expressed in Xenopus laevis oocytes, prevention of AQP2 phosphorylation at S256A (S256A-AQP2) reduced osmotic water permeability threefold compared with wild-type (WT) AQP2-injected oocytes. In contrast, prevention of AQP2 single phosphorylation at S261 (S261A), S264 (S264A), and S269 (S269A), or all three sites in combination had no significant effect on water permeability. Similarly, oocytes expressing S264D-AQP2 and S269D-AQP2, mimicking AQP2 phosphorylated at these residues, had similar water permeabilities to WT-AQP2-expressing oocytes. The use of high-resolution confocal laser-scanning microscopy, as well as biochemical analysis demonstrated that all AQP2 mutants, with the exception of S256A-AQP2, had equal abundance in the oocyte plasma membrane. Correlation of osmotic water permeability relative to plasma membrane abundance demonstrated that lack of phosphorylation at S256, S261, S264, or S269 had no effect on AQP2 unit water transport. Similarly, no effect on AQP2 unit water transport was observed for the 264D and 269D forms, indicating that phosphorylation of the COOH-terminal tail of AQP2 is not involved in gating of the channel. The use of phosphospecific antibodies demonstrated that AQP2 S256 phosphorylation is not dependent on any of the other phosphorylation sites, whereas S264 and S269 phosphorylation depend on prior phosphorylation of S256. In contrast, AQP2 S261 phosphorylation is independent of the phosphorylation status of S256. water channel; vasopressin; NDI; kidney AQUAPORIN-2 (AQP2) is a vasopressin-regulated water channel expressed in the principal cells of the kidney collecting duct. In the absence of vasopressin (AVP), AQP2-containing vesicles localize to subapical regions of the cell. Binding of AVP to the basolaterally localized type 2 vasopressin receptor initiates a signaling cascade resulting in increased intracellular cAMP, activation of PKA, phosphorylation of AQP2, and subsequent insertion of the water channel into the apical plasma membrane. This event, rendering the cell more permeable to water, is essential for water reabsorption by the collecting duct and thereby for concentrating the urine. Upon removal of AVP, AQP2 is retrieved from the membrane by endocytosis. Thus the abundance of AQP2 in the apical membrane is a regulated balance between exocytosis and endocytosis.Protein phosphorylation and dephosphorylation are common, re...

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“…8 Phosphorylation of AQP2 at S264 and S269 requires prior phosphorylation of S256. 9 Immunohistochemistry showed that pS269-AQP2 localizes exclusively in the apical plasma …”

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