Lip5 in the regulation of the Aquaporin‐2 water channel

Abstract

AVP stimulates AQP2 phosphorylation at S256 and its translocation to the apical membrane. Upon AVP removal, AQP2 is ubiquitinated at K270, internalized, and water reabsorption is reduced. To identify AQP2 interacting proteins, the AQP2 C‐terminus was used as a bait in a yeast‐2‐hybrid assay. The Lyst‐interacting protein 5 (Lip5) to identified to bind. Immunohistochemistry showed that Lip5 co‐localizes with AQP2 in collecting ducts and interacts with AQP2‐S256D and AQP2‐K270R, but not with AQP3 or AQP4. Shortening the C‐tail revealed that the proximal C‐tail (L230‐D243) in AQP2 is essential for Lip5 interaction. Also, Lip5 interacts with AQP2 via its N‐terminal microtubule interacting and trafficking (MIT) domain. Lip5 is implicated in sorting of other proteins to multivesicular bodies (MVB) and lysosomes, from where proteins can recycle to the plasma membrane or are targeted for lysosomal degradation. Indeed, knockdown of endogenous LIP5 by shRNAs reduced AQP2 degradation in cultured cells. In conclusion, (1) AQP2 is the first cargo protein shows to bind Lip5, (2) Lip5 is expressed in renal principal cells and specifically binds AQP2, independent of its PKA phosphorylation and ubiquitination state and (3) Lip5 plays a role in the targeting of AQP2 to MVBs and, possibly, in the decision process whether AQP2 is targeted for lysosomal degradation or recycling.