Downregulation of urea transporter UT-A1 activity by 14-3-3 protein

Urea transporter (UT)-A1 in the kidney inner medulla plays a critical role in the urinary concentrating mechanism and thereby in the regulation of water balance. The 14-3-3 proteins are a family of seven isoforms. They are multifunctional regulatory proteins that mainly bind to phosphorylated serine/threonine residues in target proteins. In the present study, we found that all seven 14-3-3 isoforms were detected in the kidney inner medulla. However, only the 14-3-3 gamma-isoform was specifically and highly associated with UT-A1, as demonstrated by a glutathione-S-transferase-14-3-3 pulldown assay. The cAMP/adenylyl cyclase stimulator forskolin significantly enhanced their binding. Coinjection of 14-3-3 gamma cRNA into oocytes resulted in a decrease of UT-A1 function. In addition, 14-3-3 gamma increased UT-A1 ubiquitination and protein degradation. 14-3-3 gamma can interact with both UT-A1 and mouse double minute 2, the E3 ubiquitin ligase for UT-A1. Thus, activation of cAMP/PKA increases 14-3-3 gamma interactions with UT-A1 and stimulates mouse double minute 2-mediated UT-A1 ubiquitination and degradation, thereby forming a novel regulatory mechanism of urea transport activity.