Activation of a latent nuclear localization signal in the NH2 terminus of γ-ENaC initiates feedback regulation of channel activity

Mironova E, Stockand JD. Activation of a latent nuclear localization signal in the NH2 terminus of gamma-ENaC initiates feedback regulation of channel activity. Am J Physiol Renal Physiol 298: F1188-F1196, 2010. First published February 10, 2010; doi:10.1152/ajprenal.00600.2009.-Proteolytic enzymes cleave the epithelial Na+ channel (ENaC) at several positions releasing, in part, the NH2 terminus of the gamma-subunit. Cleavage increases ENaC activity by increasing open probability; however, the role of polypeptides cleaved from the channel core remains unclear. We find that the cytosolic NH2 terminus of gamma-ENaC unexpectedly targets to the nucleus being particularly strong in nucleoli. In contrast, the cytosolic COOH terminus targets to the cytoplasm and plasma membrane in a manner similar to full-length subunits. Targeting of the cytosolic NH2 terminus of gamma-ENaC to the nucleus has functional consequences for coexpression of eGFP-fusion proteins containing this segment of the channel, but not the COOH terminus, decrease ENaC activity in a dose-dependent manner. The mechanism of this negative regulation is associated with a decrease in the functional half-life of ENaC at the plasma membrane. Inspection of the primary amino acid sequence of gamma-ENaC reveals possible nuclear localization signals (NLS) conserved at the extreme NH2 terminus and just preceding the first transmembrane domain. Disruption of the putative NLS preceding the first transmembrane domain in gamma-ENaC but not that at the extreme NH2 terminus abolishes both targeting to the nucleus and negative regulation of ENaC activity. These findings are consistent with the release of the NH2 terminus of gamma-ENaC following cleavage being functionally important for signaling to the nucleus in a manner similar to Notch signaling and release of the cytosolic COOH-terminal tail of polycystin-1.