Ion channels

Abstract:

The epithelial sodium channel (ENaC) and the secretory potassium channel (Kir1.1/ROMK) are expressed in the apical membrane of renal collecting duct principal cells where they provide the rate-limiting steps for Na(+) absorption and K(+) secretion. The cystic fibrosis transmembrane conductance regulator (CFTR) is thought to regulate the function of both ENaC and Kir1.1. We hypothesized that CFTR may provide a regulatory link between ENaC and Kir1.1. In Xenopus laevis oocytes co-expressing both ENaC and CFTR, the CFTR currents were 3-fold larger than those in oocytes expressing CFTR alone due to an increased expression of CFTR in the plasma membrane. ENaC was also able to increase Kir1.1 currents through an increase in surface expression, but only in the presence of CFTR. In the absence of CFTR, co-expression of ENaC was without effect on Kir1.1. ENaC-mediated CFTR-dependent up-regulation of Kir1.1 was reduced with a Liddle's syndrome mutant of ENaC. Furthermore, ENaC co-expressed with CFTR was without effect on the closely related K(+) channel, Kir4.1. We conclude that ENaC up-regulates Kir1.1 in a CFTR-dependent manner. CFTR may therefore provide the mechanistic link that mediates the coordinated up-regulation of Kir1.1 during the stimulation of ENaC by hormones such as aldosterone or antidiuretic hormone.

Abstract:

The amino-terminal and carboxy-terminal domains of inwardly rectifying potassium channel (Kir) subunits are both intracellular. A direct physical interaction between these two domains is involved in the response of Kir channels to regulatory factors such as G-proteins, nucleotides and intracellular pH. We have previously mapped the region within the N-terminal domain of Kir6.2 that interacts with the C-terminus. In this study we use a similar in vitro protein-protein interaction assay to map the regions within the C-terminus which interact with the N-terminus. We find that multiple interaction domains exist within the C-terminus: CID1 (amino acids (aa) 279-323), CID2 (aa 214-222) and CID3 (aa 170-204). These domains correlate with regions previously identified as making important contributions to Kir channel assembly and function. The highly conserved nature of the C-terminus suggests that a similar association with the N-terminus may be a feature common to all members of the Kir family of potassium channels, and that it may be involved in gating of Kir channels by intracellular ligands.