Membrane Biology
Thumb domains of the three epithelial Na+ channel subunits have distinct functions
The epithelial Na+ channel (ENaC) possesses a large extracellular domain formed by a β-strand core enclosed by three peripheral α-helical subdomains, which have been dubbed thumb, finger, and knuckle. Here we asked whether the ENaC thumb domains play specific roles in channel function. To this end, we examined the characteristics of channels lacking a thumb domain in an individual ENaC subunit (α, β, or γ). Removing the γ subunit thumb domain had no effect on Na+ currents when expressed in Xenopus oocytes, but moderately reduced channel surface expression.Regulation of the epithelial Na+ channel by paraoxonase-2
Paraoxonase-2 (PON-2) is a membrane-bound lactonase with unique anti-oxidative and anti-atherosclerotic properties. PON-2 shares key structural elements with MEC-6, an endoplasmic reticulum–resident molecular chaperone in Caenorhabditis elegans. MEC-6 modulates the expression of a mechanotransductive ion channel comprising MEC-4 and MEC-10 in touch-receptor neurons. Because pon-2 mRNA resides in multiple rat nephron segments, including the aldosterone-sensitive distal nephron where the epithelial Na+ channel (ENaC) is expressed, we hypothesized that PON-2 would similarly regulate ENaC expression.Specific Palmitoyltransferases Associate with and Activate the Epithelial Sodium Channel
The epithelial sodium channel (ENaC) has an important role in regulating extracellular fluid volume and blood pressure, as well as airway surface liquid volume and mucociliary clearance. ENaC is a trimer of three homologous subunits (α, β, and γ). We previously reported that cytoplasmic residues on the β (βCys-43 and βCys-557) and γ (γCys-33 and γCys-41) subunits are palmitoylated. Mutation of Cys that blocked ENaC palmitoylation also reduced channel open probability. Furthermore, γ subunit palmitoylation had a dominant role over β subunit palmitoylation in regulating ENaC.Functional Roles of Clusters of Hydrophobic and Polar Residues in the Epithelial Na+ Channel Knuckle Domain
Background: There are regulatory interactions between ENaC and extracellular factors.Results: Mutations of multiple α subunit knuckle residues activate ENaC by suppressing the inhibitory effect of Na+. Channels lacking the β or γ subunit knuckle have processing defects.Conclusion: Interactions between the α subunit knuckle and palm/finger domains regulate ENaC.Significance: Intrasubunit domain-domain interactions have important regulatory roles.Intracellular Na+ Regulates Epithelial Na+ Channel Maturation
Epithelial Na+ channel (ENaC) function is regulated by the intracellular Na+ concentration ((Na+)i) through a process known as Na+ feedback inhibition. Although this process is known to decrease the expression of proteolytically processed active channels on the cell surface, it is unknown how (Na+)i alters ENaC cleavage. We show here that (Na+)i regulates the posttranslational processing of ENaC subunits during channel biogenesis. At times when (Na+)i is low, ENaC subunits develop mature N-glycans and are processed by proteases.
