The gamma-subunit of ENaC is more important for channel surface expression than the beta-subunit |
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Authors: | Konstas Angelos-Aristeidis Korbmacher Christoph |
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Institution: | University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom. |
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Abstract: | The amiloride-sensitiveepithelial sodium channel (ENaC) plays a critical role in fluid andelectrolyte homeostasis and is composed of three homologous subunits: , , and . Only heteromultimeric channels made of ![alpha](http://ajpcell.physiology.org/math/12pt/normal/alpha.gif) ![beta](http://ajpcell.physiology.org/math/12pt/normal/beta.gif) ENaCare efficiently expressed at the cell surface, resulting in maximallyamiloride-sensitive currents. To study the relative importance ofvarious regions of the - and -subunits for the expression offunctional ENaC channels at the cell surface, we constructedhemagglutinin (HA)-tagged - -chimeric subunits composed of -and -subunit regions and coexpressed them with HA-tagged ![alpha](http://ajpcell.physiology.org/math/12pt/normal/alpha.gif) - and![alpha](http://ajpcell.physiology.org/math/12pt/normal/alpha.gif) -subunits in Xenopus laevis oocytes. The whole cellamiloride-sensitive sodium current ( Iami) andsurface expression of channels were assessed in parallel using thetwo-electrode voltage-clamp technique and a chemiluminescence assay.Because coexpression of ![alpha](http://ajpcell.physiology.org/math/12pt/normal/alpha.gif) ENaC resulted in larger Iami and surface expression compared withcoexpression of ![alpha](http://ajpcell.physiology.org/math/12pt/normal/alpha.gif) ENaC, we hypothesized that the -subunit ismore important for ENaC trafficking than the -subunit. Usingchimeras, we demonstrated that channel activity is largely preservedwhen the highly conserved second cysteine rich domains (CRD2) of the - and -subunits are exchanged. In contrast, exchanging the wholeextracellular loops of the - and the -subunits largely reducedENaC currents and ENaC expression in the membrane. This indicates thatthere is limited interchangeability between molecular regions of thetwo subunits. Interestingly, our chimera studies demonstrated that theintracellular termini and the two transmembrane domains of ENaC aremore important for the expression of functional channels at the cellsurface than the corresponding regions of ENaC. |
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