Structural and functional features of the intracellular amino terminus of DEG/ENaC ion channels

The degenerin/epithelial sodium channel (DEG/ENaC) protein family includes related ion channel subunits from organisms ranging from the simple nematode Caenorhabditis elegans to humans. Members of this protein family have been implicated in functions as diverse as touch transduction and proprioception [1–4], pain sensation and maintenance of sodium balance [5]. Several blocks of sequence are conserved in DEG/ENaC subunits, but understanding structure/function relations in this channel class is in its infancy. There is only one conserved region in the intracellular amino termini of all DEG/ENaC family members, and this region is clearly critical for channel function (Figure 1). Several channel-inactivating substitutions in C. elegans degenerins affect the conserved domain [6,7] and a substitution in this motif in human βENaC causes the salt-wasting disorder pseudohypoaldosteronism, type I [8]. This region can influence ENaC gating properties [9,10] as well as ion permeation and selectivity properties [11] and has been implicated in endocytosis [12]. Still, how the conserved amino-terminal domain actually influences channel function remains a mystery. We have analyzed all novel degenerin family members revealed by the C. elegans genome sequencing project [13] and found that, with no exception, they contain the intracellular amino-terminal conserved motif (Figure 1b). Based on the genetic properties of mutations altering key amino acids within this motif in two nematode degenerins, UNC-8 and MEC-4, we propose that this region might serve as an interaction domain that associates with other proteins that form the channel complex. unc-8 is expressed in motor neurons and command interneurons of the nematode nervous system and has been implicated in proprioception and regulation of locomotion [6]. Dominant, gain-of-function mutations in the unc-8 gene cause transient neuronal swelling and dysfunction and render the canonical sinusoidal movement of the worm severely uncoordinated. Absence of the UNC-8 protein in unc-8 loss-of-function mutant strains results in a pronounced reduction of the amplitude and wavelength of the worm's normal sinusoidal movement [6]. Interestingly, the effects of a dominant mutation in the UNC-8 protein can be completely blocked by mutating the absolutely conserved histidine residue, within the conserved amino-terminal motif, to tyrosine, highlighting the functional importance of this motif. Suppression is observed both when the histidine substitution resides in cis, on the same protein molecule as the dominant mutation or in trans, on different co-expressed molecules, as observed in heterozygote animals carrying a dominant allele on one chromosome and a histidine-substituted allele on the other [14] (see also Supplementary material). …