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Determinants of selective ion permeation in the epithelial Na+ channel.

ABSTRACT: The epithelial Na+ channel (ENaC) is a key transporter mediating and controlling Na+ reabsorption in many tight epithelia. A very high selectivity for Na+ over other cations, including K+, is a hallmark of this channel. This selectivity greatly exceeds that of the closely related acid-sensing channels (ASICs). Here, we assess the roles of two regions of the ENaC transmembrane pore in the determination of cation selectivity. Mutations of conserved amino acids with acidic side chains near the cytoplasmic end of the pore diminish macroscopic currents but do not decrease the selectivity of the channel for Na+ versus K+ In the WT channel, voltage-dependent block of Na+ currents by K+ or guanidinium+, neither of which have detectable conductance, suggests that these ions permeate only ?20% of the transmembrane electric field. According to markers of the electric field determined by Zn2+ block of cysteine residues, the site of K+ block appears to be nearer to the extracellular end of the pore, close to a putative selectivity filter identified using site-directed mutations. To test whether differences in this part of the channel account for selectivity differences between ENaC and ASIC, we substitute amino acids in the three ENaC subunits with those present in the ASIC homotrimer. In this construct, Li:Na selectivity is altered from that of WT ENaC, but the high Na:K selectivity is maintained. We conclude that a different part of the pore may constitute the selectivity filter in the highly selective ENaC than in the less-selective ASIC channel.

SUBMITTER: Yang L

PROVIDER: S-EPMC6168236 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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Determinants of selective ion permeation in the epithelial Na+ channel.

Yang Lei L Palmer Lawrence G LG

The Journal of general physiology 20180822 10

The epithelial Na+ channel (ENaC) is a key transporter mediating and controlling Na+ reabsorption in many tight epithelia. A very high selectivity for Na+ over other cations, including K+, is a hallmark of this channel. This selectivity greatly exceeds that of the closely related acid-sensing channels (ASICs). Here, we assess the roles of two regions of the ENaC transmembrane pore in the determination of cation selectivity. Mutations of conserved amino ...[more]

PMID: 30135076

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Project description:Bacterial and human voltage-gated sodium channels (Navs) exhibit similar cation selectivity, despite their distinct EEEE and DEKA selectivity filter signature sequences. Recent high-resolution structures for bacterial Navs have allowed us to learn about ion conduction mechanisms in these simpler homo-tetrameric channels, but our understanding of the function of their mammalian counterparts remains limited. To probe these conduction mechanisms, a model of the human Nav1.2 channel has been constructed by grafting residues of its selectivity filter and external vestibular region onto the bacterial NavRh channel with atomic-resolution structure. Multi-?s fully atomistic simulations capture long time-scale ion and protein movements associated with the permeation of Na+ and K+ ions, and their differences. We observe a Na+ ion knock-on conduction mechanism facilitated by low energy multi-carboxylate/multi-Na+ complexes, akin to the bacterial channels. These complexes involve both the DEKA and vestibular EEDD rings, acting to draw multiple Na+ into the selectivity filter and promote permeation. When the DEKA ring lysine is protonated, we observe that its ammonium group is actively participating in Na+ permeation, presuming the role of another ion. It participates in the formation of a stable complex involving carboxylates that collectively bind both Na+ and the Lys ammonium group in a high-field strength site, permitting pass-by translocation of Na+. In contrast, multiple K+ ion complexes with the DEKA and EEDD rings are disfavored by up to 8.3 kcal/mol, with the K+-lysine-carboxylate complex non-existent. As a result, lysine acts as an electrostatic plug that partially blocks the flow of K+ ions, which must instead wait for isomerization of lysine downward to clear the path for K+ passage. These distinct mechanisms give us insight into the nature of ion conduction and selectivity in human Nav channels, while uncovering high field strength carboxylate binding complexes that define the more general phenomenon of Na+-selective ion transport in nature.

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Determinants of selective ion permeation in the epithelial Na+ channel.

Publications

Determinants of selective ion permeation in the epithelial Na<sup>+</sup> channel.

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