Abstract
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-μs 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.
Highlights
Voltage-gated sodium (Nav) channels are widely distributed in the central and peripheral nervous systems where they participate in essential functions, including heartbeat, muscle contraction and brain activity [1, 2]
Each of these domains/subunits consists of 6 helical trans-membrane spanning segments, S1-S6, where S1-S4 make up a voltage sensor domain (VSD) and S5-S6 the pore domain (PD)
We suggest that optimal conduction requires the greater occupancy of the 3-ion state which is achieved when the crystallographic E177-S178 H-bond is lost on 2 opposite monomers, allowing the SHFS carboxylate groups to reach toward the center of the selectivity filter (SF) and coordinate 2 Na+ ions concurrently
Summary
Voltage-gated sodium (Nav) channels are widely distributed in the central and peripheral nervous systems where they participate in essential functions, including heartbeat, muscle contraction and brain activity [1, 2]. Human Navs consist of four domains, DI-DIV, linked together to form one long polypeptide chain, whereas the simpler bacterial channels are made up of four identical subunits [13] (Fig 1A) Each of these domains/subunits consists of 6 helical trans-membrane spanning segments, S1-S6, where S1-S4 make up a voltage sensor domain (VSD) and S5-S6 the pore domain (PD). Between these two latter segments is a P-loop that includes a narrow ion selectivity filter (SF) [13] (Fig 1B and 1C), which establishes an ion preference “fingerprint” (permeability Li+~Na+ > K+~Cs+~Rb+) that is the same for both bacterial and human Navs [1, 17, 18].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
