JGP_201611740 613..622

613 Introduction The opening of eukaryotic sodium channels initiates the action potential in excitable tissues (Hille, 2001). The entire action potential functional cycle includes opening, closing, and inactivation of the sodium channels (Nav), and the closely coordinated functioning of potassium and calcium ion channels and sodium potassium ATPase pumps. These processes are essential in the functioning of all the closely related sodium channel isoforms found in different human tissues (central and peripheral nervous tissues, cardiac tissues, and muscle fibers). Mutations affecting the sodium channel functional cycles produce a range of channelopathies such as epilepsy, chronic pain, and ventricular fibrillations. As a result, these channels are important targets for pharmaceutical drug discovery. A limited number of prokaryotes (mostly extremophiles) also possess sodium channels (Ren et al., 2001; Koishi et al., 2004); the biological roles of these channels are as yet unclear, but appear to be related to motility and chemotaxis, and may play a role in maintaining intracellular sodium ion concentrations and/or pH homeostasis (Ito et al., 2004). Although the quaternary structures of eukaryotic and prokaryotic sodium channels are different (the former are four-domain monomers that form pseudotetrameric structures, whereas the latter are single-domain monomers that associate to form true tetramers), their overall folds are similar, with each domain or monomer consisting of a voltage sensor (VS; transmembrane helices S1 to S4), selectivity filter (SF), and pore region (transmembrane helices S5 and S6; Ahern et al., 2016). There is considerable (∼20– 35%) sequence identity between the transmembrane regions of prokaryotic and eukaryotic Navs, and consequently, they appear to have very similar secondary structures (Charalambous and Wallace, 2011). Hence, prokaryotic sodium channels, for which several crystal structures have been elucidated (Payandeh et al., 2011, 2012; McCusker et al., 2012; Zhang et al., 2012; Bagnéris et al., 2013, 2014; Shaya et al., 2014; Arrigoni et al., 2016; Naylor et al., 2016; Sula et al., 2017), tend to be good structural proxies for human sodium channels. This is indicated by the observations that several the prokaryotic sodium channels enable sodium flux with similar characteristics to human channels (Payandeh et al., 2011; Ulmschneider et al., 2013; DeCaen et al., 2014). One of the prokaryotic channels, NavMs, has also been shown to exhibit similar binding affinities and kinetics for eukaryotic channel blockers as human sodium channels (Bagnéris et al., 2014). Hence, the prokaryotic Nav structures were also anticipated to provide insight into structure–function relationships in eukaryotic channels. A new crystal structure of the full-length prokaryotic sodium channel NavMs has recently been determined that provides new insight into the linkage between channel activation and opening. A striking feature of this structure is the presence of a novel “interaction motif” (IM), which we proposed is intimately associated with channel opening. Conserved residues in this motif are linked with mutations causing human diseases. Here, we discuss how residues from the S3 of the VS, the S4–S5 linker, and the S6 helix of the pore domain interact and the evidence that they play a dynamic role in the mechanism of channel gating. Voltage-gated sodium channels enable the translocation of sodium ions across cell membranes and play crucial roles in electrical signaling by initiating the action potential. In humans, mutations in sodium channels give rise to several neurological and cardiovascular diseases, and hence they are targets for pharmaceutical drug developments. Prokaryotic sodium channel crystal structures have provided detailed views of sodium channels, which by homology have suggested potentially important functionally related structural features in human sodium channels. A new crystal structure of a full-length prokaryotic channel, NavMs, in a conformation we proposed to represent the open, activated state, has revealed a novel interaction motif associated with channel opening. This motif is associated with disease when mutated in human sodium channels and plays an important and dynamic role in our new model for channel activation. Interpreting the functional role of a novel interaction motif in prokaryotic sodium channels