How does the β1 subunit accelerate pore opening in Nav channels? A possible mechanism could be a modulation of the kinetics of the rearrangements of the VS by the β1 subunits. We tested this hypothesis by measuring gating currents that directly report VS movement. Figure 1A shows gating current traces recorded HTS assay in Xenopus oocytes using activation protocols for both muscular (Nav1.4) and neuronal (Nav1.2) Nav
channels with or without coexpressed β1 subunits. In both channels, the kinetics of activating gating currents (see Figure S1 available online for a detailed fitting procedure) are accelerated approximately 2-fold in the presence of β1 subunits ( Figure 1B, open versus full Selleckchem VE821 symbols), in good agreement with the moderate acceleration of pore opening. These results constitute evidence for a direct modulation of the VS movement in Nav channels by the β1 subunits and provide a general
molecular basis to explain the modulatory role of these subunits on Nav channel function. The mechanism by which the β1 subunit accelerates VS kinetics in Nav channels is presently unknown to us. In the presence of the β1 subunit, the rearrangement of the VS exhibits positive cooperativity (Campos et al., 2007a and Chanda et al., 2004), which leads to accelerated VS kinetics (Chanda et al., 2004). Hence, it is tempting to speculate that the β1 subunit may act by coupling the movement of VS in adjacent domains of the Nav channel. Yet, even in the absence of the β1 subunit, the gating currents develop up to 3-fold faster in Nav channels relative to prototypical
Shaker-type Kv channels for voltages near the threshold of activation of action potentials (i.e., around −40 mV, Figures 1B and 1C). What are the molecular determinants and mechanism underlying this intrinsic kinetics difference? It is now well established that the activation of the four VSs in the α subunit of Nav channels is asynchronous: the VSs in the first three domains (DI–DIII) rearranges rapidly and controls pore opening, while the VSs in DIV rearranges with slow kinetics comparable to those of VSs found in Shaker-type Kv channels and controls fast inactivation of the sodium conductance (Chanda and Bezanilla, 2002, too Goldschen-Ohm et al., 2013 and Gosselin-Badaroudine et al., 2012). Hence, these observations suggest that the rapid VSs of Nav DI–DIII may possess specific molecular determinants that are absent in the slow VSs of Nav DIV and of Shaker-type Kv channels. In order to identify such determinants, we compared the amino acid sequence of the VSs from Nav1.4 DI–DIII to the slow VSs from Nav1.4 DIV, from Shaker-type Kv channels and also from slow-activating bacterial Nav channels (Kuzmenkin et al., 2004). Two positions bear either hydrophilic residues in rapid VSs or hydrophobic residues in slow VSs.
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