The Sodium Channel β3-Subunit Induces Multiphasic Gating in NaV1.3 and Affects Fast Inactivation via Distinct Intracellular Regions

摘要

Electrical excitability in neurons depends on the activity of membrane-bound voltage gated sodium channels (Nav) that are assembled from an ion conducting α-subunit and often auxiliary β-subunits. The α-subunit isoform Nav1.3 occurs in peripheral neurons together with the Nav β3-subunit, both of which are coordinately up-regulated in rat dorsal root ganglion neurons after nerve injury. Here we examine the effect of the β3-subunit on the gating behavior of Nav1.3 using whole cell patch clamp electrophysiology in HEK-293 cells. We show that β3 depolarizes the voltage sensitivity of Nav1.3 activation and inactivation and induces biphasic components of the inactivation curve. We detect both a fast and a novel slower component of inactivation, and we show that the β3-subunit increases the fraction of channels inactivating by the slower component. Using CD and NMR spectroscopy, we report the first structural analysis of the intracellular domain of any Nav β-subunit. We infer the presence of a region within the β3-subunit intracellular domain that has a propensity to form a short amphipathic α-helix followed by a structurally disordered sequence, and we demonstrate a role for both of these regions in the selective stabilization of fast inactivation. The complex gating behavior induced by β3 may contribute to the known hyperexcitability of peripheral neurons under those physiological conditions where expression of β3 and Nav1.3 are both enhanced.