Sodium Channel β1 Subunit-mediated Modulation of Nav1.2 Currents and Cell Surface Density Is Dependent on Interactions with Contactin and Ankyrin

Abstract

Voltage-gated sodium channels are composed of a pore-forming α subunit and at least one auxiliary β subunit. Both β1 and β2 are cell adhesion molecules that interact homophilically, resulting in ankyrin recruitment. In contrast, β1, but not β2, interacts heterophilically with contactin, resulting in increased levels of cell surface sodium channels. We took advantage of these results to investigate the molecular basis of β1-mediated enhancement of sodium channel cell surface density, including elucidating structure-function relationships for β1 association with contactin, ankyrin, and Nav1.2. β1/β2 subunit chimeras were used to assign putative sites of contactin interaction to two regions of the β1 Ig loop. Recent studies have shown that glutathione S-transferase fusion proteins containing portions of Nav1.2 intracellular domains interact directly with ankyrinG. We show that native Na v1.2 associates with ankyrinG in cells in the absence of β subunits and that this interaction is enhanced in the presence of β1 but not β1Y181E, a mutant that does not interact with ankyrin G. β1Y181E does not modulate Nav1.2 channel function despite efficient association with Nav1.2 and contactin. β1Y181E increases Nav1.2 cell surface expression, but not as efficiently as wild type β1. β1/β2 chimeras exchanging various regions of the β1 Ig loop were all ineffective in increasing Na v1.2 cell surface density. Our results demonstrate that full-length β1 is required for channel modulation and enhancement of sodium channel cell surface expression.