Sodium channel β1 subunit-mediated modulation of Nav1.2 currents and cell surface density is dependent on interactions with contactin and ankyrin

Voltage-gated sodium channels are composed of a pore-forming alpha subunit and at least one auxiliary beta subunit. Both beta1 and beta2 are cell adhesion molecules that interact homophilically, resulting in ankyrin recruitment. In contrast, beta1, but not beta2, 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 beta1-mediated enhancement of sodium channel cell surface density, including elucidating structure-function relationships for beta1 association with contactin, ankyrin, and Na(v)1.2. beta1/beta2 subunit chimeras were used to assign putative sites of contactin interaction to two regions of the beta1 Ig loop. Recent studies have shown that glutathione S-transferase fusion proteins containing portions of Na(v)1.2 intracellular domains interact directly with ankyrin(G). We show that native Na(v)1.2 associates with ankyrin(G) in cells in the absence of beta subunits and that this interaction is enhanced in the presence of beta1 but not beta1Y181E, a mutant that does not interact with ankyrinG. beta1Y181E does not modulate Na(v)1.2 channel function despite efficient association with Na(v)1.2 and contactin. beta1Y181E increases Na(v)1.2 cell surface expression, but not as efficiently as wild type beta1. beta1/beta2 chimeras exchanging various regions of the beta1 Ig loop were all ineffective in increasing Na(v)1.2 cell surface density. Our results demonstrate that full-length beta1 is required for channel modulation and enhancement of sodium channel cell surface expression.