Distinct domains of the sodium channel β3-subunit modulate channel-gating kinetics and subcellular location

Electrical excitability in neurons depends on the expression and activity of voltage-gated sodium channels in the neuronal plasma membrane. The ion-conducting α-subunit of the channel is associated with auxiliary β-subunits of which there are four known types. In the present study, we describe the first detailed structure/function analysis of the β3-subunit. We correlate the effect of point mutations and deletions in β3 with the functional properties of the sodium channel and its membrane-targeting behaviour. We show that the extracellular domain influences sodium channel gating properties, but is not required for the delivery of β3 to the plasma membrane when expressed with the α-subunit. In contrast, the intracellular domain is essential for correct subunit targeting. Our results reveal the crucial importance of the Cys²¹–Cys⁹⁶ disulphide bond in maintaining the functionally correct β3 structure and establish a role for a second putative disulphide bond (Cys²–Cys²⁴) in modulating channel inactivation kinetics. Surprisingly, our results imply that the wild-type β3 molecule can traverse the secretory pathway independently of the α-subunit.