The extracellular domain of the beta 1 subunit is both necessary and sufficient for beta 1-like modulation of sodium channel gating

The type IIA voltage-gated sodium Na+ channel from rat brain is composed of a large, pore-forming alpha subunit and the auxiliary subunits beta 1 and beta 2. When expressed in Xenopus oocytes, the beta 1 subunit modulates the gating properties of the type IIA alpha subunit, resulting in acceleration of both inactivation and recovery from inactivation and in a negative shif t in the voltage dependence of fast inactivation. The beta 1 subunit is com posed of an extracellular domain with a single immunoglobulin-libe fold, a single transmembrane segment, and a small intracellular domain. A series of chimeras with exchanges of domains between the Na+ channel beta 1 and beta 2 subunits and between pi and the structurally related protein myelin P0 w ere constructed and analyzed by two-microelectrode voltage clamp in Xenopus oocytes. Only chimeras containing the pi extracellular domain were capable of beta 1-like modulation of Na+ channel gating. Neither the transmembrane segment nor the intracellular domain was required for modulation, although mutation of Glu(158) within the transmembrane domain altered the voltage d ependence of steady-state inactivation. A truncated beta 1 subunit was engi neered in which the pi extracellular domain was fused to a recognition sequ ence for attachment of a glycosylphosphatidylinositol membrane anchor. The beta 1(ec)-glycosylphosphatidylinositol protein fully reproduced modulation of Na+ channel inactivation and recovery from inactivation by wild-type be ta 1. Our findings demonstrate that extracellular domain of the beta 1 subu nit is both necessary and sufficient for the modulation of Na+ channel gati ng.