STRUCTURE AND FUNCTION OF THE BETA-2 SUBUNIT OF BRAIN SODIUM-CHANNELS, A TRANSMEMBRANE GLYCOPROTEIN WITH A CAM MOTIF

Voltage-gated sodium channels in brain neurons are complexes of a pore -forming alpha subunit with smaller beta 1 and beta 2 subunits, cDNA c loning and sequencing showed that the beta 2 subunit is a 186 residue glycoprotein with an extracellular NH2-terminal domain containing an i mmunoglobulin-like fold with similarity to the neural cell adhesion mo lecule (CAM) contactin, a single transmembrane segment, and a small in tracellular domain, Coexpression of beta 2 with alpha subunits in Xeno pus oocytes increases functional expression, modulates gating, and cau ses up to a 4-fold increase in the capacitance of the oocyte, which re sults from an increase in the surface area of the plasma membrane micr ovilli. beta 2 subunits are unique among the auxiliary subunits of ion channels in combining channel modulation with a CAM motif and the abi lity to expand the cell membrane surface area. They may be important r egulators of sodium channel expression and localization in neurons.