Voltage-gated sodium channels in brain neurons are complexes of a pore-forming a subunit with smaller 81 and 82 subunits. cDNA cloning and sequencing showed that the 82 subunit is a 186 residue glycoprotein with an extracellular NH*-terminal domain containing an immunoglobulin-like fold with similarity to the neural cell adhesion molecule (CAM) contactin, a single transmembrane segment, and a small intracellular domain. Coexpression of 82 with a subunits in Xenopus oocytes increases functional expression, modulates gating, and causes up to a 4-fold increase in the capacitance of the oocyte, which results from an increase in the surface area of the plasma membrane microvilli. 82 subunits are unique among the auxiliary subunits of ion channels in combining channel modulation with a CAM motif and the ability to expand the cell membrane surface area. They may be important regulators of sodium channel expression and localization in neurons.