Structural and functional diversity of voltage-gated K(v)1-type potassium channels in rat brain is enhanced by the association of two different types of subunits, the membrane-bound, pore-forming alpha-subunits and a peripheral beta-subunit. We have cloned a beta-subunit (K-v beta 1) that is specifically expressed in the rat nervous system. Association of K-v beta 1 with alpha-subunits confers rapid A-type inactivation on non-inactivating K(v)1 channels (delayed rectifiers) in expression systems in vitro. This effect is mediated by an inactivating ball domain in the K-v beta 1 amino terminus.