Cytoplasmic potassium channel subunit that modulates the characteristics of the channel-forming alpha-subunits (By similarity). Modulates action potentials via its effect on the pore-forming alpha subunits (By similarity). Promotes expression of the pore-forming alpha subunits at the cell membrane, and thereby increases channel activity (By similarity). Mediates closure of delayed rectifier potassium channels by physically obstructing the pore via its N-terminal domain and increases the speed of channel closure for other family members (By similarity). Binds NADPH; this is required for efficient down-regulation of potassium channel activity (By similarity). Has NADPH-dependent aldoketoreductase activity (By similarity). Oxidation of the bound NADPH strongly decreases N-type inactivation of potassium channel activity (By similarity). Homotetramer (By similarity). Interaction with tetrameric potassium channel alpha subunits gives rise to a heterooctamer (Probable). Recruited to the cytoplasmic side of the cell membrane via its interaction with pore-forming potassium channel alpha subunits. In the inner ear, expression detected in the otocyst at embryonic day 3 and in the whole cochlea at E 6. Expressed throughout embryonic development and adulthood. The N-terminal domain of the beta subunit mediates closure of delayed rectifier potassium channels by physically obstructing the pore. Belongs to the shaker potassium channel beta subunit family.