Cytoplasmic potassium channel subunit that modulates the characteristics of the channel-forming alpha-subunits. Modulates action potentials via its effect on the pore-forming alpha subunits (Probable). 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 (PubMed:8183366, PubMed:15618540, PubMed:18222921). Promotes the closure of KCNA1, KCNA2 and KCNA5 channels (PubMed:10064591, PubMed:10650996, PubMed:16504945). Accelerates KCNA4 channel closure (PubMed:8183366). Accelerates the closure of heteromeric channels formed by KCNA1 and KCNA4 (PubMed:16504945). Accelerates the closure of heteromeric channels formed by KCNA2, KCNA5 and KCNA6 (PubMed:15618540). Enhances KCNB1 and KCNB2 channel activity (By similarity). Binds NADPH; this is required for efficient down-regulation of potassium channel activity. Has NADPH-dependent aldoketoreductase activity (PubMed:18222921). Oxidation of the bound NADPH strongly decreases N-type inactivation of potassium channel activity (PubMed:18222921, PubMed:21436029). Homotetramer (PubMed:18222921). Interaction with tetrameric potassium channel alpha subunits gives rise to a heterooctamer (Probable). Identified in potassium channel complexes containing KCNA1, KCNA2, KCNA4, KCNA5, KCNA6, KCNAB1 and KCNAB2 (PubMed:9334400, PubMed:16504945). Interacts with KCNA1 (PubMed:10064591). Interacts with the dimer formed by GNB1 and GNG2; this enhances KCNA1 binding (PubMed:10064591). Interacts with KCNA4 (By similarity). Interacts with KCNB2 and KCNA5 (By similarity). Interacts with SQSTM1 (PubMed:10477520). Part of a complex containing KCNA1, KCNA4 and LGI1; interaction with LGI1 inhibits down-regulation of KCNA1 channel activity (PubMed:16504945). Recruited to the cytoplasmic side of the cell membrane via its interaction with pore-forming potassium channel alpha subunits. Detected in brain (PubMed:9334400). Detected in hippocampus, in the middle third of the molecular layer of the dentate gyrus and in the mossy fiber zone of the CA3 region (PubMed:9334400). Detected in globus pallidus and pars reticulata of the substantia nigra (at protein level) (PubMed:9334400). Specifically expressed in the nervous system (PubMed:8183366). Detected in mesenteric arteries (PubMed:15618540). 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.