Function: Cytoplasmic potassium channel subunit that modulates the characteristics of the channel-forming alpha-subunits (PubMed:9763623, PubMed:21357749). Contributes to the regulation of nerve signaling, and prevents neuronal hyperexcitability (By similarity). Promotes expression of the pore-forming alpha subunits at the cell membrane, and thereby increases channel activity (PubMed:10896669, PubMed:16770729, PubMed:18003609, PubMed:21357749). Promotes potassium channel closure via a mechanism that does not involve physical obstruction of the channel pore (PubMed:21357749). Modulates the functional properties of KCNA4 (PubMed:9763623). Modulates the functional properties of KCNA5 (By similarity). Enhances KCNB2 channel activity (By similarity). Binds NADPH and has NADPH-dependent aldoketoreductase activity (PubMed:18672894, PubMed:21209188). Has broad substrate specificity and can catalyze the reduction of methylglyoxal, 9,10-phenanthrenequinone, prostaglandin J2, 4-nitrobenzaldehyde, 4-nitroacetophenone and 4-oxo-trans-2-nonenal (in vitro) (PubMed:18672894).