Pore-forming (alpha) subunit of voltage-gated rapidly inactivating A-type potassium channels. May contribute to I(To) current in heart and I(Sa) current in neurons. Channel properties are modulated by interactions with other alpha subunits and with regulatory subunits. Homotetramer or heterotetramer with KCND1 and/or KCND2. Interacts with DLG1. Associates with the regulatory subunits KCNIP1, KCNIP2, KCNIP3 and KCNIP4. Interacts with KCNE1, KCNE2, SCN1B and KCNAB1 (By similarity). Interaction with palmitoylated KCNIP2 and KCNIP3 enhances cell surface expression. Highly expressed in brain, in particular in the retrosplenial cortex, medial habenula, anterior thalamus, hippocampus, cerebellum and lateral geniculate and superior colliculus. Highly expressed in heart atrium (at protein level) and throughout the ventricle wall, in lung and vas deferens. The segment S4 is probably the voltage-sensor and is characterized by a series of positively charged amino acids at every third position. Regulated through phosphorylation at Ser-569 by CaMK2D. Belongs to the potassium channel family. D (Shal) (TC 1.A.1.2) subfamily. Kv4.3/KCND3 sub-subfamily.