1.A.1.2.5
Voltage-gated K⁺ channel subfamily D, member 2, Kv4.2 or KCND2, in neurons and muscle; forms complexes with auxiliary subunits and scaffolding proteins via its N-terminus, influencing trafficking, temperature-sensitivity and gating (Radicke et al. 2013).These subunits are (1) dipeptidyl-peptidase-like type II transmembrane proteins typified by DPPX-S (e.g., protein 6, P42658; 865 aas, TC#8.A.51), and (2) cytoplasmic Ca²⁺ binding proteins known as K⁺ channel interacting proteins (KChIPs; TC#8.A.82.2.2; Q6PIL6) (Seikel and Trimmer 2009).  The C-terminus interacts with KChIP2 to influence gating, surface trafficking and gene expression (Han et al., 2006; Schwenk et al., 2008). KChIPs (250 aas for mouse KChIP4a; Q6PHZ8) are homologous to domains in NADPH oxidases (5.B.1). Heteropoda toxin 2 (P58426; PDB 1EMX; TC#8.B.5.2.2) interactions with Kv4.3 and Kv4.1 give rise to differences in gating modifications (DeSimone et al., 2011).  Mutations cause autism and seizures due to a slowing of channel inactivation (Lee et al. 2014).  The stoichiometry of Kv4.2 and DPP6 is 4:4 (Soh and Goldstein 2008). Neferine, an isoquinoline alkaloid from plants, inhibits Kv4.3 channels, probably by blocking the open state (Wang et al. 2015). SUMOylating (derivatizing with a small ubiquitin-like modifier) at two distinct sites on Kv4.2 increases surface expression and decreases current amplitude (Welch et al. 2019). Modulation of voltage-gated potassium (Kv) channels by auxiliary subunits is central to the physiological function of channels in the brain and heart. Native Kv4 tetrameric channels form macromolecular ternary complexes with two auxiliary beta-subunits-intracellular Kv channel-interacting proteins (KChIPs) and transmembrane dipeptidyl peptidase-related proteins (DPPs)-to evoke rapidly activating and inactivating A-type currents, which prevent the backpropagation of action potentials (see above). Kise et al. 2021 investigated the modulatory mechanisms of Kv4 channel complexes, reporting cryo-EM structures of the Kv4.2-DPP6S-KChIP1 dodecameric complex, the Kv4.2-KChIP1 and Kv4.2-DPP6S octameric complexes, and Kv4.2 alone. The structure of the Kv4.2-KChIP1 complex revealed that the intracellular N terminus of Kv4.2 interacts with its C-terminus that extends from the S6 gating helix of the neighbouring Kv4.2 subunit. KChIP1 captures both the N and the C terminus of Kv4.2. Thus, KChIP1 prevents N-type inactivation and stabilizes the S6 conformation to modulate gating of the S6 helices within the tetramer. Unlike the reported auxiliary subunits of voltage-gated channel complexes, DPP6S interacts with the S1 and S2 helices of the Kv4.2 voltage-sensing domain, which suggests that DPP6S stabilizes the conformation of the S1-S2 helices. DPP6S may therefore accelerate the voltage-dependent movement of the S4 helices. KChIP1 and DPP6S do not directly interact with each other in the Kv4.2-KChIP1-DPP6S ternary complex. Thus, two distinct modes of modulation contribute in an additive manner to evoke A-type currents from the native Kv4 macromolecular complex (Kise et al. 2021).