Co-assembly of Kv4 α Subunits with K+ Channel-interacting Protein 2 Stabilizes Protein Expression and Promotes Surface Retention of Channel Complexes*

Abstract

Members of the KMembers of the Shal subfamily of voltage-gated K ϩ (Kv) channel pore-forming (␣) subunits encode rapidly activating and inactivating Kv channels that also recover rapidly from inactivation and are important in the generation of I A channels in neurons (1-4) and I to channels in cardiac myocytes (5, 6). Accumulating evidence suggests that functional Kv4 channels reflect the assembly of Kv4 ␣ subunits with one or more Kv channel accessory subunits and other regulatory proteins that influence channel cell surface expression and/or biophysical properties (7). The K ϩ channel-interacting proteins (KChIP), 4 members of the Neuronal Calcium Sensor superfamily (8, 9), for example, are cytosolic accessory subunits that were initially identified in a yeast two-hybrid screen using the N terminus of Kv4.2 as bait (10). Heterologous co-expression with accessory KChIP subunits increases Kv4.2 current densities, as well as altering the timeand voltage-dependent properties of currents (10 -14). Truncation of the first 40 amino acids in the Kv4.2 N terminus results in the loss of KChIP-mediated current modulation but a paradoxical increase in Kv4.2 current densities (11,14). Progressive truncation of the N terminus (up to 40 amino acids) was reported to result in progressively greater increases in current densities, although it was not determined whether the observed increase reflected increased total and/or cell surface Kv4.2 protein expression. Previous mutagenesis studies have been interpreted as suggesting that the major binding site for KChIP2 on the Kv4.2 N terminus is between residues 11 and 23 (15). Structural analysis of the N terminus of Kv4.3, crystallized in complex with the core region (conserved across all family members) of KChIP1 (16,17), however, revealed that KChIPs bind the distal 20 N-terminal residues of Kv4 ␣ subunits in a hydrophobic binding pocket.The KChIP-mediated increases in Kv4.2 current densities have been ascribed to increased trafficking of channels from the endoplasmic reticulum (ER) to the surface membrane (10,11,18), although a precise motif that regulates ER retention has yet to be identified. The amino acid motif Arg-Xaa-Arg (RXR) has been shown to play a role in ER retention of inwardly rectifying K ϩ channels (Kir) (19). For example, ATP-sensitive K ϩ channels (K ATP ) formed by the co-assembly of Kir6 ␣ subunits and sulfonylurea receptor accessory subunits are retained in the ER when either subunit is expressed alone. Subunit co-assembly, however, masks RXR retention motifs, promoting forward trafficking to the cell surface of channel complexes (20). Although the Kv4.2 N terminus contains (at positions 35-37) an RKR sequence, previous studies suggest that this sequence does not function as an ER retention motif (18). It has also been suggested that Kv4.2 alone traffics out of the ER but fails to progress beyond the Golgi complex in the absence of KChIPs (21 4 The abbreviations used are: KChIP, K ϩ channel-interacting protein; ER, endoplasmic reticulum; HP, holding potentia...

Augmentation of Kv4.2-encoded Currents by Accessory Dipeptidyl Peptidase 6 and 10 Subunits Reflects Selective Cell Surface Kv4.2 Protein Stabilization

“…Several previous studies also suggest a critical role for the KChIPs in the generation of native Kv4-encoded channels and that Kv4-KChIP protein-protein interactions are required for the stability of both proteins (25,(35)(36)(37)(38)(39). Interestingly, a similar stabilizing effect has been described for the Shaker channel accessory subunit, Sleepless, in Drosophila (40).…”

The Stoichiometry and Biophysical Properties of the Kv4 Potassium Channel Complex with K+ Channel-interacting Protein (KChIP) Subunits Are Variable, Depending on the Relative Expression Level

“…According to previous studies, the single channel conductance of Kv4 (ϳ5 picosiemens) was not affected by the co-expression of KChIP proteins (38,42). It was reported that KChIP binding to the N terminus of Kv4.2 promotes the trafficking of Kv4.2 to the plasma membrane from the endoplasmic reticulum or Golgi (22,23,39,43). Consistent with these findings, we also observed that the surface expression of Kv4.2 increased in accordance with the increase in co-expressed KChIP4 (Fig.…”

“…Does the Stoichiometry of the Kv4.2⅐KChIP4 Complex Change on the Cell Membrane?-As mentioned above, KChIP is known to promote the translocation of Kv4.2 from the endoplasmic reticulum or Golgi to the cell surface (22,23,39,43). Thus, KChIP binds to Kv4.2 in the endoplasmic reticulum and is required for the transport of the complex to the cell surface.…”

Modulation of the Voltage-gated Potassium Channel (Kv4.3) and the Auxiliary Protein (KChIP3) Interactions by the Current Activator NS5806

“…Electrophysiological studies support the idea that both the N and C termini of Kv4 channels interact to regulate the inactivation kinetics (40,42), opening the possibility that such interactions are modulated by binding of KChIPs at the N terminus (34,38). It is also proposed that the N terminus functions as a membrane transport control, which in the absence of KChIPs anchors Kv4 channels near the perinuclear region of the cell (43,44). The association of KChIPs at the T1 domain of Kv4 results in translocation to the membrane and a concomitant increase in total K ϩ current.…”