Structure of the KChIP1–Kv4.3 T1 complex. (a) Overall structure of the complex as seen from the cytosolic side. KChIPs (cyan) and Kv4.3 components (orange) are shown as ribbons. Ca²⁺ (magenta spheres) and Zn²⁺ (black spheres) ions are also indicated. A–D and E–H label Kv4.3 and KChIP1 chains, respectively. Box marks the region shown in Figure 2d. (b) Side view of the two octameric complexes in the asymmetric unit, which are apposed on their cytoplasmic faces. Kv4.3 N-terminal domains (orange and red) and KChIP1s (cyan and slate blue) are shown. Dashed box marks the docking loop.

Details of KChIP1–Kv4.3 T1N and T1 domain interactions. (a) Comparison of T1N regions from human (Kv4.1, Kv4.2 and Kv4.3) and tunicate (Kv4 C)18 Kv4 channels. Orange indicates residues from the T1 domain. Rat and human Kv4.3 sequences are identical over the depicted region. Box indicates Kv4.3 T1N Met20. (b) Ribbon diagram of site 1 shows KChIP1 (cyan) and the Kv4.3 T1N helix, T1N linker and T1 domain (orange). KChIP1 helices H1–H10 are indicated. Ca²⁺ ions are shown as magenta spheres. (c) The three aromatic residues that anchor the T1N helix to KChIP1 are indicated as sticks (orange). The KChIP1 surface is shown with carbon (cyan), nitrogen (blue), oxygen (red) and sulfur (yellow) atoms indicated. (d) Ribbon diagram of site 2 interactions. Interface residues are shown as sticks and are labeled. (e) Superposition of KChIP1 from the KChIP1–Kv4.3_1–143 complex (cyan) and the KChIP1 monomer structure17 (dark blue) reveals the conformational change in the H10 helix (green). The H10 helix from the KChIP1–Kv4.3_1–143 complex is indicated. The T1N helix is not shown. Calcium ions are shown as magenta spheres.

SAXS analysis of the KChIP1–Kv4.3 T1 complex. (a) Top two images, superposition of the ab initio SAXS model with the X-ray structure of the KChIP1–Kv4.3 T1 domain structure. KChIPs are cyan and Kv4.3 N-terminal domain is orange. Bottom graph, comparison of the data and the calculated scattering curve from the X-ray structure (χ² = 3.03). (b) Top two images, superposition of the ab initio SAXS model and a model in which the KChIPs orient along the T1 domain sides in a square, as suggested by EM microscopy22. Bottom chart, comparison of the data and the calculated scattering curve of the model (χ² = 7.06). s = (4πsin(θ))/λ.

Effects of site 2 mutations on KChIP1–Kv4.3 channel modulation and complex formation. (a) Peak currents at +40 mV for Kv4.3 alone and expressed with wild-type KChIP1 or indicated KChIP mutants. (b) Inactivation time constants at +40 mV for a. (c) Comparison of recovery from inactivation at +40 mV for b. In a, b and c, error bars show s.d. (d) MBP pull-down of Kv4.3_1–143, KChIP1 and KChIP mutants. I, column input; E, column eluate. Molecular weight markers (M; in kDa) are shown.

Calcium effects on KChIP1–Kv4.3 complex formation and channel modulation. (a) MBP pull-down of Kv4.3_1–143, KChIP1 and KChIP mutants. I, column input; E, column eluate. Molecular weight markers (M; in kDa) are shown. (b) Circular dichroism of KChIP1_37–216 in the presence and absence of calcium (see key). All EGTA samples include 1 mM CaCl₂. (c) Peak currents at +40 mV for Kv4.3 alone and expressed with wild-type KChIP1 or indicated EF mutants. (d) Inactivation time constants at +40 mV for c. (e) Comparison of recovery from inactivation at +40 mV for c. In c, d and e, asterisks indicate data for a 3.5:1 Kv4.3/KChIP1 molar ratio. Error bars show s.d.

Model of a Kv4.3–KChIP channel complex. (a) Model of the Kv4.3–KChIP complex based on the Kv1.2 structure21. KChIP1 (cyan), Kv4.3 T1 domain (orange) and the transmembrane domains (TM, dark red) are indicated. Calcium ions are magenta. Zinc ions are not visible. (b) View of model in a from the extracellular side. KChIP and voltage sensor domain (VSD) are indicated. Green sphere is a potassium ion in the pore. (c) Hypothetical model of a Kv4–KChIP–Kvβ complex. Domains are colored as in a with Kvβs indicated (dark blue). (d) Potential simultaneous engagement of the T1 docking loop by Kvβ and KChIP. Docking loop superposition for Kv1.2 (salmon) and Kv4.3 (orange) are shown. Surface shows the KChIP H1 and H2 helices (cyan) and the Kvβ interaction site (dark blue). Side chains for KChIP residues Glu40 and Lys50 and Kvβ residues Phe233 and His234 are omitted.