Structural basis and energy landscape for the Ca ²⁺ gating and calmodulation of the Kv7.2 K ⁺ channel

Significance

Ion channels are sophisticated proteins that exert control over a plethora of body functions. Specifically, the members of the Kv7 family are prominent components of the nervous systems, responsible for the ion fluxes that regulate the electrical signaling in neurons and cardiac myocytes. Albeit its relevance, there are still several questions, including the Ca ²⁺/calmodulin (CaM)-mediated gating mechanism. We found that Ca ²⁺ binding to CaM triggers a segmental rotation that allosterically transmits the signal from the cytosol up to the transmembrane region. NMR-derived analysis of the dynamics demonstrates that it occurs through a conformational selection mechanism. Energetically, CaM association with the channel tunes the affinities of the CaM lobes (calmodulation) so that the channel can sense the specific changes in [Ca ²⁺] resulting after an action potential.

Abstract

The Kv7.2 (KCNQ2) channel is the principal molecular component of the slow voltage-gated, noninactivating K ⁺ M-current, a key controller of neuronal excitability. To investigate the calmodulin (CaM)-mediated Ca ²⁺ gating of the channel, we used NMR spectroscopy to structurally and dynamically describe the association of helices hA and hB of Kv7.2 with CaM, as a function of Ca ²⁺ concentration. The structures of the CaM/Kv7.2-hAB complex at two different calcification states are reported here. In the presence of a basal cytosolic Ca ²⁺ concentration (10–100 nM), only the N-lobe of CaM is Ca ²⁺-loaded and the complex (representative of the open channel) exhibits collective dynamics on the millisecond time scale toward a low-populated excited state (1.5%) that corresponds to the inactive state of the channel. In response to a chemical or electrical signal, intracellular Ca ²⁺ levels rise up to 1–10 μM, triggering Ca ²⁺ association with the C-lobe. The associated conformational rearrangement is the key biological signal that shifts populations to the closed/inactive channel. This reorientation affects the C-lobe of CaM and both helices in Kv7.2, allosterically transducing the information from the Ca ²⁺-binding site to the transmembrane region of the channel.