Pivoting between Calmodulin Lobes in the Calmodulin/Kv7.2 Complex Triggered by Calcium

Mammalian KCNQ genes encode five Kv7 potassium channel subunits (Kv7.1-Kv7.5). Kv7.2 and Kv7.3 are expressed in the nervous system, being the principal molecular components of the slow voltage gated M-channel, which exert a strong control in neuronal excitability.Like all Kv channels, the Kv7 α subunits share a common core structure of six transmembrane segments with a voltage-sensing domain (S1-S4), a pore domain (S5-S6) and intracellular N- and C-terminal regions. The C-terminus harbours four regions that present a high probability of adopting an alpha helix configuration (helices A-D). This region binds some lipids and several proteins, including the ubiquitous calcium binding protein calmodulin (CaM).CaM mediates inhibition of Kv7.2 channels and is required for the channels to exit the endoplasmic reticulum. Both processes are enhanced by Ca2+, but the molecular details of how Ca2+ trigger channel trafficking or the reduction of M-current are unknown. The aim of this study was to explore the molecular events within CaM triggered by Ca2+ using two complementary approaches. In one, we have performed a fluorimetric assay using dansylated calmodulin (D-CaM) to characterize the interaction of individual lobes to the Kv7.2 CaM binding site. The association of the Kv7.2 with CaM was also explored using NMR spectroscopy, employing 15N-labeled CaM as a reporter. Our data show interdependency of the N- and C-lobes in the interaction and suggest that Ca2+ causes the contacts with CaM to pivot between EF-1 in the N-lobe and EF-4 in the C-lobe. In addition, Ca2+ makes CaM binding to the channel more difficult, and the channel makes CaM binding to Ca2+ more difficult.