Cryo-EM structures of the small-conductance Ca2+-activated KCa2.2 channel
Young-Woo Nam, Dohyun Im, Adriana Garcia, Marios Tringides, Hai M. Nguyen, Yan Liu, Razan Orfali, Alena Ramanishka, Grigore Pintilie, Chih‐Chia Su, Meng Cui, Diomedes E. Logothetis, Eunsun Yu, Heike Wulff, K. George Chandy, Miao Zhang
Abstract
Small-conductance Ca2+-activated K+ (KCa2.1-KCa2.3) channels modulate neuronal and cardiac excitability. We report cryo-electron microscopy structures of the KCa2.2 channel in complex with calmodulin and Ca2+, alone or bound to two small molecule inhibitors, at 3.18, 3.50, 2.99 and 2.97 angstrom resolution, respectively. Extracellular S3-S4 loops in β-hairpin configuration form an outer canopy over the pore with an aromatic box at the canopy’s center. Each S3-S4 β-hairpin is tethered to the selectivity filter in the neighboring subunit by inter-subunit hydrogen bonds. This hydrogen bond network flips the aromatic residue (Tyr362) in the filter’s GYG signature by 180°, causing the outer selectivity filter to widen and water to enter the filter. Disruption of the tether by a mutation narrows the outer selectivity filter, realigns Tyr362 to the position seen in other K+ channels, and significantly increases unitary conductance. UCL1684, a mimetic of the bee venom peptide apamin, sits atop the canopy and occludes the opening in the aromatic box. AP14145, an analogue of a therapeutic for atrial fibrillation, binds in the central cavity below the selectivity filter and induces closure of the inner gate. These structures provide a basis for understanding the small unitary conductance and pharmacology of KCa2.x channels. Small-conductance Ca2+ -activated K+ (KCa2.1-KCa2.3) channels modulate neuronal and cardiac excitability. Here, authors report cryo-EM structures of the KCa2.2 channel, providing a basis for understanding the small unitary conductance and pharmacology of KCa2.x channels.