Broken symmetry in the human BK channel
Voltage-gated and ligand-modulated ion channels play critical roles in excitable cells. To understand the interplay among voltage-sensing, ligand-binding and channel opening, the structures of ion channels in various functional states need to be determined. Here, the \"random spherically constrained\" (RSC) single-particle cryo-EM method was employed to study the human large conductance voltage- and calcium-activated potassium (hBK or hSlo1) channels reconstituted into liposomes. The hBK structure has been determined at 3.5 [A] resolution in the absence of Ca2+. Instead of the common four-fold symmetry observed in ligand-modulated ion channels, a two-fold symmetry was observed in hBK. Two opposing subunits in the Ca2+ sensing gating ring rotate around the center of each subunit, which results in the movement of the assembly and flexible interfaces and Ca2+ binding sites. Despite the significant movement, the local conformation of the assembly interfaces and Ca2+ binding sites remains the same among the four subunits.