Structures of the Mg2+ Channel Cora in the Open State by Cryo Electron Microscopy

Abstract

Magnesium ions are essential for life and play key roles in signaling and metabolism. The ∼200 kDa pentameric membrane channel CorA opens when the intracellular Mg2+ levels are low, to allow diffusion of Mg2+ into the cell, and closes again when the Mg2+ concentration is sufficiently high. X-ray crystallographic studies have reported similar conformations in both Mg2+-bound (closed) and Mg2+-free states, but EPR spectroscopic studies in lipid-reconstituted CorA show large Mg2+-driven quaternary conformational changes. Using single-particle cryo-EM, we have determined structures of CorA in the Mg2+-bound “closed” conformation plus two “open” Mg2+-free states at resolutions of 3.8 A, 7.1 A and 7.1 A respectively. Comparison of the different conformations reveals dramatic differences in channel symmetry triggered by the unbinding of Mg2+ at the inter-subunit interfaces. Highly asymmetric gating rearrangements were observed from large hinge-bending movements of the long “stalk” helix (as much as ∼35°) in two of the five subunits, leading to large reorientations (∼10 to ∼25 A) of the cytoplasmic domains, which propagate to the transmembrane segments. Our findings suggest an unprecedented gating mechanism where the reversible loss of five-fold symmetry represents the key structural signature of the transition between a single closed state and an ensemble of low-Mg2+, open conformational states.