Cryo-EM structure of the MgtE Mg2+ channel pore domain in Mg2+-free conditions reveals cytoplasmic pore opening

ABSTRACT MgtE is a Mg 2+ channel conserved in organisms ranging from prokaryotes to eukaryotes, including humans, and plays an important role in Mg 2+ homeostasis. The previously determined MgtE structures in the Mg 2+ -bound, closed state and structure-based functional analyses of MgtE revealed that the binding of Mg 2+ ions to the MgtE cytoplasmic domain induces channel inactivation to maintain Mg 2+ homeostasis. However, due to the lack of a structure of the MgtE channel, including its transmembrane domain in Mg 2+ -free conditions, the pore-opening mechanism of MgtE has remained unclear. Here, we determined the cryoelectron microscopy (cryo-EM) structure of the MgtE-Fab complex in the absence of Mg 2+ ions. The Mg 2+ -free MgtE transmembrane domain structure and its comparison with the Mg 2+ -bound, closed-state structure, together with functional analyses, showed the Mg 2+ -dependent pore opening of MgtE on the cytoplasmic side and revealed the kink motions of the TM2 and TM5 helices at the glycine residues, which are important for channel activity. Overall, our work provides structure-based mechanistic insights into the channel gating of MgtE.