Structural asymmetry in the magnesium channel CorA points to sequential allosteric regulation

Abstract

Magnesium ions (Mg2+) are essential for life, but the mechanisms regulating their transport into and out of cells remain poorly understood. The CorA-Mrs2-Alr1 superfamily of Mg2+ channels represents the most prevalent group of proteins enabling Mg2+ ions to cross membranes. Thermotoga maritima CorA (TmCorA) is the only member of this protein family whose complete 3D fold is known. Here, we report the crystal structure of a mutant in the presence and absence of divalent ions and compare it with previous divalent ion-bound TmCorA structures. With Mg2+ present, this structure shows binding of a hydrated Mg2+ ion to the periplasmic Gly-Met-Asn (GMN) motif, revealing clues of ion selectivity in this unique channel family. In the absence of Mg2+, TmCorA displays an unexpected asymmetric conformation caused by radial and lateral tilts of protomers that leads to bending of the central, pore-lining helix. Molecular dynamics simulations support these movements, including a bell-like deflection. Mass spectrometric analysis confirms that major proteolytic cleavage occurs within a region that is selectively exposed by such a bell-like bending motion. Our results point to a sequential allosteric model of regulation, where intracellular Mg2+ binding locks TmCorA in a symmetric, transport-incompetent conformation and loss of intracellular Mg2+ causes an asymmetric, potentially influx-competent conformation of the channel.