title: Structural asymmetry in the magnesium channel CorA points to sequential allosteric regulation.
abstract: Magnesium ions (Mg(2+)) are essential for life, but the mechanisms regulating their transport into and out of cells remain poorly understood. The CorA-Mrs2-Alr1 superfamily of Mg(2+) channels represents the most prevalent group of proteins enabling Mg(2+) 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 Mg(2+) present, this structure shows binding of a hydrated Mg(2+) ion to the periplasmic Gly-Met-Asn (GMN) motif, revealing clues of ion selectivity in this unique channel family. In the absence of Mg(2+), 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 Mg(2+) binding locks TmCorA in a symmetric, transport-incompetent conformation and loss of intracellular Mg(2+) causes an asymmetric, potentially influx-competent conformation of the channel.
authors.affiliation: Department of Biochemistry, University of Toronto, Toronto, ON, Canada, M5G 1L7.
authors.forename: Roland
authors.initials: R
authors.lastname: Pfoh
authors.forename: Angela
authors.initials: A
authors.lastname: Li
authors.forename: Nilmadhab
authors.initials: N
authors.lastname: Chakrabarti
authors.forename: Jian
authors.initials: J
authors.lastname: Payandeh
authors.forename: Régis
authors.initials: R
authors.lastname: Pomès
authors.forename: Emil F
authors.initials: EF
authors.lastname: Pai
mesh_terms: D020816:Amino Acid Motifs; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D027682:Cation Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018360:Crystallography, X-Ray; D008274:Magnesium / Q000737:chemistry* / Q000378:metabolism; D056004:Molecular Dynamics Simulation; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship; D020124:Thermotoga maritima / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D027682:Cation Transport Proteins; D008274:Magnesium