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, 440 (7085), 833-7

Crystal Structure of the CorA Mg2+ Transporter

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Crystal Structure of the CorA Mg2+ Transporter

Vladimir V Lunin et al. Nature.

Abstract

The magnesium ion, Mg2+, is essential for myriad biochemical processes and remains the only major biological ion whose transport mechanisms remain unknown. The CorA family of magnesium transporters is the primary Mg2+ uptake system of most prokaryotes and a functional homologue of the eukaryotic mitochondrial magnesium transporter. Here we determine crystal structures of the full-length Thermotoga maritima CorA in an apparent closed state and its isolated cytoplasmic domain at 3.9 A and 1.85 A resolution, respectively. The transporter is a funnel-shaped homopentamer with two transmembrane helices per monomer. The channel is formed by an inner group of five helices and putatively gated by bulky hydrophobic residues. The large cytoplasmic domain forms a funnel whose wide mouth points into the cell and whose walls are formed by five long helices that are extensions of the transmembrane helices. The cytoplasmic neck of the pore is surrounded, on the outside of the funnel, by a ring of highly conserved positively charged residues. Two negatively charged helices in the cytoplasmic domain extend back towards the membrane on the outside of the funnel and abut the ring of positive charge. An apparent Mg2+ ion was bound between monomers at a conserved site in the cytoplasmic domain, suggesting a mechanism to link gating of the pore to the intracellular concentration of Mg2+.

Figures

Figure 1
Figure 1. Structure of the CorA Mg2+ channel
a, Ribbon diagram of the CorA pentamericcomplex, viewed in the plane of the membrane. On the right is a single unit of the CorA channel highlighting the following structural features: Stalk helix and inner TM1 helix (turquoise), outer helix (dark blue) and willow helices (purple). These and the remaining α-helices (red) and β-sheets (yellow) are numbered according to the text. The membrane surface is indicated. b, View from the intracellular region. c, View from the periplasm. This and other figures were generated with the program PyMOL, except where indicated.
Figure 2
Figure 2. Analysis of the CorA pore
The CorA pore, highlighting the locations of key residues on one of the TM1 helices (pale brown), as discussed in the text. The view is looking down the pore from the periplasmic surface (left panel) and in the plane of the membrane (right panel, orientation in left panel rotated 60° towards the viewer). To illustrate the pore interior better, a pair of TM1 and TM2 helices was removed (highlighted in pale green and indicated by blue arrow) for the panel on the right.
Figure 3
Figure 3. Electrostatic view of CorA
Positively charged residues are coloured blue and negatively charged or hydroxyl-containing residues are coloured red. Charged residues within the CorA basic sphincter (bluehighlighted region) and cytoplasmic domain (funnel interior and willow helices, red-highlighted regions) are labelled and illustrated as stick models. For better viewing of the funnel interior, two of the CorA monomers were removed from the model.
Figure 4
Figure 4. Pore dimensions
A cut-away view displaying the solventaccessible surface and dimensions of the CorA pore. Also shown are stick models of residues lining the pore. The key residues discussed in the text are labelled.
Figure 5
Figure 5. Electron density corresponding to protein and bound magnesium
a, A portion of the soluble domain 1.85 Å electron density map in the region of Asp 89, showing electron density for the putative Mg2+ ion and the water molecules that fill the hexacoordination shell. b, A portion of the 3.9 Å difference Fourier electron density map (purple) showing the putative magnesium ion between monomers. The inset shows the residue Asp89 in one monomer (pale blue) and Asp253 in another monomer (green) and a peak in the difference Fourier electron density map.

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