Magnesium (Mg2+) is a key divalent cation in biology. It regulates and maintains numerous, physiological functions such as nucleic acid stability, muscle contraction, heart rate and vascular tone, neurotransmitter release, and serves as cofactor in a myriad of enzymatic reactions. Most importantly, it coordinates with ATP, and is thus crucial for energy production in mitochondria.*
In order to store Mg2+ in the mitochondrial lumen it is imported via Mrs2 and Alr2 ion channels that are closely related to CorA, the main Mg2+-importer in bacteria. Although these Mg2+-transport proteins do not show much sequence conservation, they all share two trans-membrane domains (TMDs) with the signature motif Glycine-Methionine-Asparagine (GMN) at the extracellular loop.*
CorA, a divalent-selective channel in the metal ion transport superfamily, is the major Mg2+-influx pathway in prokaryotes. CorA structures in closed (Mg2+-bound), and open (Mg2+-free) states, together with functional data showed that Mg2+-influx inhibits further Mg2+-uptake completing a regulatory feedback loop. While the closed state structure is a symmetric pentamer, the open state displayed unexpected asymmetric architectures.*
In the article “Real time dynamics of Gating-Related conformational changes in CorA” Martina Rangl, Nicolaus Schmandt, Eduardo Perozo and Simon Scheuring used high-speed atomic force microscopy (HS-AFM), to explore the Mg2+-dependent gating transition of single CorA channels: HS-AFM movies during Mg2+-depletion experiments revealed the channel’s transition from a stable Mg2+-bound state over a highly mobile and dynamic state with fluctuating subunits to asymmetric structures with varying degree of protrusion heights from the membrane.*
Their data shows that at Mg2+-concentration below Kd, CorA adopts a dynamic (putatively open) state of multiple conformations that imply structural rearrangements through hinge-bending in TM1. They also discuss how these structural dynamics define the functional behavior of this ligand-dependent channel.*
All Atomic Force Microscopy experiments described in the article were performed using NanoWorld Ultra-Short Cantilevers USC-F1.2-k0.15 for high-speed Atomic Force Microscopy ( HS-AFM ). Videos of CorA membranes were recorded with imaging rates of ~1–2 frames s−1 and at a resolution of 0.5 nm pixel−1.
Sample morphology of CorA reconstitutions for HS-AFM.
(a) HS-AFM overview topograph of densely packed CorA in a POPC/POPG (3:1) lipid bilayer exposing the periplasmic side and a loosely packed protein area with diffusing molecules exposing the intracellular face (full color scale: 20 nm). Left: Height histogram of the HS-AFM image with two peaks representative of the mica and the CorA surface (∆Height (peak-peak): 12 nm (20,500 height values)). The dashed line indicates the position of the cross-section analysis shown in (b). (b) Profile of the membrane shown in a), including a cartoon (top) of the membrane in side view. The height profile (~12 nm) corresponds well to the all-image height analysis (a, left) and the CorA structure (Matthies et al., 2016). (c) High-resolution image (top) and cross-section analysis along dashed line (bottom) of the periplasmic face. The height and dimension of the periplasmic face is in good agreement with the structure (left), and the periodicity (~14 nm, n = 40) corresponds well with the diameter of the intracellular face spacing the molecules on the other side of the membrane (full color scale: 2 nm). (d) HS-AFM image of densely packed CorA embedded in a DOPC/DOPE/DOPS (4:5:1) membrane. This reconstitution resulted in two stacked membrane layers, both exposing the CorA intracellular face. The dashed line indicates the position of the cross-section analysis shown in (e). Left: Height histogram of the HS-AFM image with two peaks at ~12 nm and ~17 nm (32,500 height values), corresponding to the proteins in two stacked membranes (full color scale: 20 nm). (e) Section profile of the membrane shown in d), including a cartoon (top) of the membrane in side view. (f) High-resolution view and cross-section analysis along dashed line (bottom) of the CorA intracellular face revealing the individual subunits of the pentamers (full color scale: 3 nm). Inset: 5-fold symmetrized average of CorA. The dimensions of CorA observed with HS-AFM are in good agreement with the structure (left: PDB 3JCF). The structures in (c) and (f) are shown in ribbon (top) and surface (bottom) representations, respectively.
*Martina Rangl, Nicolaus Schmandt, Eduardo Perozo, and Simon Scheuring
Real time dynamics of Gating-Related conformational changes in CorA
eLife. 2019; 8: e47322
DOI: 10.7554/eLife.47322
Please follow this external link to read the full article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6927688/
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