Hydration properties of mechanosensitive channel pores define the energetics of gating.
ABSTRACT Opening of ion channels directly by tension in the surrounding membrane appears to be the most ancient and simple mechanism of gating. Bacterial mechanosensitive channels MscL and MscS are the best-studied tension-gated nanopores, yet the key physical factors that define their gating are still hotly debated. Here we present estimations, simulations and experimental results showing that hydration of the pore might be one of the major parameters defining the thermodynamics and kinetics of mechanosensitive channel gating. We associate closing of channel pores with complete dehydration of the hydrophobic gate (occlusion by 'vapor lock') and formation of two water-vapor interfaces above and below the constriction. The opening path is the expansion of these interfaces, ultimately leading to wetting of the hydrophobic pore, which does not appear to be the exact reverse of the closing path, thus producing hysteresis. We discuss specifically the role of polar groups (glycines) buried in narrow closed conformations but exposed in the open states that change the wetting characteristics of the pore lining and stabilize conductive states of the channels.
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ABSTRACT: The magnesium ion (Mg(2+)) is the most abundant divalent cation within cells. In man, Mg(2+)-deficiency is associated with diseases affecting the heart, muscle, bone, immune, and nervous systems. Despite its impact on human health, little is known about the molecular mechanisms that regulate magnesium transport and storage. Complete structural information on eukaryotic Mg(2+)-transport proteins is currently lacking due to associated technical challenges. The prokaryotic MgtE and CorA magnesium transport systems have recently succumbed to structure determination by X-ray crystallography, providing first views of these ubiquitous and essential Mg(2+)-channels. MgtE and CorA are unique among known membrane protein structures, each revealing a novel protein fold containing distinct arrangements of ten transmembrane-spanning α-helices. Structural and functional analyses have established that Mg(2+)-selectivity in MgtE and CorA occurs through distinct mechanisms. Conserved acidic side-chains appear to form the selectivity filter in MgtE, whereas conserved asparagines coordinate hydrated Mg(2+)-ions within the selectivity filter of CorA. Common structural themes have also emerged whereby MgtE and CorA sense and respond to physiologically relevant, intracellular Mg(2+)-levels through dedicated regulatory domains. Within these domains, multiple primary and secondary Mg(2+)-binding sites serve to staple these ion channels into their respective closed conformations, implying that Mg(2+)-transport is well guarded and very tightly regulated. The MgtE and CorA proteins represent valuable structural templates to better understand the related eukaryotic SLC41 and Mrs2-Alr1 magnesium channels. Herein, we review the structure, function and regulation of MgtE and CorA and consider these unique proteins within the expanding universe of ion channel and transporter structural biology.Biochimica et Biophysica Acta 08/2013; · 4.66 Impact Factor
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ABSTRACT: The mechanosensitive channel of large conductance (MscL) is a homopentameric membrane protein that protects bacteria from hypoosmotic stress. Its mechanics are coupled to structural changes in the membrane, yet the molecular mechanism of the transition from closed to open states and the cooperation between subunits are poorly understood. To determine the early stages of channel activation, we have created a chemically addressable heteropentameric MscL, which allows us to selectively trigger only one subunit in the pentameric protein assembly. By employing a liposome leakage assay developed in house, we measured the size-exclusion limits of MscL (G22C(5) homopentamer and WT(4)G22C(1) heteropentamer). Patch-clamp, single-channel conductance recordings were used to electrically characterize the various channel substates. We show that a decrease in the hydrophobicity of a pore residue in only one subunit breaks the energy barrier for gating and increases the pore diameter up to 10 Å. A further decrease on the hydrophobicity of the same pore residue in other subunits opens the channel further, up to a diameter of 25 Å. However, it is not sufficient for full opening of the channel. This suggests the presence of supplementary mechanisms other than only the hydrophobic gate for MscL opening and closing and/or insufficient expansion of the channel by hydrophobic gating in the absence of applied membrane tension.-Mika, J. T., Birkner, J. P., Poolman, B., Koçer, A. On the role of individual subunits in MscL gating: "All for one, one for all?"The FASEB Journal 11/2012; · 5.70 Impact Factor
- Chemical Reviews 10/2012; · 41.30 Impact Factor