Can the selectivity of a mechanosensitive channel be modulated by an ion selective antechamber domain?
The goal of this study is to investigate the basis of ion specificity of Eschereschia coli mechanosensitive channel MscS:
Site-directed mutagenesis of antechamber domain pore residues to be either positively charged, negatively charged, or neutral.
Patch clamp electrophysiology to determine either cation, anion, or no preference for mutant and native MscS mechanosensitive channel proteins in Xenopus laevi oocytes.
Ion exchange between cells and membrane bound cellular compartments is an important cellular process and signal transduction mechanism. There are many well known cellular processes that are controlled by ion fluxes including development, muscular movements, and senses such as touch and hearing. The latter of these processes have been attributed to a specific class of ion channels solving one of the most basal problems in the evolutionary history of the cell, that is osmoregulation. The early cell’s solution to the problem of excess water into itself was to sense the tension and curvature of it’s membrane which is a function of the cell’s interior volume. As the volume increases, tension in the membrane increases and specialized mechanosensitive channels embedded in the membrane are pulled apart acting as a release valve for the contents of the cell to be released relieving the pressure (Kung 2005). This early mechanism is more or less homologous to many other categorically distinct senses due to the fact that many things can affect the forces in cellular membranes. Known membrane deformation actions that are sensed by mechanosensitive channels are mechanical touch (here referring to mechanical processes outside of the membrane), asymmetric membrane lipid composition, and of course osmotic homeostasis (Kung 2005). It has also been suggested that changes in membranes due to temperature changes are detected by mechanosensitive channels (Kung 2005), possibly explaining why lipid soluble compounds such as capsaicin and menthol provide sensations of hot and cold.
Discovery and structural crystallization of the bacterial MscS complex (Bass 2002) (Fig. 1 and 2), and subsequently MscL (Chang 1998), provided a mechanistic rationale for how these proteins accomplish mechanical sensing of the membrane forces. Under deformations and phase transitions membrane forces change position along the normal axis spanning the membrane, we see then that the points of forceful contact with embedded objects changes under different membrane states (Wiggins 2005). The transmembrane helices of MscS, arranged as a turbine, adopt the lowest energy configuration in correspondance with the membrane forces, changing the width of the aperture at the center allowing the passage of particles (Tang 2006, Wiggins 2005, Wiggins 2004). An apt analogy is how the aperture of a camera lens is changed by the concerted twisting of several blocking plates with a specific geometric arrangement.
Many channels act solely on a more enzymatic model where a specific substrate binding makes a conformational energy accessible causing a geometric arrangement to transiently open a gate in which the substrate can then be passed through. While some control of substrate selectivity can be managed in the channel by the size of the opening and the charge of the channel lining these structures are far less specific than substrate binding channels (Maksaev 2013, Maksaev 2012). This disadvantage is offset by an increased possible conductance which may be especially useful for releasing large volumes of substance. One then wonders how a specific stimuli such as temperature change and touch can be parsed if the underlying force is similar?
A full description of the anisotropy of a lipid bilayer is beyond the scope of the problem engaged here but indeed these different stimuli affect membranes differently (Markin 2007). Different force profiles along the normal membrane axis can be distiguished by specific transmembrane arrangements (Tang 2006, Wiggins 2005). But for two mechanosensitive channels with distinguishable affectors we have the same effect: the conductance of bulk cellular contents down a pressure and/or osmotic gradient.
The hypothesis that ion selectivity is achieved in the membrane spanning pore is irrelevant as pores such as MSL10 (Msc-Like 10; discussed further later) from Arabidopsis thaliana are lined with bulky aromatic residues such as phenylalanine (Maksaev 2012, Maksaev 2013). This suggests that size exclusion may be a factor but that charge would be unaffected, which is not the case in MSL10 which shows a strong preference for anions (Maksaev 2012, Maksaev 2013). So the question is how are these channels ion selective?
The answer may lie in the conspicuously large ( 40 Å) pomegranate shaped extramembrane cavity domain of E. coli MscS (as suggested by (Maksaev 2013)), which I will refer to as the antechamber (Fig 1 and 2). In the antechamber there are 8 pores; one between each of the subunit pair in the heptameric complex, and one at the tip of the antechamber. The pores to the hydrophilic domain may be selective enough to create an increased concentration of selected ions in the antechamber. An increased concentration of a specific ionic species in the antechamber would increase relative flux of that species in a non-specific channel. Furthermore, one could conceive further selectivity could be brought about through charged residues in the membrane spanning channel.
An understanding of how mechanosensitive channels are ion specific is important in many aspects. Firstly, a model that predicts ion selectivity in mechanosensitive channels based on the size and charge of important residues will be useful in characterizing homologous proteins across taxa. For example in Arabidopsis there are 10 proteins homologous to MscS, the MscS-Like (MSL) family, most of which are uncharacterized (Wilson 2013, Haswell 2008, Haswell 2007), not including the anion preferring MSL10 as previously (Maksaev 2012). Second, confirming the degree to which ion selectivity can occur in mechanosensitive channels will connect the higher senses of temperature and touch to an initial signaling factor, namely a specific burst of ions into some inter- or intracellular compartment (Kudla 2010, Finazzi 2014, Plieth 1999). Lastly, this sort of ion selectivity may be more easily manipulated in synthetic applications as it is more likely that changes in a non-membrane domain will not have loss of function effects (Cox 2014, Perozo 2002).
A note as to the general theoretical mechanisms of biological specificity and entropy reduction, a two-stage ergodicity reducing ion transit system is quite distinct from the usual lock and key substrate specificity mechanisms utilized by most enzymes and ion channels, and represents a fairly distinct class in biology.