The Combined Effect of Hydrophobic Mismatch and Bilayer Local Bending on the Regulation of Mechanosensitive Ion Channels

Abstract

The hydrophobic mismatch between the lipid bilayer and integral membrane proteins has well-defined effect on mechanosensitive (MS) ion channels. Also, membrane local bending is suggested to modulate MS channel activity. Although a number of studies have already shown the significance of each individual factor, the combined effect of these physical factors on MS channel activity have not been investigated. Here using finite element simulation, we study the combined effect of hydrophobic mismatch and local bending on the archetypal mechanosensitive channel MscL. First we show how the local curvature direction impacts on MS channel modulation. In the case of MscL, we show inward (cytoplasmic) bending can more effectively gate the channel compared to outward bending. Then we indicate that in response to a specific local curvature, MscL inserted in a bilayer with the same hydrophobic length is more expanded in the constriction pore region compared to when there is a protein-lipid hydrophobic mismatch. Interestingly in the presence of a negative mismatch (thicker lipids), MscL constriction pore is more expanded than in the presence of positive mismatch (thinner lipids) in response to an identical membrane curvature. These results were confirmed by a parametric energetic calculation provided for MscL gating. These findings have several biophysical consequences for understanding the function of MS channels in response to two major physical stimuli in mechanobiology, namely hydrophobic mismatch and local membrane curvature.

Fig 1. The continuum model of MscL [5] which is embedded in a continuum implicit model bilayer.

(A) Lipid with three different acyl chain length (i.e. di-10:0, di-16:0 and of di-24:1) have been simulated using finite element (FE) method for positive, zero and negative mismatches between the protein and the lipid bilayer respectively. (B) The influence of inward (negative) and outward (positive) local membrane bending on the channel deformation. Depending on the direction of curvature, different parts of the channel experience tension or compression.

Fig 2. The influence of inward (negative) and outward (positive) membrane bending on the pressure profile of di-10:0 bilayer (positive mismatch) and the MscL pore shape.

(A) Lipid bilayer deformation due to the intrinsic bilayer pressure profile (top), inward local bending (middle) outward local bending (bottom) of the membrane. (B) Inward local bending causes an asymmetry in the pressure profile such that the total tension in the inner monolayer is higher than tension in the outer monolayer. In contrast, outward bending causes more tension in the outer monolayer. (C) Compared to the equilibrium shape, inward bending results in a considerable opening in the narrowest part of the channel (shaded region which structurally encompasses L19 and V23), whereas the outward bending narrows the channel pore in this area.

Fig 3. The effect of inward and outward bending on the channel conformation when there is no protein-lipid mismatch.

(A) Membrane (POPC di-16:0) deformation after the MscL reconstitution (top panel), after an inward local bending (middle panel) and after an outward local bending (bottom panel). (B) Asymmetry in the pressure profile due to local bending. As a result of inward local bending, the total tension generated in the inner leaflet is higher than the membrane tension in the outer one. In the contrary, outward bending causes more tension in the outer monolayer compared to the inner one. (C) Compared to the equilibrium shape, inward bending results in a considerable opening in the narrowest part of the channel (shaded region which structurally encompasses the hydrophobic pore), whereas the outward bending inhibit the channel in this area.

Fig 4. The influence of inward and outward local bending on the pressure profile of POPC di-24:1 bilayer and the MscL pore shape (negative hydrophobic mismatch). MscL is equilibrated due to the intrinsic pressure profile of the lipid bilayer.

(A) Lipid bilayer deformation due to intrinsic pressure profile of lipid bilayer (top panel), inward bending (middle panel) outward bending (bottom panel). (B) The peak stress in the pressure profile is smaller in the thicker bilayers. Similar to the previous cases, inward local bending causes an asymmetry in the pressure profile such that the total tension in the inner monolayer is higher than in the outer monolayer. In contrast, outward local bending causes more tension in the outer monolayer. (C) Based on the channel pore in the equilibrium state, inward bending results in expansion of the narrowest part of the channel (shaded area), whereas the outward bending narrows the channel in this area.

Fig 5. Computational and theoretical comparison of the effect of inward local bending on the channel pore domain for three different protein-lipid mismatches. Three possible protein-lipid hydrophobic mismatches can be positive, zero and negative.

(A) A comparison has been made between the pore shapes in different lipid protein hydrophobic mismatches for inward local bending of the membrane. Overall, MscL in the bilayer with the same hydrophobic length shows a more expanded channel pore (shaded area) compared to positive and negative hydrophobic mismatches. (B) The gating free energy versus membrane has been shown for the above mentioned three possible lipid-protein hydrophobic mismatches. For a specific radius of curvature, the gating free energy is lower for zero hydrophobic mismatch compared to the other two cases. In turn, negative mismatch (thick lipid) results in lower free energy levels required for the MscL gating compared to the positive mismatch (thin lipid).