Figure 2. Membrane protein–lipid bilayer... | Rockefeller University Press

Figure 2.

Membrane protein–lipid bilayer hydrophobic coupling. (A) Hydrophobic interactions between integral membrane proteins and their host bilayer cause the lipid bilayer (with an unperturbed hydrophobic thickness d₀) and an embedded protein (with hydrophobic length l) to adapt to each other. Protein conformational changes that involve the protein–bilayer boundary, for example, a change in protein hydrophobic length, will alter the local bilayer thickness to match the channel length. Such local bilayer deformations, with their associated bilayer compression and monolayer bending, incur an energetic cost (the bilayer deformation energy, ΔG_def). The total free energy difference between different protein conformations (⁠ $Δ G_{tot}^{I \to II} = Δ G_{tot}^{II} - Δ G_{tot}^{I}$ ⁠) therefore has a contribution not only from the protein per se (⁠ $Δ G_{prot}^{I \to II} = Δ G_{prot}^{II} - Δ G_{prot}^{I}$ ⁠) but also from the difference in bilayer deformation energy associated with the two conformations (⁠ $Δ G_{bil}^{I \to II} = Δ G_{def}^{II} - Δ G_{def}^{I}$ ⁠). The protein conformational equilibrium therefore varies as a function of the bilayer elastic moduli, protein–bilayer hydrophobic mismatch, and the intrinsic lipid curvature. (B) When amphiphiles adsorb to the bilayer–solution interface, they alter bilayer properties such as thickness (d₀), intrinsic curvature (c₀), and elastic moduli, as well as ΔG_def. The associated changes in $Δ G_{bil}^{I \to II}$ will alter the conformational preference and thus its function. (C) Bilayer-spanning gA channels form by transmembrane dimerization of two β^6.3-helical subunits; channel formation is reported as changes in the current through the bilayer. The channel length is less than the unperturbed bilayer thickness, meaning that the energetic cost of channel formation—and the single-channel appearance frequency and lifetime—varies with changes in lipid bilayer properties. (D) Changes in lipid bilayer properties, such as those caused by the adsorption at the bilayer–solution interface, therefore will be reflected in changes in gA lifetimes and appearance frequencies, as indicated in the current trace.

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