The energetics of transmembrane helix insertion into a lipid bilayer

Abstract

Free energy profiles for insertion of a hydrophobic transmembrane protein α-helix (M2 from CFTR) into a lipid bilayer have been calculated using coarse-grained molecular dynamics simulations and umbrella sampling to yield potentials of mean force along a reaction path corresponding to translation of a helix across a lipid bilayer. The calculated free energy of insertion is smaller when a bilayer with a thinner hydrophobic region is used. The free energies of insertion from the potentials of mean force are compared with those derived from a number of hydrophobicity scales and with those derived from translocon-mediated insertion. This comparison supports recent models of translocon-mediated insertion and in particular suggests that: 1), helices in an about-to-be-inserted state may be located in a hydrophobic region somewhat thinner than the core of a lipid bilayer; and/or 2), helices in a not-to-be-inserted state may experience an environment more akin (e.g., in polarity/hydrophobicity) to the bilayer/water interface than to bulk water.

Figure 1

Comparison of the CG models of M2₁₉, M2₂₉, and M2₃₉. The sphere radii correspond to those used in the CG interaction potential. The N-termini are at the top. The color scheme corresponds to the particle types used in the CG model (gray, C (hydrophobic apolar), green, N (mixed polar/apolar), yellow, P (polar), red, Q− (charged, anionic), and blue, Q+ (charged, cationic)).

Figure 2

The M2₃₉ helix in its initial configuration at various points (z = +50 to −50 Å) along the z axis perpendicular to the bilayer (A), and the same helix after 80 ns of simulation (B).

Figure 3

(A) Average tilt angle (relative to the bilayer perpendicular) for each windows of the M2₁₉ CG4-PC PMF simulations, shown as a function of the restrained z value for each window. Error bars correspond to standard deviations for each window. (B) Helix tilt angle versus time for three windows (as indicated by the arrows in panel A), namely in water (W), at the water/bilayer interface (I), and in the hydrophobic core (H) of the bilayer.

Figure 4

PMF profiles (A) for M2₁₉, M2₂₉, and M2₃₉ in CG4-PC bilayers and (B) for M2₃₉ in a CG2-PC versus a CG4-PC bilayer. (The edge of the lipid bilayer is marked in gray for CG4-PC and brown for CG2-PC.) Error bars were estimated by the PMFs from different 20-ns sections of the simulations.

Figure 5

Snapshots at the end of each simulation for the central window, showing the M2 helices in bilayer-spanning orientations. (The residues are color-coded as follows: gray, hydrophobic; yellow, polar; red, anionic; and blue, cationic. The smaller brown spheres correspond to the phosphate groups of the lipids, and the N-termini of the helices are at the upper face of the bilayer.)