Abstract
Many membrane remodeling events rely on the ability of curvature-generating N-BAR membrane proteins to organize into distinctive supramolecular configurations. Experiments have revealed a conformational switch in N-BAR proteins resulting in vesicular or tubular membrane shapes, with shallow membrane immersion of the H0 amphipathic helices of N-BAR proteins on vesicles but deep H0 immersion on tubes. We develop here a minimal elastic model of the local thinning of the lipid bilayer resulting from H0 immersion. Our model predicts that the observed conformational switch in N-BAR proteins produces a corresponding switch in the bilayer-mediated N-BAR interactions due to the H0 helices. In agreement with experiments, we find that bilayer-mediated H0 interactions oppose N-BAR multimerization for the shallow H0 membrane immersion depths measured on vesicles, but promote self-assembly of supramolecular N-BAR chains for the increased H0 membrane immersion depths measured on tubes. Finally, we consider the possibility that bilayer-mediated H0 interactions might contribute to the concerted structural reorganization of N-BAR proteins suggested by experiments. Our results indicate that the membrane immersion depth of amphipathic protein helices may provide a general molecular control parameter for membrane organization.
Highlights
A remarkable feature of membrane remodeling by N-BAR proteins is that a single protein species can induce distinct types of membrane curvature, such as the isotropic and anisotropic curvatures associated with lipid bilayer vesicles and tubes, respectively
The only relevant spatial dimension for H0-induced bilayer-mediated interactions between N-BAR proteins is the distance d separating the axes of the neighboring H0 helices of the two N-BAR proteins, which we measure perpendicular to the H0 helix axes (Fig. 2)
Electron paramagnetic resonance (EPR) experiments have demonstrated a conformational switch in N-BAR proteins[22,23], with shallow membrane immersion of the H0 helices on vesicles but deep immersion on tubes
Summary
A remarkable feature of membrane remodeling by N-BAR proteins is that a single protein species can induce distinct types of membrane curvature, such as the isotropic and anisotropic curvatures associated with lipid bilayer vesicles and tubes, respectively. Vesicle-bound N-BAR proteins interact with the lipid bilayer through shallow wedging of the H0 helices with little or no scaffolding (left panel of Fig. 1B), while tube-bound N-BAR proteins show a combination of scaffolding and increased wedging depths of the H0 helices (right panel of Fig. 1B) It is unknown how the observed conformational switch in N-BAR proteins[22,23,24] generates the observed isotropic and anisotropic membrane curvatures[1,2,3,4,5,10,11,15,25]. Our model predicts that the increased H0 membrane immersion depths measured on tubes[22,23] result in bilayer-mediated H0 interactions between N-BAR proteins that promote the self-assembly of locally ordered tip-to-tail N-BAR chains[5,12,26,27,28,33] via the observed antiparallel alignment and dimerization of the H0 helices of neighboring N-BAR proteins (right panel of Fig. 1B).
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