Abstract
Cell homeostasis requires the maintenance of heterogeneous membrane compositions through both vesicular and non-vesicular transport mechanisms. Yet, our biophysical description of non-vesicular lipid transport remains incomplete. To help fill these gaps, we aimed to identify an accurate reaction coordinate for passive lipid exchange between membranes. Towards this goal, we investigated the elementary steps of lipid exchange, lipid desorption and insertion into a membrane, using molecular simulation. From over 1,000 lipid insertion trajectories of all-atom and coarse-grained lipid models, we discovered a free energy barrier for lipid insertion and identified multiple pathways characterized by splayed lipid intermediates. This barrier appears hidden when only the lipid's displacement normal to the bilayer, which has traditionally been used to describe lipid exchange, is monitored. In contrast, an accurate reaction coordinate measures the breakage and formation of hydrophobic lipid-membrane contacts, which give rise to a barrier for lipid insertion. At the transition state, hydrophobic contacts are just as likely to form as they are to break. Consistent with this fact, membrane distortions and solvent fluctuations, which can both enable and prevent hydrophobic contact formation, are observed in the transition state ensemble. Overall, our results demonstrate that the formation and breakage of hydrophobic contacts is rate limiting for passive lipid exchange and provide a foundation to understand the catalytic function of lipid transfer proteins.
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