Abstract
Heterogeneities (e.g., membrane proteins and lipid domains) and deformations (e.g., highly curved membrane regions) in biological lipid membranes cause lipid packing defects that may trigger functional sorting of lipids and membrane-associated proteins. To study these phenomena in a controlled and efficient way within molecular simulations, we developed an external field protocol that artificially enhances packing defects in lipid membranes by enforcing local thinning of a flat membrane region. For varying lipid compositions, we observed strong thinning-induced depletion or enrichment, depending on the lipid's intrinsic shape and its effect on a membrane's elastic modulus. In particular, polyunsaturated and lysolipids are strongly attracted to regions high in packing defects, whereas phosphatidylethanolamine (PE) lipids and cholesterol are strongly repelled from it. Our results indicate that externally imposed changes in membrane thickness, area, and curvature are underpinned by shared membrane elastic principles. The observed sorting toward the thinner membrane region is in line with the sorting expected for a positively curved membrane region. Furthermore, we have demonstrated that the amphipathic lipid packing sensor (ALPS) protein motif, a known curvature and packing defect sensor, is effectively attracted to thinner membrane regions. By extracting the force that drives amphipathic molecules toward the thinner region, our thinning protocol can directly quantify and score the lipid packing sensing of different amphipathic molecules. In this way, our protocol paves the way toward high-throughput exploration of potential defect- and curvature-sensing motifs, making it a valuable addition to the molecular simulation toolbox.
Highlights
Biological membranes are highly dynamic and heterogeneous lipid bilayer barriers that physically separate the functional compartments of cells (McMahon and Gallop, 2005)
Provided that the concomitant enthalpic gain is bigger than the entropic loss of demixing, lipids or proteins are effectively attracted toward the curved region of the membrane
The thickness profile in the buffer zone as a function of the x-coordinate can be described by a sigmoid function a(x), which neatly falls within one standard deviation (STD) interval (Figure 1C)
Summary
Biological membranes are highly dynamic and heterogeneous lipid bilayer barriers that physically separate the functional compartments of cells (McMahon and Gallop, 2005). Proteins or lipids can reduce or even nullify the energetic cost associated with defect formation depending on their intrinsic molecular shape (Risselada and Marrink, 2009; Pinot et al, 2014; Baoukina et al, 2018) and amphipathicity (Ouberai et al, 2013; Vanni et al, 2013). They reduce the elastic energy associated with membrane bending. Provided that the concomitant enthalpic gain is bigger than the entropic loss of demixing, lipids or proteins are effectively attracted toward the curved region of the membrane (i.e., curvature sensing)
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