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Probing lipid membrane nano-mechanics


Measuring the deformation imposed by membrane proteins and other membrane-associated objects to the lipid bilayer (the main ingredient of the cell membrane) is essential in describing their elastic interaction and the resulting influence of the host medium on the activity of the protein. Until now, however, no experimental technique had been able to characterize the deformation profile near such objects.

A team of researchers from the Laboratoire de Physique des Solides and the MSC laboratory in Paris studied the elastic properties of lipid bilayers using as probes gramicidin channels, which perturb the membranes locally and hence experience an interaction mediated by their elasticity. They measured this interaction, revealing membrane deformations on the nanometer scale, inaccessible by other techniques (Figure 1).

Figure 1 : Interaction energy V between two gramicidin channels in a phospholipid bilayer as a function of their center-to-center distance d. Inset : Structure factors S as a function of the scattering vector q for gramicidin channels in DLPC bilayers at different surface fractions η of inclusions : experimental data (red dots) and fits using the interaction energy V(d) (lines).

Using a complete elastic theory, they extracted the preferred slope of the lipid-water interface at contact with the protein and its associated anchoring strength, answering a question that had remained open for twenty years. In bilayers composed of phospholipids (the main ingredients of the cell membrane), the membrane thickness decreases steeply away from contact, sometimes even when the membrane is thicker than the channel (see Figure 2) ! This surprising conclusion provides a missing boundary condition necessary for solving the relevant elasticity equations.

Figure 2 : Schematic representation of increasingly thicker phospholipid membranes deformed by two inserted gramicidin channels. The deformation profile is magnified by a factor of three.

Using the parameters measured above and literature data for other material constants (and no adjustable parameters), they also predicted quantitatively literature results of numerical simulations and of conductivity measurements, bringing into agreement three completely different techniques in the framework of a single theoretical model and validating the use of continuum mechanics to describe lipid membranes at the nanoscale.

Reference

Coupling between inclusions and membranes at the nanoscale

Florent Bories, Doru Constantin, Paolo Galatola, and Jean-Baptiste Fournier

Physical Review Letters 120, 128104 (2018)
DOI:10.1103/PhysRevLett.120.128104

Contacts

Doru Constantin
Paolo Galatola
Jean-Baptiste Fournier