Membrane mechanics

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Cell membranes are lipid bilayers enriched in proteins, that serves as interfaces, both between the cell and the outside, and between cell compartments. Mechanically, they behave rather interestingly, halfway between a solid and a liquid.

Indeed, they have a rigidity, and resist deformations perpendicular to the membrane plane. This deformation can be measured by the curvature of the membrane (see figure, left and middle). However, in the plane of the membrane, they behave as liquid and do not resist shear. They are also usually under tension, just like the surface of a balloon, which will also penalise membrane deformations (see figure, right).

Moreover, they can exhibit heterogeneities, in the form of membrane domains, or rafts. The boundary of these domains is thus a line, the energy cost to having boundaries causes an additional tension named line tension. Eventually, membrane behaviour will involve its interaction with its environment, including the cytoskeleton.

I was able to use such simple models of membrane behaviour in several problems :

SV40is a polyomavirus, a class of virus that can infect humans with immune deficiency, and are associated to breathing and kidney diseases. Our collaborators showed experimentally that SV40 can deform the cell membrane by by recruiting GM1 lipids, in a clathrin- and calveolae-independent pathway. I showed theoretically that viruses aggregate into tubes to minimise the interface energy of these GM1-enriched domains, i.e. because of line tension. This could be confirmed experimentally by our collaborators, who also showed that this tubulation is the first step to cell infection.

As we just saw, membrane domains are extremely important to understand membrane behaviour. To better understand domain formation, we predicted theoretically the kinetics of domain growth in membranes undergoing irreversible chemical reactions, i.e. a process of maturation. We show that the kinetics of maturation (e.g. phosphorylation) can regulate the size of domains, and hence the formation of transport intermediates.

Yeast are unicellular organism that play a large role in our everyday life (in allowing us to make bread and beer for instance), but also in our lab life : they are an easily tractable model system, with a biochemistry similar to that of mammals. However, they have a specificity : they are undergoing a tremendous turgor pressure of 10 atmospheres, i.e. twice the pressure in a bottle of Champagne ! This pressure is pushing the membrane against the cell wall, thus opposing membrane inward movement, like takes place during endocytosis. We show that endocytic membrane profiles observed experimentally can be accurately fitted by a minimal mechanical model of the membrane, and that the actin machinery must exert very large pulling forces do deform the membrane. We reveal the existence of a membrane shape instability that can lead to membrane scission as a consequence of BAR-domain proteins removal. Most importantly, we show that membrane under pressure are very unstable, and behave very differently from membranes under tension, which is the usual theoretical and experimental framework.

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For more on membrane modeling, see :