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bioRxiv · 10.1101/2021.08.22.457294

Assessing the use of ellipsoidal microparticles for determining lipid membrane viscosity

Jahl, P. E.·Parthasarathy, R.
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Abstract

The viscosity of lipid membranes sets the timescales of membrane-associated flows and therefore influences the dynamics of a wide range of cellular processes. Techniques to measure membrane viscosity remain sparse, however, and reported measurements to date, even of similar systems, give viscosity values that span orders of magnitude. To address this, we improve a method based on measuring both the rotational and translational diffusion of membrane-anchored microparticles and apply this approach and one based on tracking the motion of phase-separated lipid domains to the same system of phase-separated giant vesicles. We find good agreement between the two methods, with inferred viscosities within a factor of two of each other. Our technique uses ellipsoidal microparticles, and we show that the extraction of physically meaningful viscosity values from their motion requires consideration of their anisotropic shape. The validation of our method on phase-separated membranes makes possible its application to other systems, which we demonstrate by measuring the viscosity of bilayers composed of lipids with different chain lengths ranging from 14 to 20 carbon atoms, revealing a very weak dependence of two-dimensional viscosity on lipid size. The experimental and analysis methods described here should be generally applicable to a variety of membrane systems, both reconstituted and cellular. Statement of SignificanceThe lipid bilayers that underlie cellular membranes are two-dimensional fluids whose viscosity sets timescales of flow. Lipid membrane viscosity remains poorly quantified, with a paucity of methods and considerable disagreement between values reported using different techniques. We describe a method based on measuring the Brownian diffusion of ellipsoidal microparticles which we apply to phase-separated membranes alongside a previously established method for determining membrane viscosity, finding good agreement between the two techniques. We further examine homogenous membranes composed of lipids with different chain lengths, not amenable to phase-separationbased methods, revealing a very weak dependence of viscosity on lipid size. Our approach should be applicable to a wide range of membrane systems, both in vitro and in living cells.

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Jahl, P. E., Parthasarathy, R.. 2021-08-23. Assessing the use of ellipsoidal microparticles for determining lipid membrane viscosity. https://doi.org/10.1101/2021.08.22.457294

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