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Are small features in membranes due to "modulated phases" or "microemulsions"?

Caitlin E. Cornell, Allison D. Skinkle, Shushan He, Ilya Levental, Kandice R. Levental, and Sarah L. Keller, 2018, Tuning length scales of small domains in cell-derived membranes and synthetic model membranes, Biophys. J., 115:690-701.

Suppose you observe domains arranged in small dots or stripes across the surface of a lipid membrane. What sets the length scale of those features? Is it more accurate to describe the membrane as being in a "modulated phase" or a "microemulsion"? In this paper, we tested predictions of how the size, morphology, and fluorescence levels of small domains varied with the membrane's temperature, tension, and lipid composition. Using artificial vesicles and cell-derived vesicles, we found that 1) the characteristic size of domains decreases when the temperature is increases or membrane tension is decreased, 2) stripes are favored over circular domains for lipid compositions with low energy per unit interface, 3) fluorescence levels are consistent with domain registration across both monolayer leaflets of the bilayer, and 4) small domains form in GUVs composed of lipids both with and without ester-linked lipids. No current theory for modulated phases or microemulsions fits all of these observations, suggesting a motivation to modify or enhance current theories. The lead author, Caitlin Cornell, collected her data at UW and in the lab of our outstanding collaborators, Ilya and Kandice Levental.

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