What is it about?

Every cell is bound by a limiting cell membrane built from a complex mixture of lipids and proteins. This not only acts as a physical barrier defining the inside from the outside, but it also acts as a gate-keeper of information and material flow in and out of cells. The living cell membrane is also patterned into membrane domains which impart specific functionality to membrane proteins that are localized to these regions, due to their compositional diversity. While few question the existence of such specialized regions, how these membrane domains are built has remained a major mystery in cell biology. The cell membrane itself sits sandwiched between the extra cellular matrix on one side and atop a thin layer of cytoskeleton primarily made of filamentous actin filaments and myosin motors to which it is intricately coupled. This actomyosin cortex has also been implicated in patterning the cell surface, however how this is achieved has remained unclear. In this study, we have used a combination of advanced fluorescence microscopy tools and theoretical models and to show that it is the activity of the actomyosin cortex and local lipid-lipid interactions that drive the membrane into an actively stirred state creating ‘active emulsions’ of diverse cell surface domains. We find these domains enrich specific lipids and lipid-linked proteins and hence can provide a mechanism of molecular sorting at the cell surface.

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Why is it important?

The physics of active emulsions and their observations in cellular context is an emerging frontier. Our study is both important and timely in providing one of the first evidences for active emulsions of mesoscale membrane domains at the cell surface. Moreover, our results open up newer directions and challenges to understand how active emulsions drive molecular sorting at the surface. Finally, newer probes for studying membrane order will allow us to track how these domains evolve over time and understand how cellular signaling machinery might utilise these domains.


This study is an outcome of a long and rewarding collaboration with my close theorist physics collaborators. These interactions over the years and the cross pollination of ideas have not only enriched our paper but also fueled my growth as a scientist.

Suvrajit Saha
University of California San Francisco

Read the Original

This page is a summary of: Active emulsions in living cell membranes driven by contractile stresses and transbilayer coupling, Proceedings of the National Academy of Sciences, July 2022, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2123056119.
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