Cell membranes play a crucial role in maintaining the integrity and functionality of cells. However, the mechanisms by which they fulfill these roles are not yet fully understood. Scientists from the University of Geneva (UNIGE), in collaboration with the Institut de Biologie Structurale de Grenoble (IBS) and the University of Fribourg (UNIFR), have used cryo-electron microscopy to observe how lipids and proteins interact in the plasma membrane and respond to mechanical stress. This work shows that, depending on the conditions, small membrane areas can stabilize different lipids to trigger specific cellular responses. These discoveries, published in the journal Natureconfirm the existence of well-organized lipid domains and begin to reveal the role they play in cell survival.
Cells are surrounded by a membrane – the plasma membrane – which acts as a physical barrier, but must also be malleable. These properties are conferred by the constituent components of membranes – lipids and proteins – whose molecular organization varies depending on the external environment. These dynamics are critical to membrane function, but must be well balanced to ensure that the membrane does not become too tense or too loose. The way cells sense changes in the biophysical properties of the plasma membrane is thought to involve microregions on the membrane – known as microdomains – that are thought to possess specific lipid and protein content and organization.
High-resolution cryo-electron microscopy
The team led by Robbie Loewith, professor in the Department of Molecular and Cellular Biology at UNIGE Faculty of Science, is interested in how the components of the plasma membrane interact with each other to ensure that the overall biophysical properties of the membrane remain optimized. for cell growth and survival.
‘Until now, the available techniques did not allow us to study lipids in their natural environment within membranes. Thanks to the Dubochet Center for Imaging (DCI) of the Universities of Geneva, Lausanne, Bern and the EPFL, we have been able to tackle this challenge using cryo-electron microscopy,” explains Robbie Loewith. This technique allows samples to be frozen at -200°C to retain the membranes in their original state, which can then be observed under an electron microscope.
The scientists used baker’s yeast (Saccharomyces cerevisiae), a model organism used in many research laboratories because it is very easy to grow and genetically manipulate. Furthermore, most basic cellular processes mirror those of higher organisms. This study focused on a specific membrane microdomain supported by a protein coat known as eisosomes. These structures are thought to be able to sequester or release proteins and lipids to help cells resist and/or signal damage to the membrane, using processes previously unknown.
”For the first time we have managed to purify and observe eisosomes containing plasma membrane lipids in their native state. This is a real step forward in our understanding of how they function,” explains Markku Hakala, a postdoctoral student at the Department of Biochemistry at the UNIGE Faculty of Science and co-author of the study.
Convert a mechanical signal into a chemical signal
Using cryo-electron microscopy, the scientists observed that the lipid organization of these microdomains changes in response to mechanical stimuli. ”We found that when the eisosome protein lattice is stretched, the complex arrangement of lipids in the microdomains changes. This lipid reorganization likely allows the release of sequestered signaling molecules to activate stress adaptation mechanisms. Our study reveals a molecular mechanism by which mechanical stress can be translated into biochemical signaling via protein-lipid interactions in unprecedented detail,” enthuses Jennifer Kefauver, postdoctoral researcher in the Department of Molecular and Cellular Biology and first author of the study.
This work opens many new avenues for studying the fundamental role of membrane compartmentalization – that is, the movement of proteins and lipids within membranes to form subcompartments known as microdomains. This mechanism allows cells to perform specialized biochemical functions, in particular the activation of cellular communication pathways in response to the various stresses to which they may be exposed.
