Collaboration between researchers at the University of Geneva, Institut de biologie structurale de Grenoble, and the University of Fribourg has shown how lipids and proteins in cell membranes react in response to mechanical strain and stress.
“Until now, the techniques available did not allow us to study lipids in their natural environment inside membranes,” said project lead Robbie Loewith, PhD, professor at the University of Geneva. “We have been able to meet this challenge by using cryo-electron microscopy.’’
The researchers used cryo-electron microscopy (cryo-EM) to visualize how regions of the membrane stabilize lipids under different conditions, leading to cascading cellular responses. These findings were published in Nature, in a study titled, “Cryo-EM architecture of a near-native stretch-sensitive membrane microdomain.”
The plasma membrane is primarily composed of lipids and proteins, which are organized to be functionally dynamic supporting the changing needs of the cell, while also structured to maintain cellular integrity, acting as a barrier between the intracellular components and the outside environment. The balance between these functions has been hypothesized to involve subregions of the membrane called microdomains, comprised of specific lipid and protein components.
The present study used cryo-EM on baker’s yeast (Saccharomyces cerevisiae) to precisely observe the protein and lipid structures of the membrane. This technique freezes the cells at -200°C, allowing for detailed visualization of the plasma membrane using an electron microscope. The research team focused on a specific microdomain called eisosomes.
“For the first time, we have succeeded in purifying and observing eisosomes containing plasma membrane lipids in their native state. This is a real step forward in our understanding of how they function,” explained Markku Hakala, PhD, a postdoctoral researcher at the University of Geneva.
Eisosomes scaffold the plasma membrane compartments that can sense and respond to mechanical stress. The protein coat contains BAR-domain proteins Pil1 and Lsp1 that form a lattice structure forming helical tubules bound to membrane lipids which change shape in response to mechanical stress. Using cryo-EM enabled the team to observe the structural organization of the protein coat in this microdomain and its response to stress.
“We discovered that when the eisosome protein lattice is stretched, the complex arrangement of lipids in the microdomains is altered. This reorganization of the lipids likely enables the release of sequestered signaling molecules to trigger stress adaptation mechanisms,” said Jennifer Kefauver, PhD, postdoctoral researcher and first author, University of Geneva.
The authors described how their data show how the “stretching of the Pil1/Lsp1 lattice liberates lipids that would otherwise be anchored by the Pil1/Lsp1 coat.” This change in structure provides insight into how the BAR proteins in this microdomain are functionally responsive to mechanical stress.
“Our study reveals a molecular mechanism by which mechanical stress can be converted to biochemical signaling via protein-lipid interactions in unprecedented detail,” explained Kefauver.
The authors propose that “lipid mobilization represents a general mechanism to free sequestered factors to initiate their signaling functions that are sensitive to membrane stress,” suggesting that these changes in structure may have larger implications for the internal function of cells as well as the observed membrane changes. Other microdomains are thought to have differing functions, including retaining or releasing proteins and lipids during stress or sending signals for membrane damage elsewhere in the cell.
This work enhances understanding of cellular membranes and their response to stressors. Future research into microdomain function may help elucidate details on plasma membrane function, paving the way for the development of new therapeutic strategies targeting membrane-associated disorders.
