GPI anchored proteins identify gel-like lipid domains in the membrane of the Endoplasmic Reticulum
- Authors
- Publication Date
- Jul 05, 2024
- Source
- HAL
- Keywords
- Language
- English
- License
- Unknown
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Abstract
By definition, eukaryotic cells are made up of different compartments, each with its own specific functions. This specificity is the result of differences in the protein and lipid compositions of these compartments. Lipids not only act as a physical barrier between compartments, they also play a key role in numerous mechanisms. In particular, lipids enable the organization of proteins inserted in membranes, and can thus promote the clustering of these proteins in restricted areas. In contrast to intracellular membranes, lipid domains have been widely described at the plasma membrane. Yet the ability of lipids to form domains within membranes is exploited during intracellular transport of glycosylphosphatidylinositol (GPI)-anchored proteins. These proteins have the particularity of being attached to membranes via their anchoring to GPI, a phospholipid conjugated to sugars. In polarized cells, it has been shown that prior to export to plasma membrane, GPI-anchored proteins are accumulated in rigid lipid domains in the Golgi apparatus. This partitioning enables GPI-anchored proteins to be addressed to the right pole of the cell. This mechanism highlights the role of lipid domains within intracellular membranes, but the study of such domains remains complex, notably because of the difficulty of labelling and tracking the lipids. Although lipid probes do exist, they share a number of drawbacks. Most probes rely on the binding of a fluorescent molecule to lipids, either by targeting polar groups exposed on the membrane surface or by interacting directly with the hydrophobic core of the membrane. The former can only target lipids carrying a specific group, while the latter involve the insertion of ectopic lipids into membranes, thereby modifying their properties. Moreover, hydrophobic probes are also subject to lipid diffusion and are thus transported throughout the cell, preventing the study of lipids in a specific compartment. In this project, we developed a sensor capable of tracking lipid dynamics within ER membranes. Using the RUSH (Retention Using Selective Hooks) system developed to synchronize protein transport, we retained GPI-anchored proteins in the ER. This sensor enabled us to track in the ER not only GPI-anchored proteins, but also the lipids to which these proteins are anchored. We thus studied the effect of increasing membrane stiffness on ER membranes. In response to increased membrane saturation, we observed the formation of GPI-containing domains in the ER only. This effect is potentiated by a decrease in temperature, which also induces a decrease in stiffness. We were able to characterize these domains, showing that they remain connected to the rest of the ER, but that no diffusion is possible within them. Surprisingly, the appearance of these domains did not disrupt the organization or function of the ER, suggesting that they may represent a response to increased stiffness that preserves the fluidity of ER membranes.
