The plasma membrane provides a barrier between a cell and its environment, which – at the same time – needs to be highly flexible in order to respond to mechanical or osmotic changes that require a rapid expansion of the membrane area. In animals, plasma membrane invaginations formed by caveolae have been suggested to act as tension-sensing membrane reservoirs that flatten in response to mechanical stress to increase the surface area. Yeast does not have caveolae, but many fungal and algal cells contain eisosomes, protein-based structures that generate invaginations at the plasma membrane and have been linked to control of signalling pathways. The architectural similarity of eisosomes with tension-sensing membrane invaginations prompted James Moseley and co-workers to investigate their possible role as membrane reservoirs. On page , they devise a single-cell assay to study rapid membrane expansion in yeast protoplasts, which lack a rigid cell wall. Indeed, the authors find that eisosomes are required for the survival of yeast cells under hypoosmotic stress because cells that lack the main eisosomal protein Pil1 are unable to expand their membrane and, subsequently, lyse. Furthermore, as shown here, hypoosmotic conditions result in a disassembly of eisosomes with a concomitant increase in the plasma membrane surface area. Thus, these findings point to a remarkable similarity between yeast eisosomes and caveolae, despite marked differences in their molecular composition, and also raise interesting questions, such as whether the signalling and mechanical functions of eisosomes are related.
