The plasma membrane delimits the cell, and its integrity is essential for cell survival. Lipids and proteins form domains of distinct composition within the plasma membrane. How changes in plasma membrane composition are perceived, and how the abundance of lipids in the plasma membrane is regulated to balance changing needs remains largely unknown. Here, we show that the Slm1/2 paralogues and the target of rapamycin kinase complex 2 (TORC2) play a central role in this regulation. Membrane stress, induced by either inhibition of sphingolipid metabolism or by mechanically stretching the plasma membrane, redistributes Slm proteins between distinct plasma membrane domains. This increases Slm protein association with and activation of TORC2, which is restricted to the domain known as the membrane compartment containing TORC2 (MCT; ref. ). As TORC2 regulates sphingolipid metabolism, our discoveries reveal a homeostasis mechanism in which TORC2 responds to plasma membrane stress to mediate compensatory changes in cellular lipid synthesis and hence modulates the composition of the plasma membrane. The components of this pathway and their involvement in signalling after membrane stretch are evolutionarily conserved.

Sterols and sphingolipids are limited to eukaryotic cells, and their interaction has been proposed to favor formation of lipid microdomains. Although there is abundant biophysical evidence demonstrating their interaction in simple systems, convincing evidence is lacking to show that they function together in cells. Using lipid analysis by mass spectrometry and a genetic approach on mutants in sterol metabolism, we show that cells adjust their membrane composition in response to mutant sterol structures preferentially by changing their sphingolipid composition. Systematic combination of mutations in sterol biosynthesis with mutants in sphingolipid hydroxylation and head group turnover give a large number of synthetic and suppression phenotypes. Our unbiased approach provides compelling evidence that sterols and sphingolipids function together in cells. We were not able to correlate any cellular phenotype we measured with plasma membrane fluidity as measured using fluorescence anisotropy. This questions whether the increase in liquid order phases that can be induced by sterol-sphingolipid interactions plays an important role in cells. Our data revealing that cells have a mechanism to sense the quality of their membrane sterol composition has led us to suggest that proteins might recognize sterol-sphingolipid complexes and to hypothesize the coevolution of sterols and sphingolipids.

Summary Dietary nutrient limitation (dietary restriction)

Graphical Abstract Highlights d Treatment of cells with palmitate increases saturated glycerolipids and ER stress d Di-saturated glycerolipids are key to lipotoxicity in this model d Inhibiting ER-localized GPAT enzymes protects cells from lipotoxicity d Depletion of the putative E3 ligase RNF213 protects cells from lipotoxicity In Brief Cellular lipotoxicity due to saturated fatty acids causes cell death and is thought to be a root cause of metabolic diseases. Piccolis, Bond, et al. use unbiased analyses to reveal hundreds of genes modulating cellular palmitate lipotoxicity and implicate saturated glycerolipids as a causative factor.

An unbiased mass spectrometry–based lipidomic screening method is used to analyze the major lipids of yeast deletions in protein kinase/phosphatase genes. This creates a new, rich source of biological insight. It uncovers new players in lipid homeostasis and gives a useful data set to further the understanding of lipid regulation by signaling networks.