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.

Emp24p is a type I transmembrane protein that is involved in secretory protein transport from the endoplasmic reticulum (ER) to the Golgi complex. A yeast mutant that lacks Emp24p (emp24 delta) is viable, but periplasmic invertase and the glycosylphosphatidyl‐inositol‐anchored plasma membrane protein Gas1p are delivered to the Golgi apparatus with reduced kinetics, whereas transport of alpha‐factor, acid phosphatase and two vacuolar proteins is unaffected. Oligomerization and protease digestion studies of invertase suggest that the selective transport phenotype observed in the emp24 delta mutant is not due to a defect in protein folding or oligomerization. Consistent with a role in ER to Golgi transport, Emp24p is a component of COPII‐coated, ER‐derived transport vesicles that are isolated from a reconstituted in vitro budding reaction. We propose that Emp24p is involved in the sorting and/or concentration of a subset of secretory proteins into ER‐derived transport vesicles.

In Saccharomyces cerevisiae, alpha‐factor is internalized by receptor‐mediated endocytosis and transported via vesicular intermediates to the vacuole where the pheromone is degraded. Using beta‐tubulin and actin mutant strains, we showed that actin plays a direct role in receptor‐mediated internalization of alpha‐factor, but is not necessary for transport from the endocytic intermediates to the vacuole. beta‐tubulin mutant strains showed no defect in these processes. In addition, cells lacking the actin‐binding protein, Sac6p, which is the yeast fimbrin homologue, are defective for internalization of alpha‐factor suggesting that actin filament bundling might be required for this step. The actin dependence of endocytosis shows some interesting similarities to endocytosis from the apical membrane in polarized mammalian cells.

Dysregulated mammalian target of rapamycin (mTOR) promotes cancer, but underlying mechanisms are poorly understood. We describe an mTOR-driven mouse model that displays hepatosteatosis progressing to hepatocellular carcinoma (HCC). Longitudinal proteomic, lipidomics, and metabolomic analyses revealed that hepatic mTORC2 promotes de novo fatty acid and lipid synthesis, leading to steatosis and tumor development. In particular, mTORC2 stimulated sphingolipid (glucosylceramide) and glycerophospholipid (cardiolipin) synthesis. Inhibition of fatty acid or sphingolipid synthesis prevented tumor development, indicating a causal effect in tumorigenesis. Increased levels of cardiolipin were associated with tubular mitochondria and enhanced oxidative phosphorylation. Furthermore, increased lipogenesis correlated with elevated mTORC2 activity and HCC in human patients. Thus, mTORC2 promotes cancer via formation of lipids essential for growth and energy production.