Maintaining the integrity and proper composition of the plasma membrane are critical for cell survival; therefore, cells have elaborate mechanisms for responding to various types of membrane stress or changes in membrane composition. Sphingolipids are a specific type of lipid found in the outer leaflet of the plasma membrane, and the synthesis of these lipids is promoted by the kinase complex target of rapamycin 2 (TORC2). The kinase Ypk1 and the pleckstrin homology domain–containing proteins Slm1 and Slm2 are targets of TORC2 and are implicated in transducing TORC2 signals to the synthesis of sphingolipids. Berchtold et al. investigated the mechanisms through which inhibition of sphingolipid synthesis or membrane stress induced by low osmolarity or mechanical stretch stimulated the activity of TORC2 in yeast by monitoring the phosphorylation status of the TORC2 target Ypk1. Pharmacological inhibition or genetic disruption of sphingolipid synthesis triggered an increase in the phosphorylation of Ypk1. Loss of function of Slm1 and Slm2 (Slm1/2) reduced basal Ypk1 phosphorylation, and expression of a hyperactive form of Ypk2 rescued the lethality associated with Slm1/2 loss of function. Slm1 fused to green fluorescent protein (GFP) was present in two distinct membrane compartments, a site that marks the location of endocytosis (the eisosome) and another that did not colocalize with a marker of the eisosome. The abundance of Slm1-GFP in the eisosome pool was reduced in response to inhibition of sphingolipid synthesis, and inducible anchoring of Slm1 to eisosomes, based on rapamycin-induced association of fusion proteins with FKBP and Frb, prevented its redistribution and decreased basal phosphorylation of Ypk1. Coimmunoprecipitation experiments showed that the interaction of Ypk1 or TORC2 with Slm1 increased in response to disruption of sphingolipid synthesis, suggesting that redistribution of Slm1 may facilitate its interaction with Ypk1 and TORC2. Low osmolarity also triggered an increase in Ypk1 phosphorylation, and this response was lost in cells deficient in Slm1/2. Low osmolarity and mechanical stretch stimulated the redistribution of Slm1-GFP, along with the phosphorylation of Ypk1. Thus, the authors propose that plasma membrane stress triggers the release of Slm1/2 from the eisosome, enabling it to interact with the plasma membrane–associated TORC2 and stimulate TORC2 activity toward Ypk1, which stimulates sphingolipid synthesis.
D. Berchtold, M. Piccolis, N. Chiaruttini, I. Riezman, H. Riezman, A. Roux, T. C. Walther, R. Loewith, Plasma membrane stress induces relocalization of Slm proteins and activation of TORC2 to promote sphingolipid synthesis. Nat. Cell Biol. 14, 542–547 (2012). [PubMed]