Plasma membrane stress induces relocalization of Slm proteins and activation of TORC2 to promote sphingolipid synthesis

Abstract

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.

Caveolae regulate the nanoscale organization of the plasma membrane to remotely control Ras signaling

“…Enrichment of PS on the cytoplasmic face of caveolae has been reported (Fairn 2013), lend themselves to a hypothesis whereby an individual caveola may function as an organizational unit that can flatten into the PM and release specific lipids that modulate the local lipid environment to respond to changes in cellular stress. Cross talk between mechanosensitive caveolae and cellular lipids strengthens the proposed functional similarity to eisosomes in yeast in which eisosome mechanosensation is translated into alterations in glycosphingolipid metabolism as a protective mechanism (Berchtold et al, 2012;Parton and del Pozo, 2013). We suggest that the loss of CAV1/caveolae from the PM has widereaching and dramatic effects on PM dynamics due to these lipid changes.…”

Sphingolipid Homeostasis in the Endoplasmic Reticulum and Beyond

“…Here a prominent role for Slm proteins is suggested by reports that the PH-domain-containing proteins Slm1 and Slm2 are necessary and sufficient for recruitment of Ypk1 to Torc2 and for Ypk1 activation (Fig. 4D) (Berchtold et al 2012;Niles et al 2012). Furthermore, inhibition of sphingolipid synthesis causes a rapid relocalization of Slm proteins from eisosomes to membrane domains containing Torc2, and this relocalization is required for phosphorylation of Ypk1 at Thr662 (Berchtold et al 2012).…”

“…In yeast, the Torc2 kinase complex is a positive regulator of ceramide synthase activity and therefore controls steady-state levels of LCBs and ceramides (Aronova et al 2008). Torc2 activity is in turn induced by sphingolipid depletion (Berchtold et al 2012), suggesting a potential feedback mechanism controlling ceramide production. It is not yet known how Torc2 regulates ceramide synthase.…”

“…This mechanism may be used by the yeast proteins Nce102, Slm1, and Slm2. These proteins dynamically localize to sphingolipid-dependent membrane domains, and their activity is likewise controlled by membrane domain association (Frohlich et al 2009;Berchtold et al 2012). For example, the transmembrane protein Nce102 localizes to sphingolipid-dependent domains known as eisosomes, where it inhibits the Pkh1 and Pkh2 kinases (Frohlich et al 2009).…”

Eisosome Ultrastructure and Evolution in Fungi, Microalgae, and Lichens

“…These observations were brought together by Strádalová et al (5), who showed that the membrane compartment occupied by Can1p (MCC) localized to the invaginations seen with freeze fracture EM, and by Karotki et al (6), who showed that Pil1p localizes to the cytoplasmic surfaces of these invaginations. Additional proteins also associate with these punctate domains, in some cases in a transient fashion (17), and the MCC component is reportedly enriched in ergosterol (18) and influenced by phosphoinositide (6,19,20) and sphingolipid (14,17,(21)(22)(23)(24) levels. Hence, the current yeast model (25,26) proposes that Pil1p and Lsp1p, which contain membrane curvatureinducing BAR domains (6,27,28), together with Seg1p (29,30), form a submembrane complex (6) reportedly influenced by Pil1p phosphorylation (25); this complex then either creates or associates with the MCC domains, presumably inducing an inward curvature.…”