Regulation of ceramide biosynthesis by TOR complex 2.

Ceramides and sphingoid long-chain bases (LCBs) are precursors to more complex sphingolipids and play distinct signaling roles crucial for cell growth and survival. Conserved reactions within the sphingolipid biosynthetic pathway are responsible for the formation of these intermediates. Components of target of rapamycin complex 2 (TORC2) have been implicated in the biosynthesis of sphingolipids in S. cerevisiae; however, the precise step regulated by this complex remains unknown. Here we demonstrate that yeast cells deficient in TORC2 activity are impaired for de novo ceramide biosynthesis both in vivo and in vitro. We find that TORC2 regulates this step in part by activating the AGC kinase Ypk2 and that this step is antagonized by the Ca2+/calmodulin-dependent phosphatase calcineurin. Because Ypk2 is activated independently by LCBs, the direct precursors to ceramides, our data suggest a model wherein TORC2 signaling is coupled with LCB levels to control Ypk2 activity and, ultimately, regulate ceramide formation.

Probing the membrane environment of the TOR kinases reveals functional interactions between TORC1, actin, and membrane trafficking in Saccharomyces cerevisiae.

The TOR kinases are regulators of growth in eukaryotic cells that assemble into two distinct protein complexes, TORC1 and TORC2, where TORC1 is inhibited by the antibiotic rapamycin. Present models favor a view wherein TORC1 regulates cell mass accumulation, and TORC2 regulates spatial aspects of growth, including organization of the actin cytoskeleton. Here, we demonstrate that in yeast both TORC1 and TORC2 fractionate with a novel form of detergent-resistant membranes that are distinct from detergent-resistant plasma membrane "rafts." Proteomic analysis of these TOR-associated membranes revealed the presence of regulators of endocytosis and the actin cytoskeleton. Genetic analyses revealed a significant number of interactions between these components and TORC1, demonstrating a functional link between TORC1 and actin/endocytosis-related genes. Moreover, we found that inhibition of TORC1 by rapamycin 1) disrupted actin polarization, 2) delayed actin repolarization after glucose starvation, and 3) delayed accumulation of lucifer yellow within the vacuole. By combining our genetic results with database mining, we constructed a map of interactions that led to the identification of additional genetic interactions between TORC1 and components involved in membrane trafficking. Together, these results reveal the broad scope of cellular processes influenced by TORC1, and they underscore the functional overlap between TORC1 and TORC2.

Tor kinases are in distinct membrane-associated protein complexes in Saccharomyces cerevisiae.

Tor1p and Tor2p kinases, targets of the immune-suppressive antibiotic rapamycin, are components of a highly conserved signaling network that couples nutrient availability and cell growth. To gain insight into the molecular basis underlying Tor-dependent signaling, we used cell fractionation and immunoaffinity chromatography to examine the physical environment of Tor2p. We found that the majority of Tor2p associates with a membrane-bound compartment along with at least four other proteins, Avo1p-Avo3p and Lst8p. Using immunogold electron microscopy, we observed that Tor2p, as well as Tor1p, localizes in punctate clusters to regions adjacent to the plasma membrane and within the cell interior, often in association with characteristic membranous tracks. Cell fractionation, coimmunoprecipitation, and immunogold electron microscopy experiments confirmed that Lst8 associates with both Tor2p as well as Tor1p at these membranous sites. In contrast, we find that Kog1, the yeast homologue of the mammalian Tor regulatory protein Raptor, interacts preferentially with Tor1p. These findings provide evidence for the existence of Tor signaling complexes that contain distinct as well as overlapping components. That these complexes colocalize to a membrane-bound compartment suggests an intimate relationship between membrane-mediated signaling and Tor activity.

TOR complex 1 includes a novel component, Tco89p (YPL180w), and cooperates with Ssd1p to maintain cellular integrity in Saccharomyces cerevisiae.

The Tor1p and Tor2p kinases, targets of the therapeutically important antibiotic rapamycin, function as components of two distinct protein complexes in yeast, termed TOR complex 1 (TORC1) and TORC2. TORC1 is responsible for a wide range of rapamycin-sensitive cellular activities and contains, in addition to Tor1p or Tor2p, two highly conserved proteins, Lst8p and Kog1p. By identifying proteins that co-purify with Tor1p, Tor2p, Lst8p, and Kog1p, we have characterized a comprehensive set of protein-protein interactions that define further the composition of TORC1 as well as TORC2. In particular, we have identified Tco89p (YPL180w) and Bit61p (YJL058c) as novel components of TORC1 and TORC2, respectively. Deletion of TOR1 or TCO89 results in two specific and distinct phenotypes, (i) rapamycin-hypersensitivity and (ii) decreased cellular integrity, both of which correlate with the presence of SSD1-d, an allele of SSD1 previously associated with defects in cellular integrity. Furthermore, we link Ssd1p to Tap42p, a component of the TOR pathway that is believed to act uniquely downstream of TORC1. Together, these results define a novel connection between TORC1 and Ssd1p-mediated maintenance of cellular integrity.