Roles for sphingolipids in Saccharomyces cerevisiae

Abstract

Studies using Saccharomyces cerevisiae, the common baker's or brewer's yeast, have progressed over the past twenty years from knowing which sphingolipids are present in cells and a basic outline of how they are made to a complete or nearly complete directory of the genes that catalyze their anabolism and catabolism. In addition, cellular processes that depend upon sphingolipids have been identified including protein trafficking/exocytosis, endocytosis and actin cytoskeleton dynamics, membrane microdomains, calcium signaling, regulation of transcription and translation, cell cycle control, stress resistance, nutrient uptake and aging. These will be summarized here along with new data not previously reviewed. Advances in our knowledge of sphingolipids and their roles in yeast are impressive but molecular mechanisms remain elusive and are a primary challenge for further progress in understanding the specific functions of sphingolipids.

Fig. 1

Pathways of sphingolipid metabolism in Saccharomyces cerevisiae. Metabolic intermediates and complex sphingolipids are shown in bold, genes are indicated by italics and enzyme names are in regular lettering. When grown aerobically the fatty acid in complex sphingolipids is often hydroxylated at C₂ and sometimes at C₃ (not shown), a reaction that requires Scs7 (not shown). Ceramides can be hydrolyzed by two ceramidases, Ydc1 and Ypc1 (not shown), to yield a fatty acid and an LCB.^, Structures of the indicated compounds are presented in previous publications.^, Adapted from ref.

Fig. 2

S. cerevisiae signal transduction pathways regulated by LCBs. LCBs transiently increase during a heat stress and are hypothesized to activate Pkh1 and Pkh2. As discussed in the text, Pkh2 is probably more dependent upon LCBs than is Pkh1 although this has not been defined for each cellular response. Kinase assays with purified proteins suggest that LCBs can directly trigger a small increase in Ypk1, Ypk2 and Sch9 activity (indicated by a dotted line).. Pkh1/2 phosphorylate Ypk1, Ypk2, Sch9 and Pkc1 in their activation loop (PDK1 site) but the proteins are not enzymatically active. To become active they also need to be phosphorylated in a hydrophobic region (PDK2 site) and in a turn motif in their C-terminus. Phosphorylation of these sites in Ypk2 is mediated by TORC2 and for Sch9 phosphorylation is mediated by TORC1. Ypk1/2 and Pkc1 are shown working in parallel pathways to control cell wall integrity, but data also support an alternative pathway in which Ypk1/2 work upstream of Pkc1.^, Adapted from ref.

Fig. 3

Synthesis and turnover of complex sphingolipids along with actin dynamics are regulated by PI4,5P₂ and the Slm1 and Slm2 proteins. PI4,5P₂, synthesized by Stt4 and Mss4 on the plasma membrane, in cooperation with the TORC2 and the Pkh1/2 protein kinases are proposed to activate Slm1 and Slm2. The Slm proteins then impair turnover of complex sphingolipids, particularly IPC, by inhibiting Isc1 and they also regulate the calcium/calmodulin-regulated protein phosphatase calcineurin which dephosphorylates and inactivates the Slm proteins and also interacts with Csg1/2 in an unknown manner to regulate conversion of IPC to MIPC by the Csg1/2 enzymes. Regulation of the Rho1/Pkc1 pathway by Slm1/2 works independently from regulation of sphingolipid metabolism. Genetic interactions suggest that IPC plays a role in actin organization, but the mechanism is unknown. Pkh1/2 are probably attached to eisosomes (not shown) as depicted in Fig. 2. Adapted from ref.