Spatial and Translational Regulation of Exocyst Subunits by Cell Cycle in Budding Yeast

The role of the exocytic pathway in cell wall assembly in yeast

The cell wall is a dynamic organelle which is tightly controlled for cell morphology, viability, and pathogenesis. It was previously shown that exocytosis is involved in the secretion of some components and enzymes of the cell wall. However, how the secretory pathway affects the cell wall integrity and assembly remains unclear. Here we show that the secretory pathway mutant (sec) cells were sensitive to cell wall antagonists in Saccharomyces cerevisiae, and they were lysed at restrictive conditions but can be rescued by osmotic stabilizers, indicating their cell walls were disrupted. Although glucans were reduced at the cell surface in sec mutants as speculated, the other two main cell wall components, chitins, and mannoproteins, were accumulated at the cell surface. We also found that both the protein level and the phosphorylation level of Slt2 increased in sec mutants. These results suggest that the exocytic pathway has a critical role in cell wall assembly. Our study will help to understand the mechanism of cell wall formation.

Sphingolipids are required for exocyst polarity and exocytic secretion in Saccharomyces cerevisiae

Background

Exocytosis is a process by which vesicles are transported to and fused with specific areas of the plasma membrane. Although several studies have shown that sphingolipids are the main components of exocytic compartments, whether they control exocytosis process is unclear.

Results

Here, we have investigated the role of sphingolipids in exocytosis by reducing the activity of the serine palmitoyl-transferase (SPT), which catalyzes the first step in sphingolipid synthesis in endoplasmic reticulum. We found that the exocyst polarity and exocytic secretion were impaired in lcb1-100 mutant cells and in wild type cells treated with myriocin, a chemical which can specifically inhibit SPT enzyme activity, suggesting that sphingolipids controls exocytic secretion. This speculation was further confirmed by immuno-fluorescence and electron microscopy results that small secretory vesicles were accumulated in lcb1-100 mutant cells.

Conclusions

Taken together, our results suggest that sphingolipids are required for exocytosis. Mammals may use similar regulatory mechanisms because components of the exocytic secretion apparatus and signaling pathways are conserved.