A Block in Endoplasmic Reticulum-to-Golgi Trafficking Inhibits Phospholipid Synthesis and Induces Neutral Lipid Accumulation

Abstract

Seeking to better understand how membrane trafficking is coordinated with phospholipid synthesis in yeast, we investigated lipid synthesis in several Sec؊ temperature-sensitive mutants, including sec13-1. Upon shift of sec13-1 cells to the restrictive temperature of 37°C, phospholipid synthesis decreased dramatically relative to the wild type control, whereas synthesis of neutral lipids, especially triacylglycerol (TAG), increased. When examined by fluorescence microscopy, the number of lipid droplets appeared to increase and formed aggregates in sec13-1 cells shifted to 37°C. Electron microscopy confirmed the increase in lipid droplet number and revealed that many were associated with the vacuole. Analysis of lipid metabolism in strains lacking TAG synthase genes demonstrated that the activities of the products of these genes contribute to accumulation of TAG in sec13-1 cells after the shift to 37°C. Furthermore, the permissive temperature for growth of the sec13-1 strain lacking TAG synthase genes was 3°C lower than sec13-1 on several different growth media, indicating that the synthesis of TAG has physiological significance under conditions of secretory stress. Together these results suggest that following a block in membrane trafficking, yeast cells channel lipid metabolism from phospholipid synthesis into synthesis of TAG and other neutral lipids to form lipid droplets. We conclude that this metabolic switch provides a degree of protection to cells during secretory stress.

Controlling Lipid Fluxes at Glycerol-3-phosphate Acyltransferase Step in Yeast

“…Among them are an increase in Gat1p level and the unique localization of this protein in crescent structures interacting with LPs, not observed for Gat2p. The increment in Gat1p levels became obvious as early as 4 h after shifting cells from glucose-to oleate-containing medium, reaching close to a 3-fold rise after 24 h. In agreement with our findings, a steady increase in GAT1 expression was detected by microarray analyses aimed at identifying genes with an expression profile responding to oleic acid (25). In this study, a peak in GAT1 expression was detected 9 h after shifting cells to oleate.…”

“…For comparison, we used the coatomer (nuclear pore complex and COPII) protein Sec13p fused to RFP as another ER-associated protein that has been linked to the control of the flux of fatty acid into TAG or phospholipids (2,25). Before induction, Gat1p and Gat2p uniformly labeled the ER membrane (Figs.…”

Molecular Mechanisms and Physiological Significance of Organelle Interactions and Cooperation

“…Most likely there is a rerouting of precursors such as phosphatidic acid and diacylglycerol from phospholipid to neutral lipid synthesis. Consistent with this, the anterograde inhibitor, brefeldin A, was found to cause an increase in lipid droplets at the expense of phospholipid synthesis (Gaspar et al, 2008). Brefeldin A treatment also resulted in an increase in TAG and lipid droplets in Clamydomonas and the related alga, Chlorella vulgaris .…”

Not Just Fat: The Structure and Function of the Lipid Droplet

“…It is not likely, however, that vesicular transports are necessary for LD targeting of ATGL and PAT proteins. In one study, the disruption of the SAR1-COPII pathway in S. cerevisiae was also shown to give a similar result (Soni et al 2009); a defect in the secretory pathway at the plasma membrane as well as Sec13 mutation enlarged LDs by decreasing phospholipids and increasing TG synthesis (Gaspar et al 2008). This suggests that the observed change in LDs may not be a direct consequence of aberrant vesicular trafficking.…”