Abstract
BackgroundWhen stressed, eukaryotic cells produce triacylglycerol (TAG) to store nutrients and mobilize autophagy to combat internal damage. We and others previously reported that in yeast, elimination of TAG synthesizing enzymes inhibits autophagy under nitrogen starvation, yet the underlying mechanism has remained elusive.ResultsHere, we show that disruption of TAG synthesis led to diacylglycerol (DAG) accumulation and its relocation from the vacuolar membrane to the endoplasmic reticulum (ER). We further show that, beyond autophagy, ER-accumulated DAG caused severe defects in the endomembrane system, including disturbing the balance of ER-Golgi protein trafficking, manifesting in bulging of ER and loss of the Golgi apparatus. Genetic or chemical manipulations that increase consumption or decrease supply of DAG reversed these defects. In contrast, increased amounts of precursors of glycerolipid synthesis, including phosphatidic acid and free fatty acids, did not replicate the effects of excess DAG. We also provide evidence that the observed endomembrane defects do not rely on Golgi-produced DAG, Pkc1 signaling, or the unfolded protein response.ConclusionsThis work identifies DAG as the critical lipid molecule responsible for autophagy inhibition under condition of defective TAG synthesis and demonstrates the disruption of ER and Golgi function by excess DAG as the potential cause of the autophagy defect.
Highlights
When stressed, eukaryotic cells produce triacylglycerol (TAG) to store nutrients and mobilize autophagy to combat internal damage
Block of TAG synthesis disrupts the endomembrane system As the process of autophagy relies on membrane and protein input from the rest of the endomembrane system through direct contacts and vesicular trafficking pathways [20], we first analyzed the morphology of major membrane-based organelles in order to understand the nature of autophagy defect in dga1Δ lro1Δ cells (Fig. 1a–c, Additional file 1: Fig. S2)
They were not strongly stained by BODIPY (Fig. 2b), indicating that the bulbs are separate from lipid droplets, which frequently associate with the endoplasmic reticulum (ER)
Summary
Eukaryotic cells produce triacylglycerol (TAG) to store nutrients and mobilize autophagy to combat internal damage. We and others previously reported that in yeast, elimination of TAG synthesizing enzymes inhibits autophagy under nitrogen starvation, yet the underlying mechanism has remained elusive. Polar lipids, including phospholipids, sphingolipids, and sterols, are major constituents of cellular membranes. In particular TAG, are employed as a storage medium of carbon nutrients [1, 2]. Autophagy is a stress response pathway that utilizes double-membrane autophagosomes to clean up obsolete intracellular materials in eukaryotes [10, 11]. Autophagosomes are formed from precursor membrane sacs, known as phagophores or isolation membranes. The phagophore is thought to emerge from
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