Dymond, M. K. | bioRxiv Science

bioRxiv2026

Nuclear Pct1 couples phosphatidylcholine synthesis with membrane biogenesis

Phosphatidylcholine (PC) is the major eukaryotic phospholipid and its synthesis must be homeostatically controlled to prevent excess membrane and organelle growth. Here we investigate how PC synthesis by the Kennedy pathway is coordinated with membrane biogenesis. In budding yeast, the rate-limiting enzyme Pct1 is nuclear and reversibly associates with the inner nuclear membrane (INM) in response to lipid packing defects caused by low PC. We show that the enzymes acting after Pct1 to generate PC remain at the endoplasmic reticulum (ER) during pathway activation. Relocating the final PC synthesis step to different endomembrane sites does not alter the kinetics of Pct1 release from the INM, indicating that newly made PC equilibrates with the INM rapidly. In contrast, elevated phosphatidic acid locks Pct1 at the INM, prevents pathway inactivation and drives nuclear/ER membrane proliferation. These results support a model in which nuclear Pct1 senses lipid imbalance while ER-localized enzymes supply PC; disrupting this homeostasis leads to uncontrolled membrane biogenesis.

cell biology↗

bioRxiv2024

The partitioning of fatty acids between membrane and storage lipids controls ER membrane expansion.

The biogenesis of membrane-bound organelles involves the synthesis, remodelling and degradation of their constituent phospholipids. How these pathways regulate organelle size, remains still poorly understood. Here we demonstrate that a lipid degradation pathway inhibits the expansion of the endoplasmic reticulum (ER) membrane. Phospholipid diacylglycerol acyltransferases (PDATs) use endogenous phospholipids as fatty acyl donors to generate triglyceride stored in lipid droplets. The significance of this non-canonical triglyceride biosynthetic pathway has remained elusive. We find that the activity of the yeast PDAT Lro1 is regulated by a membrane- proximal domain facing the luminal side of the ER bilayer. To reveal the biological roles of PDATs, we engineered an Lro1 variant with derepressed activity. We show that active Lro1 mediates the retraction of ER membrane expansion driven by phospholipid synthesis. Furthermore, the subcellular distribution and membrane turnover activity of Lro1 are controlled by diacylglycerol, produced by the activity of Pah1, a conserved member of the lipin family. Collectively, our findings reveal a lipid metabolic network that regulates endoplasmic reticulum biogenesis by converting phospholipids into storage lipids.

cell biology↗