Subcellular mechanisms coupling lipid synthesis and methionine metabolism

PROJECT SUMMARY Inter-organelle membrane contact sites (MCSs) are emerging as critical hubs that regulate metabolic pathways which enables cellular adaptation to external perturbations and environmental stress. Because of their roles in cellular metabolism, dysregulation of MCSs has been linked to numerous metabolic and neurodegenerative disorders as well as aging. Numerous studies to date have linked MCSs to non-vesicular lipid transfer and spatial organization of lipid metabolism. Using yeast as a model system, we found that contact sites between the Endoplasmic reticulum (ER) and the lysosome recruit lipid enzymes to spatially organize lipid synthesis and storage as a response to lipotoxic stress. Lipid biosynthesis is intimately linked to methionine metabolism. Specifically, S-adenosyl methionine (SAM) which is a direct product of methionine, is an essential methyl donor required for membrane lipid biosynthesis. However, how MCSs regulate the availability and utilization of non- lipidic metabolites such as SAM is currently unknown. Surprisingly, we found that deletion of ER-lysosome tethers in yeast significantly alters SAM levels and sensitizes yeast to methionine-deprivation. In addition to providing methyl groups for phospholipid methylation, SAM is involved in other cellular functions such as nutrient signaling, epigenetic regulation, and maintaining redox balance. Therefore, lowering SAM levels by perturbing ER-lysosome contact sites will have direct consequences on these essential cellular pathways. Based on these results, we propose that lipid synthesis at ER-lysosome contact sites regulates SAM availability, which serves as an important driver for regulating downstream cellular pathways. The central goal of this proposal is to test this hypothesis by 1) elucidating the mechanisms by which ER-lysosome tethers regulate phospholipid synthesis; 2) determining the functional consequences for altered compartmentalization of phospholipid methylation; 3) defining molecular and structural basis for lipid binding and transport by ER- lysosome tethers; and 4) identifying conserved mechanisms for SAM regulation by ER-lysosome contact sites. Collectively, the results of our studies will provide insight into the organizational aspects of SAM regulation, how this alters the lipidome, and its relationship to aging and metabolic diseases.