Nitrogen starvation leads to TOR kinase-mediated downregulation of fatty acid synthesis in the algaeChlorella sorokinianaandChlamydomonas reinhardtii

Abstract

AbstractThe accumulation of triacylglycerol (TAG) as a storage compound in eukaryotic algae has been the subject of extensive studies over the last 50 years. The model industrial algaChlorella sorokinianaaccumulates TAG and other storage compounds under nitrogen (N)-limited growth. Previously we used transcriptomics to explore the regulation of TAG synthesis inC. sorokiniana. Surprisingly, our analysis showed that the expression of several key genes encoding enzymes involved in plastidic fatty acid synthesis are significantly repressed. Metabolic labeling with radiolabeled acetate showed thatde novofatty acid synthesis is indeed downregulated under N-limitation. Likewise, inhibition of the Target of Rapamycin kinase (TOR), a key regulator of metabolism and growth, decreased fatty acid synthesis. We compared the changes in proteins and phosphoprotein abundance using a proteomics and phosphoproteomics approach inC. sorokinianacells under N-limitation or TOR inhibition and found extensive overlap between the N-limited and TOR-inhibited conditions. We also identified changes in the phosphorylation levels of TOR complex proteins, TOR-kinase and RAPTOR, under N-limitation, indicating that TOR signaling is altered. Our results indicate that under N-limitation there is significant metabolic remodeling, including fatty acid synthesis, mediated by TOR signaling. We find that TOR-mediated metabolic remodeling of fatty acid synthesis under N-limitation is conserved in the chlorophyte algaeChlorella sorokinianaandChlamydomonas reinhardtii.