Naldrett, M. | bioRxiv Science

bioRxiv2025

Lipid Metabolism and Contact Site Homologs Are Enriched at the Chloroplast Envelope-Thylakoid Interface

Biogenesis and maintenance of the photosynthetic thylakoid membrane requires transport of lipids from their site of synthesis in the chloroplast envelopes to their destination in the thylakoid. While vesicle trafficking is likely involved, we hypothesized a complementary mechanism involving direct membrane interactions. Using domain homology and proteomic profiling of chloroplast membrane fractions, we identified candidate lipid transport proteins enriched in a distinct, intermediate-density membrane population. This fraction was enriched in lipid metabolic enzymes and proteins homologous to known membrane organization factors. Several candidates, including TVP38 FAMILY PROTEIN (TVPFP), PLASMA MEMBRANE FUSION PROTEIN (PMFP), and LETM1-LIKE, localized to discrete subdomains within chloroplasts. Loss- of-function tvpfp or pmfp mutants exhibited altered chloroplast ultrastructure, including changes in thylakoid-envelope proximity, supporting their roles in maintaining membrane architecture. These findings identify a specialized chloroplast membrane region enriched in lipid-related functions and offer a foundation for elucidating the molecular architecture of these regions.

biochemistry↗

bioRxiv2023

Nitrogen starvation leads to TOR kinase-mediated downregulation of fatty acid synthesis in the algae Chlorella sorokiniana and Chlamydomonas reinhardtii

The 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 alga Chlorella sorokiniana accumulates TAG and other storage compounds under nitrogen (N)-limited growth. Previously we used transcriptomics to explore the regulation of TAG synthesis in C. 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 that de novo fatty 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 in C. sorokiniana cells 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 algae Chlorella sorokiniana and Chlamydomonas reinhardtii.

plant biology↗