Target of Rapamycin Complex 1 (TORC1), Protein Kinase A (PKA) and Cytosolic pH Regulate a Transcriptional Circuit for Lipid Droplet Formation

Abstract

Lipid droplets (LDs) are ubiquitous organelles that fulfill essential roles in response to metabolic cues. The identification of several neutral lipid synthesizing and regulatory protein complexes have propelled significant advance on the mechanisms of LD biogenesis in the endoplasmic reticulum (ER). However, our understanding of signaling networks, especially transcriptional mechanisms, regulating membrane biogenesis is very limited. Here, we show that the nutrient-sensing Target of Rapamycin Complex 1 (TORC1) regulates LD formation at a transcriptional level, by targeting DGA1 expression, in a Sit4-, Mks1-, and Sfp1-dependent manner. We show that cytosolic pH (pHc), co-regulated by the plasma membrane H+-ATPase Pma1 and the vacuolar ATPase (V-ATPase), acts as a second messenger, upstream of protein kinase A (PKA), to adjust the localization and activity of the major transcription factor repressor Opi1, which in turn controls the metabolic switch between phospholipid metabolism and lipid storage. Together, this work delineates hitherto unknown molecular mechanisms that couple nutrient availability and pHc to LD formation through a transcriptional circuit regulated by major signaling transduction pathways.

Cytosolic pH controls fungal MAPK signaling and pathogenicity

“…cerevisiae showing that pHc acts as a second messenger upstream of protein kinase A to regulate the metabolic switch between phospholipid metabolism and lipid storage (Teixeira et al, 2021). Moreover, a rapid downshift of pHc in response to acid stress was proposed to promote cell survival by triggering growth arrest (Lucena et al, 2020).…”

GATA-type transcriptional factor SpGAT1 interacts with SpMIG1 and promotes lipid accumulation in the oleaginous yeast $$Saitozyma \ podzolica$$ zwy-2-3

“…3 a, b); however, this phenomenon did not appear in the low C/N ratio (low glucose concentration) medium, probably because SpMIG1 was localized in the cytoplasm by SNF1 phosphorylation under low C/N ratio conditions. Actually, upstream regulatory genes (TOR1, SIT4, URE2) of GAT1 in the TOR signaling pathway regulated yeast lipid metabolism [ 21 ]. TORC1 and carbon source uptake regulator SNF1 (an upstream regulator of MIG1) had a close regulatory interaction [ 37 , 38 ].…”

“…Inhibition of the target of rapamycin complex 1 (TORC1) triggers lipid accumulation in fungi, and rapamycin (TORC1 inhibitor) therapy increases TAG accumulation in and oleaginous microorganisms (like Chlamydomonas reinhardtii , , and Trichosporon oleaginosus ) [ 17 – 20 ]. Both TORC1 inhibition and deletion of GAT 1, GLN 3, and SIT 4 in the TOR pathway affect lipid droplet replenishment in [ 17 , 21 ]. In nitrogen-limited or non-preferred nitrogen source conditions, inhibited TORC1 interacts with Tap42-Sit4 complexes to dephosphorylate GAT1, and dephosphorylated GAT1 enters the nucleus to regulate NCR-related gene expression and improve nitrogen sources absorption [ 22 ].…”

Hello from the other side: Membrane contact of lipid droplets with other organelles and subsequent functional implications

“…In the budding yeast, Dga1 is transcriptionally controlled by the Target of Rapamycin Kinase Complex 1 (TORC1) downstream effectors Sit4 (the yeast homologue of mammalian protein phosphatase PP2A/PP6) and the transcription factor Sfp1, which shares key functions with the mammalian proto-oncogene MYC in ribosomal protein gene expression and cell growth. Importantly, TORC1 and protein kinase A (PKA) act coordinately to inhibit LD accumulation, in a mechanism involving vacuolar ATPase (V-ATPase) and the plasma membrane protein Pma1 H + -ATPase pump [23].…”