Decoding the role of ADAptor2 (ADA2) of HAT complex in autophagy and phospholipid metabolism to maintain ER homeostasis and triterpene regulation

Abstract In yeast, transcriptional adaptor 2 (ADA2; SAGA complex subunit ADA2), a member of histone acetyltransferase (HAT) complex, regulates transcription through cell signalling, but its precise role in cellular metabolism remains unclear. In this study, genetic loss of ADA2 ( ada2 Δ) induces squalene (SQ) accumulation, indicating aberrant triterpene metabolism, coupled with endoplasmic reticulum (ER)/nuclear ER (nER) expansion. Lipid analyses of ada2 Δ revealed elevated phosphatidic acid (PA) and phosphatidylcholine (PC) levels, indicating disrupted phospholipid metabolism. The expanded ER causes basal autophagy elevation, cellular recycling, and nER phagy, suggesting a regulatory role for ADA2 in autophagy. Downregulation of phosphatidate cytidylyltransferase ( CDS1 ) and inositol-3-phosphate synthase ( INO1 ), coupled with elevated PA and PC in ada2 Δ, points to a significant disruption in cytidine-diphosphate–diacylglycerol and phosphatidylinositol pathway. Overexpression of CDS1 or INO1 , or the inositol supplementation, in ada2 Δ restores SQ, basal autophagy and ER phagy. The observed target of rapamycin Ser/Thr kinase complex (TORC1) activity in ada2 Δ is due to the high PA content. Rapamycin-mediated inhibition of TORC1 reduced SQ, PA and ER expansion while increasing lipid droplets. In contrast, a rapamycin-treated ada2 Δ pah1 Δ strain retained high PA, SQ and ER expansion, underscoring the functional role of TORC1–nuclear envelope morphology protein 1 (Nem1)/sporulation-specific protein SPO7 (Spo7)–Pah1 axis. Notably, SQ levels remained unchanged in a rapamycin-treated ada2 Δ atg39 Δ strain, suggesting that loss of nER-phagy receptor, Atg39, impairs the effectiveness of TORC1 inhibition. In conclusion, our data unveiled a critical role for Ada2 in maintaining the intricate relationship between lipid and triterpene/sterol metabolism and connecting autophagy and ER homeostasis.