Snf1/AMPK fine-tunes TORC1 signaling in response to glucose starvation

Abstract

The AMP-activated protein kinase (AMPK) and the target of rapamycin complex 1 (TORC1) are central kinase modules of two opposing signaling pathways that control eukaryotic cell growth and metabolism in response to the availability of energy and nutrients. Accordingly, energy depletion activates AMPK to inhibit growth, while nutrients and high energy levels activate TORC1 to promote growth. Both in mammals and lower eukaryotes such as yeast, the AMPK and TORC1 pathways are wired to each other at different levels, which ensures homeostatic control of growth and metabolism. In this context, a previous study (Hughes Hallet et. al, 2015) reported that AMPK in yeast, i.e. Snf1, prevents the transient TORC1 reactivation during the early phase following acute glucose starvation, but the underlying mechanism has remained elusive. Using a combination of unbiased mass spectrometry (MS)-based phosphoproteomics, genetic, biochemical, and physiological experiments, we show here that Snf1 temporally maintains TORC1 inactive in glucose-starved cells primarily through the TORC1-regulatory protein Pib2. Our data, therefore, extend the function of Pib2 to a hub that integrates both glucose and, as reported earlier, glutamine signals to control TORC1. We further demonstrate that Snf1 phosphorylates the TORC1 effector kinase Sch9 within its N-terminal region and thereby antagonizes the phosphorylation of a C-terminal TORC1-target residue within Sch9 itself that is critical for its activity. The consequences of Snf1-mediated phosphorylation of Pib2 and Sch9 are physiologically additive and sufficient to explain the role of Snf1 in short-term inhibition of TORC1 in acutely glucose-starved cells.

Mitochondria complex I deficiency in Candida albicans arrests the cell cycle at S phase through suppressive TOR and PKA pathways

“…The activity of TORC1 protein kinase is positively regulated by intracellular nutrients (e.g. amino acids, glucose, and lipids) (Caligaris et al 2023a ). In nutrient-rich medium—YPD, exposure to a low dose of rapamycin (10–40 ng/ml) led to growth resistance in all respiration mutants (Fig.…”

“…This upregulation likely stems from the consistently high Snf1 response observed in nuo1Δ/Δ , which was irrespective of nutrient availability (Zhang et al 2018 ). The Snf1 and TOR1 pathways act as antagonistic regulators of eukaryotic cell growth, with TOR1 activated by nutrient abundance to promote growth and Snf1 activated by energy depletion to inhibit growth (Caligaris et al 2023a ). The hyperactive Snf1 response in nuo1Δ/Δ aligns with normal SCH9 and TOR1 responses thus leads to a SCH9 -independent RIM15 elevation mechanism.…”

Metabolic regulation of misfolded protein import into mitochondria

“…Reg1 is the regulatory subunit of Glc7-Reg1 protein phosphatase 1 complex that dephosphorylates Snf1 and promotes its inhibitory conformation ( 24-27 ). Either glucose limitation or loss of Reg1 in glucose-rich medium (HG: 2% glucose) result in constitutive activation of Snf1 and relief from glucose repression of transcription ( 24-28 ) (Figure 1I). We found that Δreg1 cells exhibited significantly less accumulation of FlucSM in mitochondria than WT cells, and likewise, wild-type cells that grew in low glucose medium (LG: 0.1% glucose plus 3% glycerol) showed significantly lower FlucSM spGFP compared to cells in HG (Figure 1G and H).…”

Changing course: Glucose starvation drives nuclear accumulation of Hexokinase 2 in S. cerevisiae

“…This mobility change was due to phosphorylation, as incubation with phosphatase generated a single band that migrated like Tda1 in glucose-grown cells ( Fig 9G ). Recent studies identified several Snf1-dependent phosphorylation sites in Tda1, so perhaps the increased phosphorylation of Tda1 under glucose starvation conditions is partially regulated by Snf1 [ 67 ]. We propose that Tda1 is required for Hxk2 nuclear accumulation in glucose-starved cells.…”