Abstract AMPK and mTORC1 are nutrient-sensitive protein kinases that form a fundamental negative feedback loop that governs cell growth and proliferation. AMPK is an αβγ heterotrimer that is directly phosphorylated by mTORC1 on α2 S345 to suppress AMPK activity and promote cell proliferation under nutrient stress conditions. Using mass spectrometry, we generated precise phosphorylation profiles of all 12 AMPK complexes expressed in proliferating human cells. Of the 18 phosphorylation sites detected, seven were sensitive to pharmacological mTORC1 inhibition, including four in the AMPK γ2 isoform NH 2 -terminal domain and α2 S377 which is located in the nucleotide-sensing motif. In particular, β1 S182 and β2 S184 were found to be mTORC1 substrates in vitro and near-maximally or substantially phosphorylated under cellular growth conditions. β S182 phosphorylation was elevated in α1-containing complexes, relative to α2, an effect partly attributable to the non-conserved α-subunit serine/threonine-rich loop. While mutation of β1 S182 to a non-phosphorylatable Ala had no effect on basal and ligand-stimulated AMPK activity, β2-S184A mutation increased nuclear AMPK activity and enhanced cell proliferation under nutrient stress. We conclude that mTORC1 governs the nuclear activity of AMPK to regulate transcription factors involved in metabolism and cell survival during nutrient shortage.
