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1.  The autophagy initiating kinase ULK1 is regulated via opposing phosphorylation by AMPK and mTOR 
Autophagy  2011;7(6):645-646.
The serine/threonine kinase ULK1 is a mammalian homolog of Atg1, part of the Atg1 kinase complex, which is the most upstream component of the core autophagy machinery conserved from yeast to mammals. In budding yeast, activity of the Atg1 kinase complex is inhibited by TORC1 (target of rapamycin complex 1), but how the counterpart ULK1 complex in mammalian cells is regulated has been unknown. Our laboratories recently discovered that AMPK associates with, and directly phosphorylates, ULK1 on several sites and this modification is required for ULK1 activation after glucose deprivation. In contrast, when nutrients are plentiful, the mTORC1 complex phosphorylates ULK1, preventing its association and activation by AMPK. These studies have revealed a molecular mechanism of ULK1 regulation by nutrient signals via the actions of AMPK and mTORC1.
PMCID: PMC3359466  PMID: 21460621
autophagy; ULK1; AMPK; mTOR; 14-3-3
2.  Phosphorylation of ULK1 (hATG1) by AMP-activated protein kinase connects energy sensing to mitophagy 
Science (New York, N.Y.)  2010;331(6016):456-461.
Adenosine monophosphate-activated protein kinase (AMPK) is a conserved sensor of intracellular energy activated in response to low nutrient availability and environmental stress. In a screen for conserved substrates of AMPK, we identified ULK1 and ULK2, mammalian orthologs of the yeast protein kinase Atg1, which is required for autophagy. Genetic analysis of AMPK or ULK1 in mammalian liver and C. elegans revealed a requirement for these kinases in autophagy. In mammals, loss of AMPK or ULK1 resulted in aberrant accumulation of the autophagy adaptor p62 and defective mitophagy. Reconstitution of ULK1-deficient cells with a mutant ULK1 that cannot be phosphorylated by AMPK revealed that such phosphorylation is required for mitochondrial homeostasis and cell survival following starvation. These findings uncover a conserved biochemical mechanism coupling nutrient status with autophagy and cell survival.
PMCID: PMC3030664  PMID: 21205641
3.  AMPK Regulates the Circadian Clock by Cryptochrome Phosphorylation and Degradation 
Science (New York, N.Y.)  2009;326(5951):437-440.
Circadian clocks coordinate behavioral and physiological processes with daily light-dark cycles by driving rhythmic transcription of thousands of genes. Whereas the master clock in the brain is set by light, pacemakers in peripheral organs, such as the liver, are reset by food availability, although the setting, or “entrainment,” mechanisms remain mysterious. Studying mouse fibroblasts, we demonstrated that the nutrient-responsive adenosine monophosphate–activated protein kinase (AMPK) phosphorylates and destabilizes the clock component cryptochrome 1 (CRY1). In mouse livers, AMPK activity and nuclear localization were rhythmic and inversely correlated with CRY1 nuclear protein abundance. Stimulation of AMPK destabilized cryptochromes and altered circadian rhythms, and mice in which the AMPK pathway was genetically disrupted showed alterations in peripheral clocks. Thus, phosphorylation by AMPK enables cryptochrome to transduce nutrient signals to circadian clocks in mammalian peripheral organs.
PMCID: PMC2819106  PMID: 19833968
4.  AMPK phosphorylation of raptor mediates a metabolic checkpoint 
Molecular cell  2008;30(2):214-226.
AMPK is a highly conserved sensor of cellular energy status that is activated under conditions of low intracellular ATP. AMPK responds to energy stress by suppressing cell growth and biosynthetic processes, in part through its inhibition of the rapamycin-sensitive mTOR (mTORC1) pathway. AMPK phosphorylation of the TSC2 tumor suppressor contributes to suppression of mTORC1, however TSC2-deficient cells remain responsive to energy stress. Using a proteomic and bioinformatics approach, we sought to identify additional substrates of AMPK that mediate its effects on growth control. We report here that AMPK directly phosphorylates the mTOR binding partner raptor on two well-conserved serine residues, and this phosphorylation induces 14-3-3 binding to raptor. The phosphorylation of raptor by AMPK is required for the inhibition of mTORC1 and cell cycle arrest induced by energy stress. These findings uncover a novel conserved effector of AMPK that mediates its role as a metabolic checkpoint coordinating cell growth with energy status.
PMCID: PMC2674027  PMID: 18439900

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