AMPK: An Energy-Sensing Pathway with Multiple Inputs and Outputs

AMPK is an energy-sensing protein kinase activated by phosphorylation of Thr172 within its catalytic α subunit. It binds AMP and/or ADP, both signals of energy stress, via its regulatory γ subunit. This activates the kinase by promoting Thr172 phosphorylation, inhibiting Thr172 dephosphorylation and allosteric activation. AMP binding to the γ subunit causes its interaction with the α linker region of the α subunit. This pulls the autoinhibitory domain on the α subunit away from the kinase domain, triggering activation. AMPK is also activated by the binding of synthetic and naturally occurring drug-like molecules that bind in the allosteric drug and metabolite (ADaM) site between the α and β subunits. AMPK has a well-defined recognition motif that has been established both by hypothesis-driven approaches and by various unbiased screens. It now has over 60 well-validated substrates. AMP-activated protein kinase (AMPK) is a key regulator of energy balance expressed ubiquitously in eukaryotic cells. Here we review the canonical adenine nucleotide-dependent mechanism that activates AMPK when cellular energy status is compromised, as well as other, noncanonical activation mechanisms. Once activated, AMPK acts to restore energy homeostasis by promoting catabolic pathways, resulting in ATP generation, and inhibiting anabolic pathways that consume ATP. We also review the various hypothesis-driven and unbiased approaches that have been used to identify AMPK substrates and have revealed substrates involved in both metabolic and non-metabolic processes. We particularly focus on methods for identifying the AMPK target recognition motif and how it can be used to predict new substrates. AMP-activated protein kinase (AMPK) is a key regulator of energy balance expressed ubiquitously in eukaryotic cells. Here we review the canonical adenine nucleotide-dependent mechanism that activates AMPK when cellular energy status is compromised, as well as other, noncanonical activation mechanisms. Once activated, AMPK acts to restore energy homeostasis by promoting catabolic pathways, resulting in ATP generation, and inhibiting anabolic pathways that consume ATP. We also review the various hypothesis-driven and unbiased approaches that have been used to identify AMPK substrates and have revealed substrates involved in both metabolic and non-metabolic processes. We particularly focus on methods for identifying the AMPK target recognition motif and how it can be used to predict new substrates. region of the AMPK α subunit that connects the α-AID to the C-terminal domain, important in the mechanism of regulation by AMP. conserved sequences within the α linker that interact with the AMPK γ subunit when AMP is bound at site 3. the domain that follows the kinase domain in AMPK α subunits and that inhibits the kinase domain in the absence of AMP. kinase domains on the α subunits of AMPK that are related to the catalytic domains in other members of the ‘eukaryotic’ protein kinase (ePK) family. a binding pocket located between the α-KD and the β-CBM where drugs such as A-769662 bind and where naturally occurring metabolites are speculated to bind. a sequence motif, first found in the enzyme cystathionine-β-synthase, that always occurs as tandem repeats; a single twin repeat often binds a ligand containing adenosine, such as AMP, ATP, or S-adenosylmethionine. amino acid sequence surrounding a phosphorylation site that promotes targeting of that residue by a given protein kinase.