Regulation of AMP-activated protein kinase by a pseudosubstrate sequence on the γ subunit

The AMP-activated protein kinase ( AMPK) system monitors cellular energy status by sensing AMP and ATP, and is a key regulator of energy balance at the cellular and whole-body levels. AMPK exists as heterotrimeric alpha beta gamma complexes, and the gamma subunits contain two tandem domains that bind the regulatory nucleotides. There is a sequence in the first of these domains that is conserved in c subunit homologues in all eukaryotes, and which resembles the sequence around sites phosphorylated on target proteins of AMPK, except that it has a nonphosphorylatable residue in place of serine. We propose that in the absence of AMP this pseudosubstrate sequence binds to the active site groove on the a subunit, preventing phosphorylation by the upstream kinase, LKB1, and access to downstream targets. Binding of AMP causes a conformational change that prevents this interaction and relieves the inhibition. We present several lines of evidence supporting this hypothesis.