Upon formation of a trimeric LKB1:STRAD:MO25 complex, LKB1 phosphorylates and activates AMPK. If the AMP:ATP ratio rises, this activation is maintained and AMPK activates the TSC complex by phosphorylating TSC2. Active TSC activates the intrinsic GTPase activity of Rheb, resulting in GDP-loaded Rheb and
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The cytosolic AMPK complex is activated by phosphorylation. LKB1 phosphorylates AMPK heterotrimer on Thr174 of the alpha 1 subunit (or Thr172 on alpha 2 subunit) leading to activation of AMPK (if cellular AMP/ATP ratio is high) (Hawley SA et al, 2003; Woods A et al, 2003; Shaw RJ et al, 2004). Signals leading to this phosphorylation event can be mediated by exercise, leptin and adiponectin, the hypothalamic-sympathetic nervous system (SNS), and alpha adrenergic receptors, as demonstrated in studies of rat and human skeletal muscle (Minoksohi et al, 2002, Kahn et al, 2005). The details of AMPK activation in response to these stimuli will be annotated in the future. Nuclear AMPK may well be a substrate for LKB1 but, to date, there is no clear evidence for this.
TSC2 (in the TSC complex) functions as a GTPase-activating protein and stimulates the intrinsic GTPase activity of a small G-protein Rheb. This results in the conversion of Rheb-GTP into Rheb-GDP and in the inhibition of the mTOR activation by GTP-bound Rheb (Inoki K et al, 2003; Tee AR et al, 2003).
If AMP:ATP ratio rises, AMP (instead of ATP) is bound by the AMPK-gamma subunit, which inhibits the dephosphorylation of the AMPK-alpha subunit resulting in activation of AMPK. It is not clear, as of yet, whether AMP binds to unphosphorylated AMPK.
Upon complex formation with STRAD and MO25, LKB1 (also known as serine/threonine kinase 11, STK11) is mostly cytosolic. LKB1 attains 20x activity towards the substrates belonging to the subfamily of AMPK-like kinases (5'AMP-activated protein kinases).
Activated AMPK (phosphorylated on the alpha subunit and AMP bound) phosphorylates TSC2 on Ser1387, thereby activating the GAP activity of the TSC complex via an unknown mechanism.
Activated AMPK (phosphorylated on Thr172/Thr174 and AMP bound) phosphorylates Raptor on Ser 722 and Ser 792. These phosphorylations are required for inhibition of mTORC1 activity in response to energy stress (Gwinn DM et al, 2008).
inhibition of mTOR pathway.
Original Pathway at Reactome: http://www.reactome.org/PathwayBrowser/#DB=gk_current&FOCUS_SPECIES_ID=48887&FOCUS_PATHWAY_ID=380972
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