The LKB1 tumour suppressor protein kinase functions to activate two isoforms of AMPK (AMP-activated protein kinase) and 12 members of the AMPK-related family of protein kinases. The highly conserved C-terminal residues of LKB1 are phosphorylated (Ser431) by PKA (cAMP-dependent protein kinase) and RSK (ribosomal S6 kinase) and farnesylated (Cys433) within a CAAX motif. To better define the role that these post-translational modifications play, we created homozygous LKB1^S431A/S431A and LKB1^C433S/C433S knockin mice. These animals were viable, fertile and displayed no overt phenotypes. Employing a farnesylationspecific monoclonal antibody that we generated, we established by immunoprecipitation that the vast majority, if not all, of the endogenous LKB1 is prenylated. Levels of LKB1 localized at the membrane of the liver of LKB1^C433S/C433S mice and their fibroblasts were reduced substantially compared with the wildtype mice, confirming that farnesylation plays a role in mediating membrane association. Although AMPK was activated normally in the LKB1^S431A/S431A animals, we unexpectedly observed in all of the examined tissues and cells taken from LKB1 ^C433S/C433S mice that the basal, as well as that induced by the AMP-mimetic AICAR (5-amino-4-imidazolecarboxamide riboside), AMPK activation, phenformin and muscle contraction were significantly blunted. This resulted in a reduced ability of AICAR to inhibit lipid synthesis in primary hepatocytes isolated from LKB1^C433S/C433S mice. The activity of several of the AMPK-related kinases analysed [BRSK1 (BR serine/threonine kinase 1), BRSK2, NUAK1 (NUAK family, SNF1-like kinase 1), SIK3 (salt-inducible kinase 3) and MARK4 (MAP/microtubule affinity-regulating kinase 4)] was not affected in tissues derived from LKB1^S431A/S431A or LKB1^C433S/C433S mice. Our observations reveal for the first time that farnesylation of LKB1 is required for the activation of AMPK. Previous reports have indicated that a pool ofAMPKis localized at the plasma membrane as a result ofmyristoylation of its regulatory AMPKß subunit. This raises the possibility that LKB1 farnesylation and myristoylation of AMPKß might promote the interaction and co-localization of these enzymes on a two-dimensional membrane surface and thereby promote efficient activation of AMPK.