BACKGROUND: AMP-activated protein kinase (AMPK) is a heterotrimeric enzyme that is evolutionarily conserved from yeast to mammals and functions to maintain cellular and whole body energy homeostasis. Studies in experimental animals demonstrate that activation of AMPK in skeletal muscle protects against insulin resistance, type 2 diabetes and obesity. The regulatory gamma(3) subunit of AMPK is expressed exclusively in skeletal muscle; however, its importance in controlling overall AMPK activity is unknown. While evidence is emerging that gamma subunit mutations interfere specifically with AMP activation, there remains some controversy regarding the impact of gamma subunit mutations. Here we report the first gain-of-function mutation in the muscle-specific regulatory gamma(3) subunit in humans. METHODS AND FINDINGS: We sequenced the exons and splice junctions of the AMPK gamma(3) gene (PRKAG3) in 761 obese and 759 lean individuals, identifying 87 sequence variants including a novel R225W mutation in subjects from two unrelated families. The gamma(3) R225W mutation is homologous in location to the gamma(2)R302Q mutation in patients with Wolf-Parkinson-White syndrome and to the gamma(3)R225Q mutation originally linked to an increase in muscle glycogen content in purebred Hampshire Rendement Napole (RN-) pigs. We demonstrate in differentiated muscle satellite cells obtained from the vastus lateralis of R225W carriers that the mutation is associated with an approximate doubling of both basal and AMP-activated AMPK activities. Moreover, subjects bearing the R225W mutation exhibit a approximately 90% increase of skeletal muscle glycogen content and a approximately 30% decrease in intramuscular triglyceride (IMTG). CONCLUSIONS: We have identified for the first time a mutation in the skeletal muscle-specific regulatory gamma(3) subunit of AMPK in humans. The gamma(3)R225W mutation has significant functional effects as demonstrated by increases in basal and AMP-activated AMPK activities, increased muscle glycogen and decreased IMTG. Overall, these findings are consistent with an important regulatory role for AMPK gamma(3) in human muscle energy metabolism.