Title: AMPK β Subunit Targets Metabolic Stress Sensing to Glycogen
Abstract: AMP-activated protein kinase (AMPK) is a multisubstrate enzyme activated by increases in AMP during metabolic stress caused by exercise, hypoxia, lack of cell nutrients [1Hardie D.G. Carling D. The AMP-activated protein kinase–fuel gauge of the mammalian cell?.Eur. J. Biochem. 1997; 246: 259-273Crossref PubMed Scopus (1138) Google Scholar], as well as hormones, including adiponectin and leptin [2Yamauchi T. Kamon J. Minokoshi Y. Ito Y. Waki H. Uchida S. Yamashita S. Noda M. Kita S. Ueki K. et al.Adiponectin stimulates glucose utilization and fatty-acid oxidation by activating AMP-activated protein kinase.Nat. Med. 2002; 8: 1288-1295Crossref PubMed Scopus (3440) Google Scholar, 3Minokoshi Y. Kim Y.B. Peroni O.D. Fryer L.G. Muller C. Carling D. Kahn B.B. Leptin stimulates fatty-acid oxidation by activating AMP-activated protein kinase.Nature. 2002; 415: 339-343Crossref PubMed Scopus (1670) Google Scholar]. Furthermore, metformin and rosiglitazone, frontline drugs used for the treatment of type II diabetes, activate AMPK [4Fryer L.G. Parbu-Patel A. Carling D. The anti-diabetic drugs rosiglitazone and metformin stimulate AMP-activated protein kinase through distinct signaling pathways.J. Biol. Chem. 2002; 277: 25226-25232Crossref PubMed Scopus (910) Google Scholar]. Mammalian AMPK is an αβγ heterotrimer with multiple isoforms of each subunit comprising α1, α2, β1, β2, γ1, γ2, and γ3, which have varying tissue and subcellular expression [5Stapleton D. Mitchelhill K.I. Gao G. Widmer J. Michell B.J. Teh T. House C.M. Fernandez C.S. Cox T. Witters L.A. et al.Mammalian AMP-activated protein kinase subfamily.J. Biol. Chem. 1996; 271: 611-614Crossref PubMed Scopus (562) Google Scholar, 6Chen Z. Heierhorst J. Mann R.J. Mitchelhill K.I. Michell B.J. Witters L.A. Lynch G.S. Kemp B.E. Stapleton D. Expression of the AMP-activated protein kinase beta1 and beta2 subunits in skeletal muscle.FEBS Lett. 1999; 460: 343-348Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar]. Mutations in the AMPK γ subunit cause glycogen storage disease in humans [7Arad M. Benson D.W. Perez-Atayde A.R. McKenna W.J. Sparks E.A. Kanter R.J. McGarry K. Seidman J.G. Seidman C.E. Constitutively active AMP kinase mutations cause glycogen storage disease mimicking hypertrophic cardiomyopathy.J. Clin. Invest. 2002; 109: 357-362Crossref PubMed Scopus (460) Google Scholar], but the molecular relationship between glycogen and the AMPK/Snf1p kinase subfamily has not been apparent. We show that the AMPK β subunit contains a functional glycogen binding domain (β-GBD) that is most closely related to isoamylase domains found in glycogen and starch branching enzymes. Mutation of key glycogen binding residues, predicted by molecular modeling, completely abolished β-GBD binding to glycogen. AMPK binds to glycogen but retains full activity. Overexpressed AMPK β1 localized to specific mammalian subcellular structures that corresponded with the expression pattern of glycogen phosphorylase. Glycogen binding provides an architectural link between AMPK and a major cellular energy store and juxtaposes AMPK to glycogen bound phosphatases.