Mammalian AMP-activated Protein Kinase Subfamily

Abstract

The mammalian 5′-AMP-activated protein kinase (AMPK) is related to a growing family of protein kinases in yeast and plants that are regulated by nutritional stress. We find the most prominent expressed form of the hepatic AMPK catalytic subunit (α1) is distinct from the previously cloned kinase subunit (α2). The α1 (548 residues) and α2 (552 residues) isoforms have 90% amino acid sequence identity within the catalytic core but only 61% identity elsewhere. The tissue distribution of the AMPK activity most closely parallels the low abundance 6-kilobase α1 mRNA distribution and the α1 immunoreactivity rather than α2, with substantial amounts in kidney, liver, lung, heart, and brain. Both α1 and α2 isoforms are stimulated by AMP and contain noncatalytic β and γ subunits. The liver α1 isoform accounts for approximately 94% of the enzyme activity measured using the SAMS peptide substrate. The tissue distribution of the α2 immunoreactivity parallels the α2 8.5-kilobase mRNA and is most prominent in skeletal muscle, heart, and liver. Isoforms of the β and γ subunits present in the human genome sequence reveal that the AMPK consists of a family of isoenzymes. The mammalian 5′-AMP-activated protein kinase (AMPK) is related to a growing family of protein kinases in yeast and plants that are regulated by nutritional stress. We find the most prominent expressed form of the hepatic AMPK catalytic subunit (α1) is distinct from the previously cloned kinase subunit (α2). The α1 (548 residues) and α2 (552 residues) isoforms have 90% amino acid sequence identity within the catalytic core but only 61% identity elsewhere. The tissue distribution of the AMPK activity most closely parallels the low abundance 6-kilobase α1 mRNA distribution and the α1 immunoreactivity rather than α2, with substantial amounts in kidney, liver, lung, heart, and brain. Both α1 and α2 isoforms are stimulated by AMP and contain noncatalytic β and γ subunits. The liver α1 isoform accounts for approximately 94% of the enzyme activity measured using the SAMS peptide substrate. The tissue distribution of the α2 immunoreactivity parallels the α2 8.5-kilobase mRNA and is most prominent in skeletal muscle, heart, and liver. Isoforms of the β and γ subunits present in the human genome sequence reveal that the AMPK consists of a family of isoenzymes.