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 (␣ 1 ) is distinct from the previously cloned kinase subunit (␣ 2 ). The ␣ 1 (548 residues) and ␣ 2 (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 ␣ 1 mRNA distribution and the ␣ 1 immunoreactivity rather than ␣ 2 , with substantial amounts in kidney, liver, lung, heart, and brain. Both ␣ 1 and ␣ 2 isoforms are stimulated by AMP and contain noncatalytic ␤ and ␥ subunits. The liver ␣ 1 isoform accounts for approximately 94% of the enzyme activity measured using the SAMS peptide substrate. The tissue distribution of the ␣ 2 immunoreactivity parallels the ␣ 2 8.5-kilobase mRNA and is most prominent in skeletal muscle, heart, and liver. Isoforms of the ␤ and ␥ subunits present in the human genome sequence reveal that the AMPK consists of a family of isoenzymes.The 5Ј-AMP-activated protein kinase (AMPK) 1 was initially identified as a protein kinase regulating hydroxymethylglutaryl-CoA reductase (1). Subsequently, the AMPK was shown to phosphorylate hepatic acetyl-CoA carboxylase (2) and adipose hormone-sensitive lipase (3). The AMPK appears to act as a metabolic stress-sensing protein kinase switching off biosynthetic pathways when cellular ATP levels are depleted and when 5Ј-AMP rises in response to fuel limitation and/or hypoxia (4). Partial amino acid sequencing of hepatic AMPK (5, 6) revealed that it consists of 3 subunits, the catalytic subunit ␣ (63 kDa), and two noncatalytic subunits, ␤ (40 kDa) and ␥ (38 kDa).The AMPK is a member of the yeast SNF1 protein kinase subfamily that includes protein kinases present in plants, nematodes, and humans (5-9). The AMPK catalytic subunit, ␣, has strong sequence identity to the catalytic domain of the yeast protein kinase SNF1, which is involved in the induction of invertase (SUC2) under conditions of nutritional stress due to glucose starvation (10). Both Snf1p and the AMPK require phosphorylation by an activating protein kinase for full activity (11). The sequence relationship between Snf1p and AMPK led to the finding that these enzymes share functional similarities, both phosphorylating and inactivating yeast acetyl-CoA carboxylase (5,11,12). Nevertheless, the AMPK does not complement SNF1 in yeast (11), indicating that their full range of functions are not identical. The noncatalytic ␤ and ␥ subunits of AMPK are also related to proteins that interact with Snf1p; the ␤ subunit is related to the SIP1/SIP2/GAL83 family of transcription regulators and the ␥ subunit to SNF4 (also called CAT3) (6, 13). EXPERIMENTAL PROCEDURESPeptide Sequencing-Peptides were derived from rat and porcine ␣ 1 subunit of the AMPK, by in situ proteolysis (5), and sequenced on either an Ap...

Role for AMP-activated protein kinase in glucose-stimulated insulin secretion and preproinsulin gene expression

“…Similarly, metformin, a widely used insulin sensitizer, has also very recently been shown to activate AMPK and to suppress gluconeogenesis in isolated liver cells [73], and could thus also inhibit insulin secretion directly. However, it should be stressed that the expression of AMPK catalytic subunit isoforms is different in β-cells (α1 α2) [10] than in muscle (α1 α2) [12] or liver cells (α1 $ α2) [29]. Given the existence of other possible differences in the relative abundances of the β-and γ-subtypes in these tissues, isoform-selective activators or inhibitors of AMPK may thus provide new therapeutic tools for the treatment of type 2 diabetes mellitus.…”

“…Recently, we have demonstrated that AMP-activated protein kinase (AMPK) is involved in the regulation of gene expression by glucose in this cell type [10]. AMPK is a multisubstrate, heterotrimeric serine\threonine protein kinase, consisting of a catalytic α-subunit and non-catalytic β-and γ-subunits [11,12]. Enzyme activity is regulated both allosterically by AMP, and through reversible phosphorylation at Thr-172 of the α subunit by an upstream kinase (AMPK kinase, or AMPKK) [13][14][15].…”

Immunological evidence that HMG‐CoA reductase kinase‐A is the cauliflower homologue of the RKIN1 subfamily of plant protein kinases

“…and SNF4 gene products, and is involved in yeast in the response to nutritional stress (starvation for glucose) [9][10][11][12]. A number of DNA sequences encoding proteins related to Snflp have been cloned from higher plants, including RKINI from rye [13], BKIN12 and BKIN2 from barley [14,15], AKINIO from A. thaliana [16], and NPK5 from tobacco [17].…”

The AMP-regulated kinase family: Enigmatic targets for diabetes therapy

“…Considered principally as an "emergency response" enzyme in mammals, activated only during severe metabolic stress during which intracellular 5'AMP levels were raised as those of ATP fell, AMPK was substantially ignored by all but a very few groups for the next twenty years (Stapleton et al, 1996;Hardie et al, 1998).…”