The hydrolysis of ATP drives virtually all of the energy-requiring processes in living cells. A prerequisite of living cells is that the concentration of ATP needs to be maintained at sufficiently high levels to sustain essential cellular functions. In eukaryotic cells, the AMPK (AMP-activated protein kinase) cascade is one of the systems that have evolved to ensure that energy homoeostasis is maintained. AMPK is activated in response to a fall in ATP, and recent studies have suggested that ADP plays an important role in regulating AMPK. Once activated, AMPK phosphorylates a broad range of downstream targets, resulting in the overall effect of increasing ATP-producing pathways whilst decreasing ATP-utilizing pathways. Disturbances in energy homoeostasis underlie a number of disease states in humans, e.g. Type 2 diabetes, obesity and cancer. Reflecting its key role in energy metabolism, AMPK has emerged as a potential therapeutic target. In the present review we examine the recent progress aimed at understanding the regulation of AMPK and discuss some of the latest developments that have emerged in key areas of human physiology where AMPK is thought to play an important role.

Phosphorylation regulates the function of ligand-gated ion channels such as the N-methyl D-aspartate (NMDA) receptor. Here we report a mechanism for modulation of the phosphorylation state and function of the NMDA receptor via an inhibitory scaffolding protein, RACK1. We found that RACK1 binds both the NR2B subunit of the NMDA receptor and the nonreceptor protein tyrosine kinase, Fyn. RACK1 inhibits Fyn phosphorylation of NR2B and decreases NMDA receptor-mediated currents in CA1 hippocampal slices. Peptides that disrupt the interactions between RACK1, NR2B, and Fyn induce phosphorylation and potentiate NMDA receptor-mediated currents. Therefore, RACK1 is a regulator of NMDA receptor function and may play a role in synaptic plasticity, addiction, learning, and memory.

The AMP-activated protein kinase (AMPK) is a member of a growing family of related kinases, including the SNF1 complex in yeast, which respond to nutritional stress. AMPK is a heterotrimeric complex of a catalytic subunit (␣) and two regulatory subunits (␤ and ␥), and proteins related to all three subunits have been identified in the SNF1 complex. We have used the two-hybrid system in order to identify proteins interacting with the catalytic subunit (␣2). Using this approach, we have isolated a novel AMPK␤ isoform, which we designate AMPK␤2. The N-terminal region of ␤2 differs significantly from that of the previously characterized isoform (␤1), suggesting that this region could play a role in isoform-specific AMPK activity. Comparison of the Cterminal sequences of ␤1 and ␤2 with their related proteins in yeast identifies two highly conserved regions predicted to be involved in binding of the ␣ and ␥ subunits. The expression of ␤1 and ␤2 was examined in a number of tissues, revealing that the ␤1 isoform is highly expressed in liver with low expression in skeletal muscle, whereas the opposite pattern is observed for the ␤2 isoform. These results suggest that the ␤ isoforms have tissue-specific roles, which may involve altered responses to upstream signaling and/or downstream targeting of the AMPK complex.In mammals, the AMP-activated protein kinase (AMPK) 1 plays a major role in the response to metabolic stress (1-3). AMP activates AMPK via a number of independent mechanisms, including activation of an upstream kinase (AMPKK), which in turn phosphorylates and activates AMPK (4, 5). The effects of AMP are antagonized by high concentrations of ATP so that it appears that the kinase responds to the AMP/ATP ratio, rather than AMP itself (4). Once activated, AMPK phosphorylates a number of enzymes involved in biosynthetic pathways causing their inactivation and preventing further ATP utilization. These findings have led to the hypothesis that the AMPK system has evolved to monitor the energy status, or fuel supply, within the cell (2-4).Molecular characterization of AMPK has revealed that it is composed of three distinct subunits: a catalytic subunit, ␣ (molecular mass approximately 63 kDa); and two regulatory subunits, ␤ (30 kDa) and ␥ (36 kDa) (6 -10). In vitro binding studies indicate that the ␣ and ␤ subunits and the ␤ and ␥ subunits interact directly, whereas the ␣ and ␥ subunits do not form a stable interaction (10). The formation of the heterotrimeric complex may therefore be mediated, at least in part, by the ␤ subunit. Proteins related to all three subunits have been identified in the SNF1 kinase complex in Saccharomyces cerevisiae, which is involved in the derepression of glucose-repressible genes (11). AMPK␣ is 47% identical to Snf1p (we refer to the individual subunits as Snf1p, etc., and the complex as SNF1) (6), AMPK␥ is 35% identical to Snf4p (10, 12), and AMPK␤ is related to the Sip1p/Sip2p/Gal83p family of proteins (10,12). In addition to their primary sequence similarities, the AMPK and SNF1 complexe...