Competitive interaction between ATP and GTP regulates mitochondrial ATP-sensitive potassium channels

Mitochondrial ATP-sensitive K+ channels (mitoKATP) have been recently characterized structurally, and possess a protein through which K+ enters mitochondria (MitoKIR), and a regulatory subunit (mitoSUR). The mitoSUR regulatory subunit is an ATP-binding cassette (ABC) protein isoform 8 (ABCB8). Opening these channels is known to be cardioprotective, but the molecular and physiological mechanisms that activate them are not fully known. Here, to better understand the molecular and physiological mechanisms of activators (GTP) and in-hibitors (ATP) on the activity of mitoKATP, we exposed isolated mitochondria to both nucleotides. We also used molecular docking directed to the nucleotide-binding domain of human ABCB8/mitoSUR to test a comparative model of ATP and GTP effects. As expected, we find that ATP dose-dependently inhibits mitoKATP activity (IC50 = 21.24 & PLUSMN; 1.4 & mu;M). However, simultaneous exposure of mitochondria to GTP dose-dependently (EC50 = 13.19 & PLUSMN; 1.33 & mu;M) reversed ATP inhibition. Pharmacological and computational studies suggest that GTP reverses ATP activity competitively. Docking directed to the site of crystallized ADP reveals that both nucleotides bind to mitoSUR with high affinity, with their phosphates directed to the Mg2+ ion and the walker A motif of the protein (SGGGKTT). These effects, when combined, result in GTP binding, ATP displacement, mitochondrial ATP -sensitive K+ transport, and lower formation of reactive oxygen species. Overall, our findings demonstrate the basis for ATP and GTP binding in mitoSUR using a combination of biochemical, pharmacological, and compu-tational experiments. Future studies may reveal the extent to which the balance between ATP and GTP actions contributes toward cardioprotection against ischemic events.