Regulation of the ATP-sensitive K channel Kir6.2 by ATP and PIP₂

ATP-sensitive K (K_ATP) channels are blocked by ATP and activated by PIP₂. Both negatively-charged ligands are presumed to bind to positively-charged residues on the N-and C-termini of the channel's cytoplasmic domain. Evidence summarized here suggests that the channel's interaction with ATP and PIP₂ is regulated by separate groups of residues, involving both direct charge-charge interactions and allosteric effects. ATP interaction is regulated by R50 in the N-terminus and by K185, R192 and R201 in the C-terminus. R192 and R201 mutations decrease channel sensitivity to ATP, ADP and AMP to a similar extent, implying that they regulate interaction with either the α phosphate group, common to all three adenine nucleotides, or the adenosine moiety. K185 mutations, and to a lesser extent R50 mutations, decrease ATP and ADP sensitivity without markedly affecting AMP sensitivity, implying that they regulate interaction with the β phosphate of ATP and ADP. In addition, when open probability decreases due to rundown, ATP sensitivity increases in R50, K185 and R192, but not in R201 mutants. Combining these observations with recent structural data, we hypothesize the following scenario: 1) the ATP binding site is located at the outside of the channel's cytoplasmic domain away from the pore. 2) When the channel is open, R50 and K185 interact directly with the α phosphate of ATP, whereas R192, which appears to be removed from the ATP binding site, modulates the initial interaction with ATP allosterically. 3) When the channel closes, R201 is in position to interact with the α phosphate of ATP to stabilize the closed state. 4) PIP₂ also interacts with the channel's cytoplasmic domain, but at distinct positively-charged residues located above the ATP binding site and near to the plasma membrane. These residues include R54 in the N-terminus and R176, R177 and R206 in the C-terminus. Thus, the binding domains of ATP and PIP₂ in the N- and C-termini do not appear to overlap.