Proposed mechanism of phosphoinositide and LC-CoA gating in Kir channels. (A) A homology model of Kir6.2 in the closed state showing the location of the four bound PIP₂ molecules at the membrane/cytoplasm interface (position of PIP₂ was determined by a computational docking procedure (Haider et al., 2007). The negative charges on the headgroups (oxygen atoms in red) interact with a positively charged cluster in the cytoplasmic domains, for clarity the multiple residues that comprise the binding site are not shown, for details see Haider et al. (2007). (B) Previous models suggested that opening of the Kir channel is produced by electrostatic attraction between PIP₂ and its binding site that induces a major movement of the cytoplasmic domain toward the membrane. (C) In addition to being activated by PIP₂, PI(3,4)P₂, and PI(3,4,5)P₃ (not depicted), K_ATP channels are activated by oleoyl-CoA, a long-chain acyl-CoA ester as measured in inside-out patches from Xenopus oocytes. BSA is used to rapidly remove oleoyl-CoA. (D) Kir1.1 is activated by PI(4,5)P₂ but inhibited by oleoyl-CoA and PI(3,4)P₂ and inhibition could be reversed by PIP₂. (E) In contrast to the original model (B) we propose that all phosphoinositides and LC-CoAs can bind to the PIP₂ binding site on the closed Kir subunit. However, only certain bound lipids (e.g., PIP₂) are subsequently able to initiate the allosteric changes required for channel opening. Those that cannot induce channel opening (LC-CoA and PI(3,4)P₂) inhibit channel activity by competitive displacement of PIP₂. The chemical structures of PIP₂ and LC-CoA are shown at the top of the figure. The critical phosphate on LC-CoA required for its ability to induce opening in K_ATP channels and inhibition in, e.g., Kir1.1 channels is highlighted in blue. Panel A is adapted from the results of Haider et al. (2007). C and D are adapted from Rapedius et al. (2005).