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The regulation of ion channel activity by specific lipid molecules is widely recognized as an integral component of electrical signalling in cells1,2. In particular, phosphatidylinositol 4,5-bisphosphate (PIP2), a minor yet dynamic phospholipid component of cell membranes, is known to regulatemany different ion channels2.8. PIP2 is the primary agonist for classical inward rectifier (Kir2) channels, through which this lipid can regulate a cell's resting membrane potential2,7.9. However, the molecular mechanism by which PIP2 exerts its action is unknown. Here we present the X-ray crystal structure of a Kir2.2 channel in complex with a short-chain (dioctanoyl) derivative of PIP2. We found that PIP2 binds at an interface between the transmembrane domain (TMD) and the cytoplasmic domain (CTD). The PIP2-binding site consists of a conserved non-specific phospholipidbinding region in the TMD and a specific phosphatidylinositolbinding region in the CTD. On PIP2 binding, a flexible expansion linker contracts to a compact helical structure, the CTD translates 6[Angstrom] and becomes tethered to the TMD and the inner helix gate begins to open. In contrast, the small anionic lipid dioctanoyl glycerol pyrophosphatidic acid (PPA) also binds to the non-specific TMDregion, but not to the specific phosphatidylinositol region, and thus fails to engage the CTD or open the channel. Our results show how PIP2 can control the resting membrane potential through a specific ion-channel-receptor.ligand interaction that brings about a large conformational change, analogous to neurotransmitter activation of ion channels at synapses.
PIP2 influences the metabolic state of cells by at least three distinct pathways (Supplementary Fig. 1a, b): first, as the prototypical second messenger being cleaved into diacyl glycerol and inositol triphosphate10,11; second, as a localization signal targeting soluble proteins to the plasma membrane12-14; and third, as a signalling molecule capable of agonizing an ion channel2-8,15,16. This latter role, in which an ion channel is activated by PIP2, was first discovered in 1998 when it was shown that PIP2 acted alone to open a Kir channel8.
Figure 1a, b shows the influence of PIP2 on the function of Kir2.2 from chicken. Following excision of an inside-out membrane patch from a Xenopus oocyte expressing Kir2.2 channels, initially large inward K1 currents diminish over time. The diminution occurs because PIP2 is depleted from the membrane's inner leaflet8. The K1 currents can be restored partially...