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Abstract
The regulation of ion channel activity by specific lipid molecules is widely recognized as an integral component of electrical signalling in cells. In particular, phosphatidylinositol 4,5-bisphosphate (PIP^sub 2^), a minor yet dynamic phospholipid component of cell membranes, is known to regulate many different ion channels. PIP^sub 2^ is the primary agonist for classical inward rectifier (Kir2) channels, through which this lipid can regulate a cell's resting membrane potential. However, the molecular mechanism by which PIP^sub 2^ 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 PIP^sub 2^. We found that PIP^sub 2^ binds at an interface between the transmembrane domain (TMD) and the cytoplasmic domain (CTD). The PIP^sub 2^-binding site consists of a conserved non-specific phospholipid-binding region in the TMD and a specific phosphatidylinositol-binding region in the CTD. On PIP^sub 2^ binding, a flexible expansion linker contracts to a compact helical structure, the CTD translates 6 Å 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 TMD region, but not to the specific phosphatidylinositol region, and thus fails to engage the CTD or open the channel. Our results show how PIP^sub 2^ 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. [PUBLICATION ABSTRACT]