Cryo-EM analysis of PIP regulation in mammalian GIRK channels.

G-protein-gated inward rectifier potassium (GIRK) channels are regulated by G proteins and PIP. Here, using cryo-EM single particle analysis we describe the equilibrium ensemble of structures of neuronal GIRK2 as a function of the C8-PIP concentration. We find that PIP shifts the equilibrium between two distinguishable structures of neuronal GIRK (GIRK2), extended and docked, towards the docked form. In the docked form the cytoplasmic domain, to which G binds, becomes accessible to the cytoplasmic membrane surface where G resides. Furthermore, PIP binding reshapes the G binding surface on the cytoplasmic domain, preparing it to receive G. We find that cardiac GIRK (GIRK1/4) can also exist in both extended and docked conformations. These findings lead us to conclude that PIP influences GIRK channels in a structurally similar manner to Kir2.2 channels. In Kir2.2 channels, the PIP-induced conformational changes open the pore. In GIRK channels, they prepare the channel for activation by G.