Gating of a G protein-sensitive Mammalian Kir3.1 Prokaryotic Kir Channel Chimera in Planar Lipid Bilayers

Kir3 channels control heart rate and neuronal excitability through GTP-binding (G) protein and phosphoinositide signaling pathways. These channels were the first characterized effectors of the beta gamma subunits of G proteins. Because we currently lack structures of complexes between G proteins and Kir3 channels, their interactions leading to modulation of channel function are not well understood. The recent crystal structure of a chimera between the cytosolic domain of a mammalian Kir3.1 and the transmembrane region of a prokaryotic KirBac1.3 (Kir3.1 chimera) has provided invaluable structural insight. However, it was not known whether this chimera could form functional K+ channels. Here, we achieved the functional reconstitution of purified Kir3.1 chimera in planar lipid bilayers. The chimera behaved like a bona fide Kir channel displaying an absolute requirement for PIP2 and Mg2+-dependent inward rectification. The channel could also be blocked by external tertiapin Q. The three-dimensional reconstruction of the chimera by single particle electron microscopy revealed a structure consistent with the crystal structure. Channel activity could be stimulated by ethanol and activated G proteins. Remarkably, the presence of both activated G alpha and G beta gamma subunits was required for gating of the channel. These results confirm the Kir3.1 chimera as a valid structural and functional model of Kir3 channels.