The beta gamma subunits of G proteins gate a potassium channel by pivoted bending of a transmembrane segment

Molecular understanding of ion channel gating remains unclear despite intense research efforts. Here, we use a novel proline scanning mutagenesis approach to identify a segment within the pore-lining TM2 transmembrane helix of G protein-sensitive K+ channels that controls gating. Together with the approach of homology modeling of the GIRK4 structure, we localize the TM2 helix bundle crossing as the gate physiologically controlled by the βγ subunits of G proteins. Additionally, we show that the flexibility afforded by a glycine residue in the middle of the TM2 is crucial for Gβγ gating. In contrast, flexibility introduced in the intracellular part of TM2 disrupts Gβγ gating. These findings lead us to propose a novel structural model of channel gating: Gβγ binding exerts a force through the inflexible intracellular part of the TM2 to bend it at the middle of the helix at a hinge position, produced by the conserved glycine; the structural change produced by this force leads to an enlargement of the helix bundle crossing and thereby to gating of the channel. The fact that the most conserved residue in the TM2 helix among ∼500 potassium and cyclic nucleotide gated channels is the glycine residue we identified serving as a hinge, suggests that the model of “pivoted bending” is a general gating mechanism.