Structure of a KirBac potassium channel with an open bundle-crossing indicates a mechanism of channel gating

KirBac channels are prokaryotic homologs of mammalian inwardly-rectifying (Kir) potassium channels and recent crystal structures of both Kir and KirBac channels have provided a major insight into their unique structural architecture. However, all of the available structures are closed at the helix bundle-crossing and therefore the structural mechanisms that control opening of their primary activation gate remain unknown. In this study, we engineered the inner pore-lining helix (TM2) of KirBac3.1 to trap the bundle-crossing in an apparently open conformation, and determined the crystal structure of this mutant channel to 3.05 Å resolution. Contrary to previous speculation, this novel structure suggests a mechanistic model in which rotational ‘twist’ of the cytoplasmic domain is coupled to opening of the bundle-crossing gate via a network of inter- and intra-subunit interactions that involves the TM2 C-linker, slide-helix, G-loop and the CD-loop.