Novel Mechanism of Gating in the TrkH-TrkA Complex

The superfamily of K+ transporters (SKT) is ubiquitous in bacteria, fungi and plants. SKT proteins are required for survival of bacteria in low K+ conditions and are involved in salt regulation in fungi and plants. Bacterial SKTs have two components, a membrane embedded protein that resembles an ion channel and a cytosolic protein that regulates channel gating [1]. Crystal structures of two bacterial SKT systems were reported recently [2,3]. In both structures, the membrane embedded component forms a homodimer onto which a homotetrameric ring of the cytosolic protein docks. Single-channel activities of one of the complexes, the TrkH (membrane embedded) -TrkA (cytosolic) complex, were recorded and analyzed: ATP or ATP analogs such as AMPPNP activates the channel while ADP closes it [2]. The structure of the TrkH-TrkA complex is likely in a closed conformation because it was crystallized in the presence of NADH, a ligand that does not activates the channel. In order to understand how ATP or its analogs induces channel opening, we solved the structure of the TrkH-TrkA complex in the presence of AMPPNP to 3.29 A by x-ray crystallography. When compared to the previous structures, the new structure shows that each TrkA protomer binds to two AMPPNP molecules, and that the TrkA tetramer assumes an elongated conformation that likely induces a change in the TrkH. Conformational changes in the TrkH involve significant changes in the dimer interface and are different from any other channels of known structure. These new observations and hypotheses will be validated and tested by mutational and functional analyses.[1] Levin EJ, Zhou M. Recent progress on the structure and function of the TrkH/KtrB ion channel. Curr Opin Struct Biol. 2014;27:95-101.[2] Cao Y, Pan Y, Huang H, et al. Gating of the TrkH ion channel by its associated RCK protein TrkA. Nature. 2013;496(7445):317-22.[3] Vieira-pires RS, Szollosi A, Morais-cabral JH. The structure of the KtrAB potassium transporter. Nature. 2013;496(7445):323-8.