All living organisms control their intracellular concentrations of ions, in particular those of sodium and potassium [1, 2]. This is complicated in bacteria, plants, fungi and archaea by the fact that the cells of these organisms do not control their extracellular environment. As a consequence these cells have evolved mechanisms of adaptation to environmental changes such as high salinity and drought that are not present in animal cells. One of these mechanisms of adaptation involves the Trk/Ktr/HKT membrane proteins, a superfamily of Na+ and/or K+ transporters.

We have determined the X-ray crystal structure of the KtrAB K+ transporter (Figure 60) from the bacterium Bacillus subtilis at 3.5 Å [3]. It is composed of two different polypeptides, the KtrB membrane protein which is responsible for the permeation (transport) of potassium and sodium ions, and the KtrA regulatory protein, a soluble protein responsible for regulation of transporter activity. KtrB assembles as a dimer and interacts with one of the faces of the octameric ring formed by KtrA (Figure 60).

Crystal structure of the KtrAB K+ transporter of Bacillus subtilis

Fig. 60: Crystal structure of the KtrAB K+ transporter of Bacillus subtilis. KtrB subunits are represented as ribbons and are coloured in light and dark blue. K+ ions are shown as magenta spheres. The horizontal line represents the limits of the membrane bilayer. KtrA is shown in surface representation with its subunits in yellow and red; ATP molecules can be seen through the partially transparent surface in their binding sites facing the inside of the octameric ring.

Strikingly, each KtrB subunit is composed of 4 repeats which display the TM-Ploop-TM architecture first described for the KcsA potassium channel [4]. These 4 repeats assemble around a central axis that defines the ion pore. The KtrA octameric ring is formed as a tetramer of dimers where each dimer adopts the RCK domain fold, also first described in a potassium channel [5]. Each subunit binds an ATP molecule.

We have shown that KtrAB activity is increased upon ATP binding and decreased when bound to ADP. Importantly, ATP is not hydrolysed. The two ligands act as signalling molecules so that the activity of the ion transporter is probably controlled by a change in the intracellular concentration ratio of ADP/ATP.

Conformation of the KtrA octameric rings

Fig. 61: Conformation of the KtrA octameric rings. a) KtrA-ATP ring with its alternating subunits coloured in yellow and red and ATP molecules in blue facing the inside of the ring; blue arrows indicate the direction of the conformational changes relative to the ADP-bound state. The KtrA dimeric unit is delineated by a yellow border. The black square indicates the 4-fold symmetry axis. b) KtrA-ADP ring with ADP molecules coloured in cyan; arrows indicate the direction of the conformational changes relative to the ATP-bound state. The black ellipse indicates the 2-fold symmetry axis.

We have also determined the X-ray crystal structure of the isolated KtrA octameric ring bound to ATP and ADP at resolutions of 3.0 and 2.8 Å respectively. While the ATP bound structure is identical to the structure of KtrA as seen in the KtrAB complex, the ADP bound structure shows a different conformation (Figure 61). Two of the four subunits forming a ring face have moved apart resulting in an expansion of the ring and a change in its symmetry, the ring is now formed by a dimer of tetramers. Despite the strong structural and functional relationship between KtrAB and K+ channels, this ADP bound structure implies a mechanism of activation which is very different. In K+ channels, activation by RCK domain rings is dependent on expansion of the ring [5]. In KtrAB, activation by KtrA appears to be associated with a contraction of the ring.

Principal publication and authors

R.S. Vieira-Pires, A. Szollosi and J.H. Morais-Cabral, Nature 496, 323–328 (2013).

IBMC, Instituto de Biologia Molecular e Celular, Universidade do Porto (Portugal)


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