Structural characterization of ion translocation by the chimeric potassium uptake system KdpFABC

We elucidated a unique transport mechanism by a protein complex that can transport potassium with high affinity and selectivity against a steep gradient. Potassium regulation is vital for bacterial cells. A shortage of potassium leads to a failure of metabolic processes. In this thesis we explored the KdpFABC complex located in the membrane, which is synthesized when the external potassium concentration is very low, and acts as an emergency potassium uptake system. This complex consists of two main subunits, the transporter KdpB which consumes energy and the ion uptake channel KdpA. Uptake channels provide selectivity and affinity and transporters mediate transport against the gradient. Usually, transporters and uptake channels can function independently. Therefore, our main question was how these units teamed-up. We found a unique tunnel between KdpA and KdpB that mediated movement of potassium between the two units. Furthermore, we found a lipid between KdpA and KdpB, that was identified as a cardiolipin, which was shown to be important for the function of the complex. When the bacterial cell returns to a potassium rich environment, there is a need for KdpFABC to be repressed. We found that upon modification of a specific residue, two distinctive inhibitory conformations of the complex could be obtained. One of these conformations irreversibly locks the complex. Altogether, the analysis presented in this thesis provide a deep understanding of the KdpFABC complex, and can be used to better understand surviving mechanisms of bacterial cells.