The structural and mechanistic basis of K(+) translocation by the KtrAB system

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The superfamily of K(+) transporters (SKT) units proteins from all kingdoms of life but animals that translocate K(+) and/or Na(+) over the plasma membrane. These proteins are key components of osmotic regulation, pH homeostasis and resistance to high salinity and dryness. SKT proteins are closely related to K(+) channels like KcsA but also show several striking differences which are assigned to an altered function. Within the SKT proteins eukaryotic members consist of only the translocating subunit while prokaryotic members are regulated by at least one additional cytoplasmic subunit. The current hypothesis is that the translocating subunits of prokaryotic members alone show channel-like activities while the regulatory subunits may mediate transporter activity to the system as needed to fulfill its physiological role. Here, we will study the Na(+)-dependent K(+)-translocating KtrAB system of bacteria, in which KtrB is the K(+)-translocating subunit and KtrA mediates Na(+) dependency, K(+) specificity and an increase in uptake velocity. KtrB was shown to contain a unique sequence motif for gating but the structure - function relationship of the system remained unsolved. By use of X-ray crystallography, EPR measurements, transport and electrophysiological measurements we will elucidate the questions (i) which structural and by that also functional impact KtrA has on KtrB, (ii) which mechanisms control K(+) flux within the system on a molecular level and (iii) whether Na(+) acts as ligand to activate channeling or is co-transported as coupling ion.