Molecular basis for the control of K(+) uptake via KtrAB and KimA by cyclic di-AMP

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The potassium homeostasis is essential for bacterial survival and controlled by the orchestrated function of various K(+) importers and exporters. While different ligands and stimuli have been thought to regulate those, recently, cyclic di-AMP has been identified as overarching regulatory signaling molecule in different Gram-positive bacteria. The production of cyclic di-AMP appears to depend on the K(+) (availability. At an excess of external potassium an increased intracellular cyclic di-AMP level was detected. Vice versa, the uptake and release of potassium was shown to be modulated both by the control of gene expression and protein activity by cyclic di-AMP. In Bacillus subtilis cyclic di-AMP directly binds to K(+) importers KtrAB and KimA leading to their inhibition. However, the underlying mechanism of protein deactivation remains elusive. Here, we will shed light on the molecular principles of cyclic di-AMP binding and transport inhibition. Since K(+) channel KtrAB and potassium-proton symporter KimA supposedly do not share any structural similarities, different binding motifs and regulatory mechanisms are expected. The binding sites and structural consequences of cyclic di-AMP binding will be solved by single particle cryo-electron microscopy and X-ray crystallography. Pulsed EPR spectroscopy will elucidate the nucleotide-dependent conformational dynamics of KtrAB and KimA. Complementary mutational studies in combination with functional assays will identify residues required for nucleotide binding and the subsequent regulation of activity.