Crystal structure of a two-subunit TrkA octameric gating ring assembly

Interaction of unphosphorylated PtsN with the K+/H+ antiporter YcgO inhibits its activity in Escherichia coli

Genetic studies in Escherichia coli have implicated the unphosphorylated version of PtsN (unphospho-PtsN), the terminal phospho-acceptor of the PtsP-PtsO-PtsN phosphorelay, as a negative regulator of potassium (K+) efflux mediated by YcgO. YcgO is a protein belonging to the CPA1 family of monovalent cation/proton antiporters. Here we show that in vivo, YcgO comprises an approximately 383 amino acid N-terminal transmembrane domain and a 195 amino acid C-terminal cytoplasmic region (CTR). Copurification studies show that unphospho-PtsN specifically interacts with YcgO, and phosphorylation of PtsN leads to marked attenuation of the interaction. Genetic and biochemical analyses of a class of mutations in YcgO that lead to constitutive activation of YcgO identify the CTR as the site of interaction between unphospho-PtsN and YcgO and indicate that the putative CorC domain in the CTR may serve as the site of interaction. Our studies are supportive of a model which postulates that the unphospho-PtsN:CorC interaction may inhibit the activation of YcgO by a putative RCK domain in the CTR, leading to the inhibition of the K+/H+ antiport activity of YcgO.

c-di-AMP determines the hierarchical organization of bacterial RCK proteins

In bacteria, intracellular K+ is involved in the regulation of membrane potential, cytosolic pH, and cell turgor as well as in spore germination, environmental adaptation, cell-to-cell communication in biofilms, antibiotic sensitivity, and infectivity. The second messenger cyclic-di-AMP (c-di-AMP) has a central role in modulating the intracellular K+ concentration in many bacterial species, controlling transcription and function of K+ channels and transporters. However, our understanding of how this regulatory network responds to c-di-AMP remains poor. We used the RCK (Regulator of Conductance of K+) proteins that control the activity of Ktr channels in Bacillus subtilis as a model system to analyze the regulatory function of c-di-AMP with a combination of in vivo and in vitro functional and structural characterization. We determined that the two RCK proteins (KtrA and KtrC) are neither physiologically redundant or functionally equivalent. KtrC is the physiologically dominant RCK protein in the regulation of Ktr channel activity. In explaining this hierarchical organization, we found that, unlike KtrA, KtrC is very sensitive to c-di-AMP inactivation and lack of c-di-AMP regulation results in RCK protein toxicity, most likely due to unregulated K+ flux. We also found that KtrC can assemble with KtrA, conferring c-di-AMP regulation to the functional KtrA/KtrC heteromers and potentially compensating KtrA toxicity. Altogether, we propose that the central role of c-di-AMP in the control of the K+ machinery, by modulating protein levels through gene transcription and by regulating protein activity, has determined the evolutionary selection of KtrC as the dominant RCK protein, shaping the hierarchical organization of regulatory components of the K+ machinery.

Activation of a nucleotide-dependent RCK domain requires binding of a cation cofactor to a conserved site

RCK domains regulate the activity of K⁺ channels and transporters in eukaryotic and prokaryotic organisms by responding to ions or nucleotides. The mechanisms of RCK activation by Ca²⁺ in the eukaryotic BK and bacterial MthK K⁺ channels are well understood. However, the molecular details of activation in nucleotide-dependent RCK domains are not clear. Through a functional and structural analysis of the mechanism of ATP activation in KtrA, a RCK domain from the B. subtilis KtrAB cation channel, we have found that activation by nucleotide requires binding of cations to an intra-dimer interface site in the RCK dimer. In particular, divalent cations are coordinated by the γ-phosphates of bound-ATP, tethering the two subunits and stabilizing the active state conformation. Strikingly, the binding site residues are highly conserved in many different nucleotide-dependent RCK domains, indicating that divalent cations are a general cofactor in the regulatory mechanism of many nucleotide-dependent RCK domains.