Tanja Gerharz

The structure of the periplasmic ligand-binding domain of the sensor kinase CitA reveals the first extracellular PAS domain

The integral membrane sensor kinase CitA of Klebsiella pneumoniae is part of a two-component signal transduction system that regulates the transport and metabolism of citrate in response to its environmental concentration. Two-component systems are widely used by bacteria for such adaptive processes, but the stereochemistry of periplasmic ligand binding and the mechanism of signal transduction across the membrane remain poorly understood. The crystal structure of the CitAP periplasmic sensor domain in complex with citrate reveals a PAS fold, a versatile ligand-binding structural motif that has not previously been observed outside the cytoplasm or implicated in the transduction of conformational signals across the membrane. Citrate is bound in a pocket that is shared among many PAS domains but that shows structural variation according to the nature of the bound ligand. In CitAP, some of the citrate contact residues are located in the final strand of the central β-sheet, which is connected to the C-terminal transmembrane helix. These secondary structure elements thus provide a potential conformational link between the periplasmic ligand binding site and the cytoplasmic signaling domains of the receptor.

Subunits act independently in a cyclic nucleotide‐activated K+ channel

Ion channels gated by cyclic nucleotides have crucial roles in neuronal excitability and signal transduction of sensory neurons. Here, we studied ligand binding of a cyclic nucleotide‐activated K+ channel from Mesorhizobium loti and its isolated cyclic nucleotide‐binding domain. The channel and the binding domain alone bind cyclic AMP with similar affinity in a non‐cooperative manner. The cAMP sensitivities of binding and activation coincide. Thus, each subunit in the tetrameric channel acts independently of the others. The binding and gating properties of the bacterial channel are distinctively different from those of eukaryotic cyclic nucleotide‐gated channels.