Multiple solute binding proteins for gamma-aminobutyrate and 5-aminovalerate in Pseudomonas aeruginosa : assessment of their potential role in sensor kinase activation

The canonical mode of receptor activation consists in the binding of signals or signal-loaded solute binding proteins (SBPs) to sensor domains. Many sensor histidine kinases (SHK), that are activated by SBP binding, are encoded next to their cognate sbp gene. To assess to what degree this is a general rule, we studied three SBPs of Pseudomonas aeruginosa PAO1 that are encoded in the vicinity of genes encoding the AgtS (PA0600) and AruS (PA4982) SHKs. Ligand screening using compound libraries and microcalorimetric studies showed that the SPBs PA0602 and PA4985 both bound preferentially GABA (KD=2.3 and 0.58 microM, respectively), followed by 5-aminovalerate (KD=30 and 1.6 microM, respectively) and ethanoldiamine (KD=2.3 and 0.58 microM, respectively), whereas AgtB (PA0604) recognized exclusively 5-aminovaleric acid (KD=2.9 microM). However, microcalorimetric titrations of the AgtS sensor domain with AgtB or PA0602 in the absence or presence of ligands did not reveal binding. By analogy, bacterial two-hybrid assays failed to show an interaction of PA4985 with the AruS-sensor domain. Vicinal sbp and shk genes are thus not always functionally linked. We previously identified PA0222 as a GABA-specific SBP. The existence of three SBPs for GABA may be related to the role of GABA as an inducer of P. aeruginosa virulence.