Bacteria adapt to the variable environmental conditions through various signal transduction mechanisms achieved through the action of one-component systems, two-component systems and chemosensory pathways (1). The stimulation of the latter pathway is achieved by signal binding to the ligand binding domain (LBD) of the chemoreceptors, which in turn generates a molecular stimulus that modulates CheA autophosphorylation and consequently transphosphorylation of the CheY response regulator. Chemosensory pathways mediate either flagellum-based chemotaxis, type IV pili based motility or are involved in the regulation of alternative cellular processes.
Chemotaxis of the human pathogen Pseudomonas aeruginosa to amino acids has been previously studied and is mediated primarily by three paralogous chemoreceptors, termed PctA, PctB (2) and PctC (3). While PctA responds to 18 different proteinogenic L-amino acids, PctB binds preferentially to L-glutamine, which is one of the two amino acids that is not recognized by PctA. PctC binds with high preference to gamma-aminobutyrate (GABA), which is a compound omnipresent in nature and that exerts multiple biological functions such as human neurotransmitter, plant hormone or growth substrate for bacteria.
We report here high resolution structures of the LBDs of these three paralogous receptor in complex with several of their ligands. These 3 domains adopt a double CACHE fold and ligand recognition occurs in the membrane distal CACHE moiety. These structures are different to the 4-helix bundle type of LBD of the E. coli chemoreceptors, which are the traditional models in the study of chemoreceptors. The ligand specificity of LBD homologues in other species cannot be inferred by overall LBD sequence comparison. However, the molecular detail of ligand recognition, as revealed in our study, will provide the basis for the development of specific bioinformatic algorithms to identify homologues with similar ligand profiles. All crystals were obtained using the capillary counter-diffusion technique. The use of capillaries allowed us to obtain the PctA-Met complex from the PctA-Ile crystal by diffusing an excess of methionine within a capillary containing the crystals of the complex.
The author would like to acknowledge the support of the MICINN (Spain) projects BIO2013-4297-P and ESRF projects Mx1629 and Mx1739.