The structural basis for signal promiscuity in a bacterial chemoreceptor

Abstract

Signalling through chemosensory pathways is typically initiated by thebinding of signal molecules to the chemoreceptor ligand binding domain(LBD). The PcaY_PP chemoreceptor from Pseudomonas putida KT2440 ischaracterized by an unusually broad signal range, and minimal requisitesfor signal binding are the presence of a C6-membered ring and that of acarboxyl group. Previous studies have shown that only some of the multi-ple signals recognized by this chemoreceptor are of apparent metabolicvalue. We report here high-resolution structures of PcaY_PP-LBD in theabsence and presence of four cognate chemoeffectors and glycerol. Thedomain formed a four-helix bundle (4HB), and both ligand binding sites ofthe dimer were occupied with the high-affinity ligands protocatechuate andquinate, whereas the lower-affinity ligands benzoate and salicylate werepresent in only one site. Ligand binding was verified by microcalorimetrictitration of site-directed mutants revealing important roles of an arginineand number of polar residues that establish an extensive hydrogen bondingnetwork with bound ligands. The comparison of the apo and holo struc-tures did not provide evidence for this receptor employing a transmem-brane signalling mechanism that involves piston-like shifts of the finalhelix. Instead, ligand binding caused rigid-body scissoring movements ofboth monomers of the dimer. Comparisons with the 4HB domains of theTar and Tsr chemoreceptors revealed significant structural differences.Importantly, the ligand binding site in PcaY_PP-LBD is approximately8 A removed from that of the Tar and Tsr receptors. Data indicate a sig-nificant amount of structural and functional diversity among 4HBdomains.