McpQ is a specific citrate chemoreceptor that responds preferentially to citrate/metal ion complexes

Abstract

Chemoreceptors are at the beginning of chemosensory pathways that mediate chemotaxis. Pseudomonas putida KT2440 is predicted to have 27 chemoreceptors, most of which uncharacterized. We have previously identified McpS as chemoreceptor for Krebs cycle intermediates. Citrate is primarily present in the environment as metal complex, which, however, is not recognized by McpS. We show here that the McpS paralogue McpQ recognizes specifically citrate and citrate/metal complexes. The McpQ ligand binding domain (McpQ-LBD) binds citrate/metal complexes with higher affinity than citrate. McpQ-LBD is present in a monomer-dimer equilibrium and citrate and particularly citrate/Mg binding stabilize the dimer. The bacterium showed much stronger responses to citrate/Mg than to citrate and mcpQ inactivation caused a dramatic reduction in chemotaxis. Responses to Krebs cycle intermediates are thus mediated by the broad range McpS and McpQ that responds specifically to an intermediate not recognized by McpS. Interesting parallels exist to the paralogous amino acid chemoreceptors of Pseudomonas aeruginosa and Bacillus subtilis. Whereas one paralogue recognizes most amino acids, the remaining paralogue binds specifically one of the few acids not recognized by the broad range receptors. Therefore, chemotaxis to compound families by the concerted action of broad and narrow range receptors may represent a general mechanism.

Identification of a Chemoreceptor in Pseudomonas aeruginosa That Specifically Mediates Chemotaxis Toward α-Ketoglutarate

“…The molar mass extracted was 45.8 kDa, which may point to a virtual intermediate species resulting from the fast equilibrium between the monomers and dimers. Such virtual species have been observed previously for the analysis of the homologous domain of the P. putida KT2440 McpQ chemoreceptor (Martin-Mora et al, 2016). When the experiment was conducted in the presence of 1 mM αKG a single peak with s 20,w = 3.58 S was observed that translates to a species with a molecular mass of 58.7 kDa, close to the sequence derived mass of the protein dimer (61 kDa).…”

“…Equilibrium studies showed that αKG binding reduces the dimer self-dissociation constant from 55 to 5.9 μM. Chemoeffector mediated LBD dimer stabilization appears to be a general feature of 4-helix bundle and HBM domains since similar observations have been made for Tar-LBD (Milligan and Koshland, 1993; Yu et al, 2015), McpQ (Martin-Mora et al, 2016), McpS (Lacal et al, 2010a). …”

“…Examples of the former group are receptors for different L -amino acids (Taguchi et al, 1997; Glekas et al, 2012; Oku et al, 2012; Brennan et al, 2013; Rico-Jimenez et al, 2013; Webb et al, 2016), cyclic organic acids (Luu et al, 2015), purines (Fernandez et al, 2016), polyamines (Corral-Lugo et al, 2016), aromatic hydrocarbons (Lacal et al, 2011b), C4–C6 organic acids (Lacal et al, 2010a; Parales et al, 2013) or C2- and C3-organic acids (Garcia et al, 2015). Chemoreceptors that appear to respond to a single compound include the citrate specific chemoreceptors Tcp of Salmonella typhimurium (Yamamoto and Imae, 1993) and McpQ of P. putida KT2440 (Martin-Mora et al, 2016), the malate specific receptor PA2652 in P. aeruginosa or the GABA specific McpG of P. putida KT2440 (Reyes-Darias et al, 2015a). Here we report with McpK another chemoreceptor that binds specifically a single compound, αKG.…”

“…In contrast to the dCACHE domain that recognizes ligands in the monomeric state (Rico-Jimenez et al, 2013), the HBM and 4-helix bundle domain need to be dimeric for ligand recognition (Milburn et al, 1991; Lacal et al, 2010a; Martin-Mora et al, 2016). This is due to the fact that the ligand binding sites are at the dimer interface and that amino acids from both monomers of the dimer are involved in ligand binding (Milburn et al, 1991; Pineda-Molina et al, 2012).…”

“…The 3D structure of the cluster II LBD of McpS revealed that it corresponds to a novel bacterial sensor domain composed of two structural modules that each can bind directly signal molecules (Pineda-Molina et al, 2012). Other HBM domain containing receptors are the citrate specific McpQ of P. putida KT2440 (Martin-Mora et al, 2016) as well as McfS (Parales et al, 2013) of P. putida F1 and McpS of P. fluorescens Pf0-1 (Oku et al, 2014) that mediate chemotaxis to organic acids. HBM domains were found to form part of chemoreceptors and sensor kinases and were found in bacteria and archaea (Ortega and Krell, 2014).…”