Chemotaxis systems enable microbes to sense their immediate environment, moving towards beneficial stimuli and away from those that are harmful. In an effort to better understand the chemotaxis system of Sinorhizobium meliloti , a symbiont of the legume alfalfa, the cellular stoichiometries of all ten chemotaxis proteins in S. meliloti were determined. A combination of quantitative immunoblot and mass spectrometry revealed that the protein stoichiometries in S. meliloti varied greatly from those in Escherichia coli and Bacillus subtilis . To compare protein ratios to other systems, values were normalized to the central kinase CheA. All S. meliloti chemotaxis proteins exhibited increased ratios to varying degrees. The ten-fold higher molar ratio of adaptor proteins CheW1 and CheW2 to CheA might result in the formation of rings in the chemotaxis array that only consist of CheW instead of CheA and CheW in a 1:1 ratio. We hypothesize that the higher ratio of CheA to the main response regulator CheY2 is a consequence of the speed-variable motor in S. meliloti , instead of a switch-type motor. Similarly, proteins involved in signal termination are far more abundant in S. meliloti , which utilizes a phosphate-sink mechanism based on CheA retro-phosphorylation to inactivate the motor response regulator versus CheZ-catalyzed dephosphorylation as in E. coli and B. subtilis . Finally, the abundance of CheB and CheR, which regulate chemoreceptor methylation, was increased when compared to CheA, indicative of variations in the adaptation system of S. meliloti . Collectively, these results mark significant differences in the composition of bacterial chemotaxis systems. IMPORTANCE The symbiotic soil bacterium Sinorhizobium meliloti contributes greatly to host-plant growth by fixing atmospheric nitrogen. The provision of nitrogen as ammonium by S. meliloti leads to increased biomass production of its legume host alfalfa and diminishes the use of environmentally harmful chemical fertilizers. To better understand the role of chemotaxis in host-microbe interaction, a comprehensive catalogue of the bacterial chemotaxis system is vital, including its composition, function, and regulation. The stoichiometry of chemotaxis proteins in S. meliloti has very few similarities to the systems in E. coli and B. subtilis . In addition, total amounts of proteins are significantly lower. S. meliloti exhibits a chemotaxis system distinct from known models by incorporating new proteins as exemplified by the phosphate sink mechanism.
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In Situ Conformational Changes of the Escherichia coli Serine Chemoreceptor in Different Signaling States
ABSTRACT Tsr, the serine chemoreceptor in Escherichia coli , transduces signals from a periplasmic ligand-binding site to its cytoplasmic tip, where it controls the activity of the CheA kinase. To function, Tsr forms trimers of homodimers (TODs), which associate in vivo with the CheA kinase and CheW coupling protein. Together, these proteins assemble into extended hexagonal arrays. Here, we use cryo-electron tomography and molecular dynamics simulation to study Tsr in the context of a near-native array, characterizing its signaling-related conformational changes at both the individual dimer and the trimer level. In particular, we show that individual Tsr dimers within a trimer exhibit asymmetric flexibilities that are a function of the signaling state, highlighting the effect of their different protein interactions at the receptor tips. We further reveal that the dimer compactness of the Tsr trimer changes between signaling states, transitioning at the glycine hinge from a compact conformation in the kinase-OFF state to an expanded conformation in the kinase-ON state. Hence, our results support a crucial role for the glycine hinge: to allow the receptor flexibility necessary to achieve different signaling states while also maintaining structural constraints imposed by the membrane and extended array architecture. IMPORTANCE In Escherichia coli , membrane-bound chemoreceptors, the histidine kinase CheA, and coupling protein CheW form highly ordered chemosensory arrays. In core signaling complexes, chemoreceptor trimers of dimers undergo conformational changes, induced by ligand binding and sensory adaptation, which regulate kinase activation. Here, we characterize by cryo-electron tomography the kinase-ON and kinase-OFF conformations of the E. coli serine receptor in its native array context. We found distinctive structural differences between the members of a receptor trimer, which contact different partners in the signaling unit, and structural differences between the ON and OFF signaling complexes. Our results provide new insights into the signaling mechanism of chemoreceptor arrays and suggest an important functional role for a previously postulated flexible region and glycine hinge in the receptor molecule.
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- Award ID(s):
- 1713784
- NSF-PAR ID:
- 10109349
- Date Published:
- Journal Name:
- mBio
- Volume:
- 10
- Issue:
- 4
- ISSN:
- 2150-7511
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1128/mBio.00973-19
