Independence and interdependence: signal transduction of two chemosensory receptors important for the regulation of gliding motility in Myxococcus xanthus
The Myxococcus xanthus Dif and Frz chemosensory pathways play important roles in the regulation of gliding motility. The Dif system regulates the production of exopolysaccheride (EPS), which is essential for social motility and fruiting body formation. The Frz pathway controls reversal frequency, which is fundamental for directed movement by this surface-gliding bacterium. In addition, both pathways are involved in the chemotactic response towards several phosphatidylethanolamine (PE) species such that the Dif pathway is required for excitation while the Frz pathway is essential for adaptation. In this study we addressed three crucial questions regarding the signal processing of these two chemosensory pathways by focusing on DifA and FrzCD, the MCP homologs from their respective pathways.
First, the receptor protein in the Dif pathway, DifA, lacks a perisplasmic domain, the typical signal-sensing structure. To examine whether DifA shares similar transmembrane signaling mechanism with typical transmembrane sensor proteins (MCPs and sensor kinases), we constructed a chimeric protein that is composed of the N-terminus of NarX (nitrate sensor kinase) and the C-terminus of DifA. This NarX-DifA chimera restores the DifA functionality (EPS production, agglutination, S-motility and development) to a "difA mutant in a nitrate-dependent manner, suggesting DifA shares a similar transmembrane signaling mechanism with typical MCPs and sensor kinases despite its unorthodox structure.
Second, the M. xanthus chemotaxis is still controversial. It has been argued that the taxis-like response in this slowly gliding bacterium could result from physiological effects of certain chemicals. To study motility regulation by the Frz pathway, we constructed two chimeras between the N-terminus of NarX and C-terminus of FrzCD, which is the receptor protein of the Frz pathway. The two chimeras, NazDF and NazDR, are identical except that NazDR contains a G51R mutation in the otherwise wild-type NarX sensory module. This G51R mutation was shown to reverse the signaling output of a NarX-Tar chimera to nitrate. We discovered that nitrate specifically decreased the reversal frequency of NazDF-expressing cells and increased that of NazDR-expressing cells. These results show that directional motility in M. xanthus can be regulated independently of cellular metabolism and physiology. Surprisingly, the NazDR strain failed to adapt to nitrate in temporal assays, as did the wild type to known repellents. Therefore, the lack of temporal adaptation to negative stimuli is an intrinsic property in M. xanthus motility regulation.
Third, the Dif and Frz pathways are both involved in the chemotactic response towards certain PE molecules such that the Dif pathway is required for excitation and while the Frz system is essential for adaptation. In addition, 12:0 PE, known to be sensed by DifA, results in increased FrzCD methylation. These findings suggested that in the regulation of PE response, two pathways communicate with each other to mediate adaptation. Here we provided evidence to indicate that DifA does not undergo methylation during EPS regulation and PE chemotaxis. On the other hand, using mutants expressing the NarX-DifA chimera, it was found that signal transduction through DifA, DifC (CheW-like) and DifE (CheA-like) modulates FrzCD methylation. Surprisingly, the attractant 12:0 PE can modulate FrzCD methylation in two ways distinguishable by the dependency on DifA, DifC and DifE. The DifACE-independent mechanism, which may result from specific sensing of 12:0 PE by FrzCD, increases FrzCD methylation as expected. Unexpectedly, 12:0 PE decreases FrzCD methylation with the DifACE-dependent mechanism. This "opposite" FrzCD methylation by DifACE-dependent signaling was supported by results from NafA-expressing mutants because nitrate, which acts as a repellent, increases FrzCD methylation. Based on these findings, we proposed a model for chemotaxis toward 12:0 PE (and 16:1 PE). In this model, DifA and FrzCD both sense the same signal and activate the pathways of excitation (Dif) and adaptation (Frz) independently. The two pathways communicate with each other via methylation crosstalk between DifACE and FrzCD in such a way that processes of excitation and adaptation can be coordinated.