Characterization of CdbS, a PilZ domain protein involved in chromosome organization and segregation during heat shock stress in Myxococcus xanthus
The second messenger c-di-GMP regulates a wide variety of processes in bacteria that are often related to changes in lifestyle. Unexpectedly, we recently reported a link between c-di-GMP and chromosome organization. Specifically, the DNA-binding protein CdbA binds c-di-GMP, is essential for viabilit...
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|Summary:||The second messenger c-di-GMP regulates a wide variety of processes in bacteria that are often related to changes in lifestyle. Unexpectedly, we recently reported a link between c-di-GMP and chromosome organization. Specifically, the DNA-binding protein CdbA binds c-di-GMP, is essential for viability, and important for chromosome organization and segregation in Myxococcus xanthus. CdbA is highly abundant and binds >500 sites on the chromosome but its depletion causes no or only modest changes in transcription. Based on these findings, we proposed that CdbA is a nucleoid-associated protein whose activity is modulated by c-di-GMP.
Most nucleoid-associated proteins are not essential. Therefore, to explore the CdbA essentiality, suppressor mutants that were viable in the absence of CdbA were isolated. Among eight suppressors, seven had mutations in mxan_4328 that encodes a stand-alone PilZ domain protein, henceforth CdbS. The inactivation of cdbS completely suppressed the lethal CdbA depletion phenotype, and cdbS in otherwise wild-type cells was dispensable for viability. Notably, CdbA depletion, without affecting transcription of cdbS, resulted in a four-fold increased CdbS level. Moreover, overexpression of cdbS phenocopied the CdbA depletion phenotype. These observations support that the defects caused by CdbA depletion are the result of CdbS over-accumulation. In vitro, purified CdbS binds c-di-GMP, but the function of CdbS is independent of c-di-GMP binding in vivo. In in vivo pull-down experiments with an active CdbS-FLAG protein, significantly enriched proteins included five chaperones and co-chaperones including two PilZ-Hsp70 proteins, henceforth CsdK1 and CsdK2, a DnaJ homolog and a GrpE homolog. csdK1 as well as csdK2 were transcriptionally upregulated in response to CdbA depletion, and the resulting increased CsdK1 and CsdK2 accumulation lead to an elevated CdbS level. Searching for a physiological function of this system, we found that CdbS accumulation increased in response to high temperature stress at 37°C in a CsdK1- and CsdK2-dependent manner and caused accelerated cell death at this temperature.
In total, our data support that increased CdbS accumulation caused by either CdbA depletion or high temperature stress, by an unknown mechanism, results in chromosome segregation and organization defects, thereby causing cell division inhibition and cell death. We speculate that the CdbA/CsdK1/CsdK2/CdbS system could be linked to c-di-GMP signaling and that altered cellular levels of c-di-GMP level modulate DNA binding by CdbA and, ultimately, the cellular level of CdbS. Finally, we speculate that if this system is aberrantly or excessively activated it has detrimental effects on cell viability.|