During heat stress in Myxococcus xanthus, the CdbS PilZ domain protein, along with two PilZ-DnaK chaperones, perturbs chromosome organization and accelerates cell death

Abstract

C-di-GMP is a bacterial second messenger that regulates diverse processes in response to environmental or cellular cues. The nucleoid-associated protein (NAP) CdbA in Myxococcus xanthus binds c-di-GMP and DNA in a mutually exclusive manner in vitro. CdbA is essential for viability, and CdbA depletion causes defects in chromosome organization, leading to a block in cell division and, ultimately, cell death. Most NAPs are not essential; therefore, to explore the paradoxical cdbA essentiality, we isolated suppressor mutations that restored cell viability without CdbA. Most mutations mapped to cdbS, which encodes a stand-alone c-di-GMP binding PilZ domain protein, and caused loss-of-function of cdbS. Cells lacking CdbA and CdbS or only CdbS were fully viable and had no defects in chromosome organization. CdbA depletion caused post-transcriptional upregulation of CdbS accumulation, and this CdbS over-accumulation was sufficient to disrupt chromosome organization and cause cell death. CdbA depletion also caused increased accumulation of CsdK1 and CsdK2, two unusual PilZ-DnaK chaperones. During CdbA depletion, CsdK1 and CsdK2, in turn, stabilized CdbS, thereby enabling its increased accumulation and toxicity. Moreover, we demonstrate that heat stress, possibly involving an increased cellular c-di-GMP concentration, induces the CdbA/CsdK1/CsdK2/CdbA system, causing a CsdK1- and CsdK2-dependent increase in CdbS accumulation. Thereby this system accelerates heat stress-induced chromosome mis-organization and cell death. Collectively, this work describes a unique system that contributes to regulated cell death in M. xanthus and suggests a link between c-di-GMP signaling and regulated cell death in bacteria. Author summaryThe nucleotide-based second messenger c-di-GMP in bacteria controls numerous processes in response to environmental or cellular cues. Typically, these processes are related to lifestyle transitions between motile and sessile behaviors. However, c-di-GMP also regulates other processes. In Myxococcus xanthus, CdbA is a DNA-binding and nucleoid-associated protein that helps to organize the large chromosome. CdbA binds c-di-GMP and DNA in a mutually exclusive manner. While other nucleoid-associated proteins are not essential, CdbA is essential. Here, we show that the crucial function of CdbA is to maintain the level of the c-di-GMP-binding PilZ-domain protein CdbS appropriately low. The CdbS level is not only increased upon depletion of CdbA but also in response to heat stress. Under both conditions, the increased CdbS level perturbs chromosome organization and ultimately causes cell death. The CdbA/CdbS system represents a unique system that contributes to regulated cell death in M. xanthus and suggests a link between c-di-GMP signaling and regulated cell death.