Recruitment of a cytoplasmic response regulator to the cell pole is linked to its cell cycle-regulated proteolysis

Abstract

The response regulator CtrA, which silences the Caulobacter origin of replication and controls multiple cell cycle events, is specifically proteolyzed in cells preparing to initiate DNA replication. At the swarmer-to-stalked cell transition and in the stalked compartment of the predivisional cell, CtrA is localized to the cell pole just before its degradation. Analysis of the requirements for CtrA polar localization and CtrA proteolysis revealed that both processes require a motif within amino acids 1-56 of the CtrA receiver domain, and neither process requires CtrA phosphorylation. These results strongly suggest that CtrA polar localization is coupled to its cell cycle-regulated proteolysis. The polarly localized DivK response regulator promotes CtrA localization and proteolysis, but it does not directly recruit CtrA to the cell pole. Mutations in the divJ and pleC histidine kinases perturb the characteristic asymmetry of CtrA localization and proteolysis in the predivisional cell. We propose that polar recruitment of CtrA evolved to ensure that CtrA is degraded only in the stalked half of the predivisional cell, perhaps by localizing a proteolytic adaptor protein to the stalked pole. This is an example of controlled proteolysis of a cytoplasmic protein that is associated with its active recruitment to a specific subcellular address.Caulobacter ͉ CtrA ͉ ClpXP I n Caulobacter crescentus, DNA replication occurs once per cell division, and the phases of the cell cycle are linked to observable morphological changes (reviewed in refs. 1 and 2). Motile swarmer cells (Fig. 1) in the G 1 phase of the cell cycle develop into stalked cells ( Fig. 1) and initiate chromosome replication. During the swarmer-to-stalked cell (SW-ST) (or G 1 -S) transition, each cell sheds its polar flagellum and builds a stalk at the same site. As DNA replication proceeds, stalked cells elongate and become predivisional cells with distinct poles. A new flagellum is built at the pole opposite the stalk, so that each cell division produces a motile swarmer cell and a stalked cell. Before cell separation, a diffusion barrier is established between the swarmer and stalked compartments of the predivisional cell so that they contain distinct sets of signal transduction proteins that yield progeny with different replicative fates (3): the stalked progeny can initiate DNA replication immediately, whereas the swarmer cell must first differentiate into a new stalked cell.A key signal transduction protein that regulates Caulobacter cell cycle progression is the essential response regulator CtrA (4). CtrA directly induces or represses the transcription of Ϸ55 operons in the Caulobacter genome, including genes needed for flagellum and pili biosynthesis, DNA methylation, cell division, chemotaxis, and metabolism (5, 6). However, CtrA also represses chromosome replication by binding to five sites within the replication origin (7). Because CtrA has multiple functions, its activity is tightly controlled by three mechanisms: cell cycleregulated transcription (...

Direct and adaptor-mediated substrate recognition by an essential AAA+ protease

“…Degradation of CtrA is a highly specific event, only occurring before initiation of DNA replication. Localization and proper cell-cycle degradation of CtrA in vivo appear linked and require the presence of its N-terminal receiver domain and RcdA (14,33). Our results, however, show clearly that CtrA is efficiently degraded in an RcdA-independent reaction by purified ClpXP alone, with kinetics that account for the intracellular rate of CtrA degradation (Fig.…”

A phospho-signaling pathway controls the localization and activity of a protease complex critical for bacterial cell cycle progression

“…The regulated proteolysis of CtrA at the swarmer-to-stalked cell transition and in the stalked compartment of the predivisional cell requires multiple steps. These steps include the interaction with the RcdA degradation factor (3), the localization of both RcdA and CtrA to the stalked pole (3,8,11), the concomitant polar localization of ClpXP protease complex (3), and the physical interaction among ClpX, RcdA, and CtrA at that cell pole. Here, we have shown that the localization of ClpXP is essential for CtrA proteolysis, the CpdR single-domain response regulator mediates the dynamic polar localization and activity of the ClpXP protease, and the localization function of CpdR depends on its phosphorylation state.…”

“…ClpXP and its CtrA substrate are simultaneously localized to the stalked cell pole both at the swarmer-to-stalked cell transition and in the stalked cell compartment of the predivisional cell (3,8). We identify a single-domain response regulator, CpdR, that in its unphosphorylated state directs CtrA proteolysis by controlling ClpXP polar localization and activity.…”

Linear ordering and dynamic segregation of the bacterial chromosome

“…The Shapiro laboratory also demonstrated roles for Smc (structural maintenance of chromosomes), MreB, and topoisomerase IV in chromosomal organization in three other recent reports in PNAS (20)(21)(22). The widely conserved Smc has been shown in various systems to condense DNA and aid in successful segregation (23).…”