The ParA-like protein AgmE positively regulates cell division in Myxococcus xanthus
Correct positioning of the division plane is a prerequisite for the generation of daughter cells with a normal chromosome complement and with a correct size. So far, all bacterial systems, which contribute to correctly place the division plane, regulate formation of the FtsZ cytokinetic ring at m...
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|Summary:||Correct positioning of the division plane is a prerequisite for the generation of daughter cells with a normal chromosome complement and with a correct size. So far, all bacterial systems, which contribute to correctly place the division plane, regulate formation of the FtsZ cytokinetic ring at mid-cell. Myxococcus xanthus belongs to the δ-proteobacteria and divides by binary fission. However, the mechanisms that ensure proper cell division are not known. In contrast, M. xanthus has been studied in detail because of its complex life cycle and for its gliding motility. M. xanthus possesses two motility systems referred to as the social (S) and the adventurous (A)- motility systems. While analyzing the adventurous gliding motility E protein (AgmE), which is a member of the ParA/Soj family of ATPases, we discovered that an in-frame deletion of the agmE gene results in the formation of filamentous cells and chromosome-free mini-cells. We determined that an ΔagmE mutant is neither impaired in chromosome replication nor in chromosome segregation. Moreover, an ΔagmE mutant cells displayed fewer division sites and frequently these sites were not located to mid-cell. These data strongly suggest that AgmE has a central function in cell division and in placing the division plane at mid-cell. Consequently, FtsZ localization was investigated. In WT, FtsZ localizes in a speckled-pattern in small-sized cells, i.e. early in the cell cycle, whereas in larger cells, i.e. later in the cell cycle, FtsZ localizes at mid-cell. On the contrary, in an ΔagmE mutant FtsZ localizes in a speckled-pattern independently of cell length and cell cycle. We hypothesized that AgmE is required to properly localize FtsZ and could act either positively by directing FtsZ to mid-cell or by stabilizing the FtsZ cytokinetic ring or negatively by inhibiting FtsZ cytokinetic ring formation at the poles.To distinguish between these hypotheses, AgmE localization was studied. AgmE was found to localize in three distinct patterns, which correlates with cell length, i.e. the cell cycle. In small-sized cells, AgmE localizes in a patchy pattern, as cell size increases AgmE localizes to a single focus slightly off mid-cell and when the cells have reached a specific cell length AgmE localizes at mid-cell. Genetic data suggest that the cell cycle dependent localization of AgmE depends on ATPase activity. The localization of AgmE at mid-cell implies that AgmE acts positively on FtsZ localization or stabilizes the FtsZ cytokinetic ring. To distinguish between these models, co-localization studies were carried out. These analyses demonstrated that FtsZ and AgmE co-localize at mid-cell. Intriguingly, some cells displayed mid-cell localization of AgmE in the absence of mid-cell localization of FtsZ. Moreover, in vitro analyses showed that AgmE interact directly with FtsZ. On the basis of the localization of AgmE to mid-cell and the observation that AgmE localize to mid-cell before FtsZ, we propose that AgmE is a novel cell division regulator that acts positively to direct FtsZ to mid-cell. A systematic analysis of the M. xanthus proteome revealed a unique combination of cell division regulators, i.e. M. xanthus encodes an orthologue of DivIVA and lacks orthologues of MinCDE, MipZ, SlmA and Noc. These observations suggest that cell division regulators are yet to be discovered in M. xanthus. In this study, we have shown that M. xanthus possesses the orphan ParA-like protein AgmE, which is involved in cell division. Importantly, AgmE is the first example of a protein shown to positively regulate FtsZ localization. Interestingly, an in-frame deletion of divIVA has no obvious effect on cell division and a DivIVA-mCherry protein localizes in a speckled pattern. Moreover, MXAN0636, the downstream gene of agmE, seems to be an additional component for the regulation of cell division in M. xanthus.|