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Critical Role for the Unique N-Terminus of Chlamydial MreB in Directing Its Membrane Association and Interaction with Elements of the Divisome

Chlamydiae lack the conserved central coordinator protein of cell division FtsZ, a tubulin-like homolog. Current evidence indicates Chlamydia uses the actin-like homolog, MreB, to substitute for the role of FtsZ. Interestingly, we observed MreB as a ring at the septum in dividing cells of Chlamydia. We hypothesize that MreB, to substitute for FtsZ in Chlamydia, must possess unique properties compared to canonical MreB orthologs. Sequence differences between chlamydial MreB and orthologs in other bacteria revealed that chlamydial MreB possesses an extended N-terminal region and the conserved amphipathic helix found in other bacterial MreBs. The extended N-terminal region was sufficient to restore the localization of a truncated E. coli MreB mutant lacking its amphipathic helix to the membrane and was crucial for interactions with cell division components RodZ and FtsK, though the region was not required for homotypic interactions. Importantly, the N-terminal region was sufficient to direct GFP to the membrane when expressed in Chlamydia. A mutant N-terminal region with reduced amphipathicity was unable to perform these functions. From these data, the extended N-terminal region of chlamydial MreB is critical for localization and interactions of this protein. Our data provide mechanistic support for chlamydial MreB to serve as a substitute for FtsZ. Importance Chlamydia trachomatis is an obligate intracellular pathogen, causing sexual transmitted diseases and trachoma. Studying chlamydial physiology, especially its cell division mechanism, is important for developing novel therapeutic strategies for the treatment of these diseases. Since chlamydial cell division has unique features, including a polarized cell division process independent of FtsZ, a canonical cell division coordinator, studying the subject is helpful for understanding undefined aspects of chlamydial growth. In this study, we characterized MreB, a substitute for FtsZ, as a cell division coordinator. It forms a filamentous ring at the septum, like FtsZ in E. coli. We show that the localization of MreB is dependent upon the amphipathic nature of its extended N-terminus. Furthermore, this region is crucial for its interaction with other proteins involved in cell division. Given these results, chlamydial MreB may function as a scaffold for cell divisome proteins at the septum and regulate cell division in this organism.