Establishment and maintenance of cell polarity in Myxococcus xanthus
Cell polarity, the asymmetric distribution of proteins within cellular space, underlies key processes in all cells. Motile polarized cells have a front-rear polarity axis that can change dynamically in response to external signals. The rod-shaped M. xanthus cells move with well-defined front-rear po...
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|Cell polarity, the asymmetric distribution of proteins within cellular space, underlies key processes in all cells. Motile polarized cells have a front-rear polarity axis that can change dynamically in response to external signals. The rod-shaped M. xanthus cells move with well-defined front-rear polarity. In response to signaling by the Frz chemosensory system this polarity is inverted, and cells reverse their direction of movement. Front-rear polarity is established by a polarity module consisting of the small GTPase MglA, its cognate GEF RomR/RomX and GAP MglB. All four proteins localize asymmetrically to the cell poles with RomR/RomX and MglB mostly at the lagging pole and MglA mostly at the leading pole. In response to Frz signaling, the four proteins switch poles and front-rear polarity is inverted.
We used a combination of quantitative experiments and data-driven theory to uncover the design principles underlying the emergence of polarity in M. xanthus. By studying each of the polarity proteins in isolation, using RomR as a proxy for the RomR/RomX complex, and their effects as we systematically reconstruct the system, using precise in vivo techniques to quantify subcellular protein localization, we deduced the network of effective interactions between the polarity proteins. At the core of this interaction network are two positive feedbacks whereby RomR stimulates its own polar recruitment and RomR and MglB mutually recruit one another to the poles. At the same time, a negative feedback is established through MglA, which is recruited by RomR but inhibits RomR/MglB mutual recruitment. Moreover, we identify the MglC protein as important for the RomR/MglB positive feedback, allowing the GEF/GAP pairing at the lagging pole and the establishment of the asymmetry.
Our results further show that continuous cycling of MglA is crucial for the emergence of polarity and in the regulation of polarity switching during reversals. Through FRAP experiments and Photoactivatble protein fusions, we reveal that MglB, MglC and RomR participate in a tripartite cluster in which turnover is regulated by MglA activity.
We rationalize the localization pattern of the GEF and GAP as providing stable asymmetry while remaining responsive and capable of polarity inversions in response to Frz signaling during cellular reversals. Our results not only have implications for the understanding of polarity and motility in M. xanthus but also for dynamic cell polarity more broadly in bacteria as well as in eukaryotic cells.