Abstract
Migrating cells employ sophisticated signal transduction systems to respond to their environment and polarize towards attractant sources. Bacterial cells also regulate their polarity dynamically to reverse their direction of movement. In Myxococcus xanthus, a GTP-bound Ras-like G-protein, MglA, activates the motility machineries at the leading cell pole. Reversals are provoked by pole-to-pole switching of MglA, which is under the control of a chemosensory-like signal transduction cascade (Frz). It was previously known that the asymmetric localization of MglA at one cell pole is regulated by MglB, a GTPase Activating Protein (GAP). In this process, MglB specifically localizes at the opposite lagging cell pole and blocks MglA localization at that pole. However, how MglA is targeted to the leading pole and how Frz activity switches the localizations of MglA and MglB synchronously remained unknown. Here, we show that MglA requires RomR, a previously known response regulator protein, to localize to the leading cell pole efficiently. Specifically, RomR-MglA and RomR-MglB complexes are formed and act complementarily to establish the polarity axis, segregating MglA and MglB to opposite cell poles. Finally, we present evidence that Frz signaling may regulate MglA localization through RomR, suggesting that RomR constitutes a link between the Frz-signaling and MglAB polarity modules. Thus, in Myxococcus xanthus, a response regulator protein governs the localization of a small G-protein, adding further insight to the polarization mechanism and suggesting that motility regulation evolved by recruiting and combining existing signaling modules of diverse origins.
Highlights
In living organisms, cell polarization underlies many developmental and cellular processes, such as budding in yeast, cell migration and bacterial differentiation [1,2,3]
Previous works suggested an ordered pathway where Frz activates MglAB pole-to-pole switching to switch the localization of downstream motility system specific regulators such as FrzS (S-motility), AglZ and RomR (A-motility) [16,17,27,29]. To confirm these studies in a definitive manner and identify localization interdependencies between these proteins, we systematically analyzed the localization of functional YFP/mCherry fusions to MglA, MglB, FrzS, AglZ and RomR ([16], Figure S1, S2, S3) in all single mutants
Most of the results were consistent with previous reports and confirmed that MglA and MglB are required to establish a polarity axis for motility: in the mglA mutant, AglZ-YFP became diffuse and failed to accumulate both at the pole and at periodic sites; FrzS-GFP, RomR-mCh and MglB-YFP localized only to one cell pole (Figure 1C [16,17,27,29])
Summary
Cell polarization underlies many developmental and cellular processes, such as budding in yeast, cell migration and bacterial differentiation [1,2,3]. Polarization mechanisms ensure the asymmetric positioning of subcellular organelles and its transmission upon cell division [4]. Due to their small sizes bacterial cells have long been thought to be unorganized compartments, proven a misconception with the discovery that bacterial cells contain subcellular structures and micro-compartments [5,6]. In some cases polar localization must be dynamically regulated to segregate cell division inhibitors [9,10], degrade a cell cycle regulator [15], or invert the direction of cell movement [16,17]. We identify a regulator directing dynamic pole-specific activation of motility complexes in the bacterium Myxococcus xanthus
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