Regulation of type IV pili formation and function by the small GTPase MglA in Myxococcus xanthus
Myxococcus xanthus cells are rod-shaped and move in the direction of their long axis, using two distinct motility systems. Adventurous gliding (A-) depends on the Agl/Glt motility complexes that assemble at the leading pole, adhere to the substratum, and disassemble at the lagging pole. Social (S-)...
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|Summary:||Myxococcus xanthus cells are rod-shaped and move in the direction of their long axis, using two distinct motility systems. Adventurous gliding (A-) depends on the Agl/Glt motility complexes that assemble at the leading pole, adhere to the substratum, and disassemble at the lagging pole. Social (S-) motility depends on type IV pili (T4P) that localize at the leading cell pole. T4P are anchored in the cell envelope and pull cells forward through cycles of extension, surface adhesion and retraction powered by the T4P machine. This machine includes 10 proteins spanning the outer membrane, periplasm, inner membrane and cytoplasm. Most of the proteins in the machine form stationary complexes at the two poles while the two ATPases PilB and PilT localize mostly at the leading and lagging pole, respectively. M. xanthus cells occasionally stop and resume movement in the opposite direction. These reversals are regulated by the Frz chemosensory system. During reversals the old lagging pole becomes the new leading and vice versa, the two motility systems invert polarity and after a reversal, T4P are formed at the new leading pole. Thus, T4P can assemble at both poles but at any point in time, T4P only assemble at one pole. The mechanism(s) underlying unipolar T4P formation during cell movement remains unknown.
The small GTPase MglA is essential for motility in M. xanthus. MglA cycles between the active MglA-GTP state, which is essential for motility, and the inactive MglA-GDP state. MglA is activated by the RomR/RomX complex, which has guanine nucleotide exchange factor (GEF) activity, and is inhibited by MglB, which is a GTPase activating protein (GAP). MglA-GTP mostly localizes to the leading pole while MglB as well as RomR/RomX localize in bipolar, asymmetric pattern with the large cluster at the lagging pole. RomR/RomX recruits MglA-GTP to the leading pole while MglB excludes MglA-GTP from the lagging pole by converting MglA-GTP to MglAGDP. Among these four proteins, only MglA is essential for T4P-dependent motility. However, the precise function of MglA for the T4P-dependent motility remains unclear.
Here, we demonstrate that MglA-GTP stimulates T4P formation and function while MglB ensures T4P unipolarity by excluding MglA-GTP from the lagging pole. Moreover, we identify the TPR domain-containing protein SgmX and show that it is important for T4P formation. Epistasis analyses support that MglA-GTP and SgmX act in the same genetic pathway and that SgmX acts downstream of MglA-GTP. In vitro analyses support that SgmX interacts directly with MglA-GTP. Additionally, SgmX stimulates polar accumulation of the PilB extension ATPase. Based on these findings, we propose a model in which MglA-GTP stimulates T4P assembly via direct interaction with SgmX, which in turn interacts with PilB to stimulate T4P extension.|
|Physical Description:||148 Pages|