Lotte Sogaard-Andersen
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Bacterial development and differentiation
Regulation of motility: Type IV pili & the A-engine
M. xanthus moves by gliding motility and possesses two gliding engines. The S-engine depends on retraction of uni-polarly localized type IV pili while it remains unclear how the A-engine functions. Occasionally, M. xanthus cells undergo cellular reversals in which the leading pole becomes the lagging pole. During a cellular reversal the two gliding engines switch polarity. During the aggregation stage of fruiting body formation, the C-signal induces a decrease in the cellular reversal frequency and essentially switches cell behaviour from an oscillatory mode to a unidirectional mode in which cells are moving towards nascent fruiting bodies. This effect of the C-signal suggests that the C-signal inhibits polarity switching of the two gliding engines. To understand at the molecular level how the C-signal brings about this effect on the gliding engines, we have focused on elucidating the mechanisms underlying polarity switching of the two motility systems.
During reversals, the pole at which type IV pili are formed switches. Type IV pili function depends on the assembly of a macromolecular complex consisting of 10-15 proteins that span the cell envelope and are localized to the cytoplasm, inner membrane, periplasm and the outer membrane. Recently, we have shown that the outer membrane protein PilQ, the inner membrane protein PilC, and the cytoplasmic protein PilM localize to clusters at both poles in a symmetric pattern and these clusters remain stationary during reversals. Conversely, the PilB and PilT motor ATPases that energize extension and retraction, respectively, localize to opposite poles with PilB predominantly at the piliated and PilT predominantly at the non-piliated pole, and these proteins switch poles during reversals. In other words, type IV pili pole-to-pole switchings involve the disassembly of the type IV pili machinery at the old leading pole and reassembly of this machinery at the new leading pole. Our observations also suggest that the spatial separation of PilB and PilT in combination with a noisy PilT accumulation at the piliated pole allow the temporal separation of extension and retraction. Currently, we are focusing on analyses to understand how type IV pili are targeted to the poles.
During a reversal the polarity of the A-engine also switches. We have identified a structurally unique response regulator, RomR, which is required for the A-engine to function. Moreover, we have shown that RomR localizes asymmetrically to the cell poles with a large cluster at the lagging cell pole and a small cluster at the leading pole. RomR localization is dynamic and in parallel with a cell reversal the large RomR cluster relocates to the new lagging cell pole. We are currently focusing on analyses to understand the function of RomR in A-motility and how it is targeted to the poles.
Type IV pili pole-to-pole switchings depend on the disassembly and reassembly of the type IV pili molecular machine