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Abstract:
Bacteria can move across surfaces using type IV pili (T4P), which
undergo cycles of extension, adhesion, and retraction. The T4P
localization pattern varies between species; however, the underlying
mechanisms are largely unknown. In the rod-shaped Myxococcus xanthus
cells, T4P localize at the leading cell pole. As cells reverse their
direction of movement, T4P are disassembled at the old leading pole and
then form at the new leading pole. Thus, cells can form T4P at both
poles but engage only one pole at a time in T4P formation. Here, we
address how this T4P unipolarity is realized. We demonstrate that the
small Ras-like GTPase MglA stimulates T4P formation in its GTP-bound
state by direct interaction with the tetratricopeptide repeat (TPR)
domain-containing protein SgmX. SgmX, in turn, is important for polar
localization of the T4P extension ATPase PilB. The cognate MglA GTPase
activating protein (GAP) MglB, which localizes mainly to the lagging
cell pole, indirectly blocks T4P formation at this pole by stimulating
the conversion of MglA-GTP to MglA-GDP. Based on these findings, we
propose a model whereby T4P unipolarity is accomplished by stimulation
of T4P formation at the leading pole by MglA-GTP and SgmX and indirect
inhibition of T4P formation at the lagging pole by MglB due to its MglA
GAP activity. During reversals, MglA, SgmX, and MglB switch polarity,
thus laying the foundation for T4P formation at the new leading pole and
inhibition of T4P formation at the new lagging pole.
