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Abstract

The rod-shaped Myxococcus xanthus cells move with defined front-rear
polarity using polarized motility systems. A polarity module consisting
of the small GTPase MgIA, its cognate GTPase activating protein (GAP)
MgIB and RomR establishes this polarity. Agl-Glt gliding motility
complexes assemble and disassemble at the leading and lagging pole,
respectively. These processes are stimulated by MgIA-GTP at the leading
and MgIB at the lagging pole. Here, we identify RomX as an integral
component of the polarity module. RomX and RomR form a complex that has
MgIA guanine nucleotide exchange factor (GEF) activity and also binds
MgIA-GTP. In vivo RomR recruits RomX to the leading pole forming the
RomR-RomX complex that stimulates MgIA-GTP formation and binding,
resulting in a high local concentration of MgIA-GTP. The spatially
separated and opposing activities of the RomR-RomX GEF at the leading
and the MgIB GAP at the lagging cell pole establish front-rear polarity
by allowing the spatially separated assembly and disassembly of Agl-Glt
motility complexes. Our findings uncover a regulatory system for
bacterial cell polarity that incorporates a nucleotide exchange factor
as well as an NTPase activating protein for regulation of a
nucleotide-dependent molecular switch and demonstrate a spatial
organization that is conserved in eukaryotes.