Abstract
Myxococcus xanthus possesses a form of surface motility powered by the retraction of the type IV pilus (T4P). Additionally, exopolysaccharide (EPS), the major constituent of bacterial biofilms, is required for this T4P-mediated motility in M. xanthus as the putative trigger of T4P retraction. The results here demonstrate that the T4P assembly ATPase PilB functions as an intermediary in the EPS regulatory pathway composed of the T4P upstream of the Dif signaling proteins in M. xanthus. A suppressor screen isolated a pilB mutation that restored EPS production to a T4P− mutant. An additional PilB mutant variant, which is deficient in ATP hydrolysis and T4P assembly, supports EPS production without the T4P, indicating PilB can regulate EPS production independently of its function in T4P assembly. Further analysis confirms that PilB functions downstream of the T4P filament but upstream of the Dif proteins. In vitro studies suggest that the nucleotide-free form of PilB assumes the active signaling conformation in EPS regulation. Since M. xanthus PilB possesses conserved motifs with high affinity for c-di-GMP binding, the findings here suggest that c-di-GMP can regulate both motility and biofilm formation through a single effector in this surface-motile bacterium.
Highlights
Myxococcus xanthus is a motile bacterium well adapted to life on a solid surface
We propose that PilB* and PilBWA mutations changed the structure of the protein to favor its active signaling conformation and that this conformational change may have reduced the stability of PilB in vivo coincidentally
As the signals in this range are sensitive to changes in protein secondary structure[55], the results indicate that the secondary structure content is unaffected by the mutations in PilBWA and PilBWB or by ligand binding
Summary
Myxococcus xanthus is a motile bacterium well adapted to life on a solid surface. While a unicellular organism, it possesses a social or multicellular life style during both its vegetative and developmental phases[1, 2]. In M. xanthus, the preferred anchor for T4P retraction is EPS either associated with the cell surface or deposited on a solid substratum by this bacterium[26, 27]. A mutation known to eliminate the ATPase activity of PilB and its ability to support T4P assembly[21] was found to strongly suppress the EPS defect of the pilA deletion strain. This observation indicates that the role of PilB in EPS regulation can be independent of its role as the T4P assembly ATPase. Our results support the conclusion that PilB functions in a signaling capacity with dual roles in the regulation of motility and biofilm formation in M. xanthus
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