Formation of a complex between HD-GYP, GGDEF and PilZ domain proteins regulates motility in Xanthomonas campestris

RpfG is a member of a class of wide spread bacterial two-component regulators with an HD-GYP cyclic di-GMP phosphodiesterase domain. In the plant pathogen Xanthomonas campestris pv. campestris (Xcc), RpfG together with the sensor kinase RpfC regulates the synthesis of a range of virulence factors as a response to the cell-cell Diffusible Signaling Factor (DSF). RpfG regulates many different virulence factors by divergent pathways. Physical interaction of RpfG with two diguanylate cyclase (GGDEF) domain proteins controls motility. This is a dynamic interaction that depends upon DSF signaling and involves the conserved GYP motif in the HD-GYP domain. Here we use synthetic peptide overlay technology and yeast two-hybrid analysis in conjunction with alanine substitution mutagenesis to define a motif within the GGDEF domain proteins required for interaction. We show that regulation of motility by the GGDEF domain proteins depends upon this motif. Furthermore, we show by Y2H that both GGDEF domain proteins bind a specific PilZ domain adaptor protein, and this interacts with the pilus motor proteins PilU and PiIT. The results support a model in which DSF signaling influences motility through the interaction of proteins that affect pilus action. The motif required for HD-GYP domain interaction is conserved in a number of GGDEF domain proteins, suggesting that regulation via interdomain interactions may be of broad relevance.