P161: Structure of the PelD cyclic diguanylate effector involved in PEL polysaccharides production in Pseudomonas aeruginosa PAO1

The second messenger bis-(3’-5’) cyclic dimeric guanosine monophosphate (c-di-GMP3) plays a vital role in the global regulation in bacteria. Here, we describe structural and biochemical characterization of a novel c-di-GMP effector PelD that is critical to the formation of pellicles by Pseudomonas aeruginosa. We present high-resolution structures of a cytosolic fragment of PelD in apo form and its complex with c-di-GMP. The structure contains a bi-domain architecture composed of a GAF domain and a GGDEF domain, with the latter binding to one molecule of c-di-GMP. The GGDEF domain has a degenerate GGDEF motif but a conserved RxxD motif (I-site), which we show binds c-di-GMP with a Kd of 0.5 µM. We identified a series of residues that are crucial for c-di-GMP binding, and confirmed the roles of these residues through biochemical characterization of site-specific variants. The structures of PelD represent a novel class of c-di-GMP effector and expand the knowledge of scaffolds that mediate c-di-GMP recognition.