Insights into biofilm dispersal regulation from the crystal structure of the PAS-GGDEF-EAL region of RbdA from Pseudomonas aeruginosa .

RbdA is a positive R egulator of b iofilm d ispersal of Pseudomonas aeruginosa Its cytoplasmic region (cRbdA) comprises a N-terminal PAS domain followed by a diguanylate cyclase (GGDEF) and an EAL domain, whose phosphodiesterase activity is allosterically stimulated by GTP binding to the GGDEF domain. We report crystal structures of cRbdA and of two binary complexes: with GTP/Mg2+ bound to the GGDEF active site and with the EAL domain bound to the c-di-GMP substrate. These structures unveil a 2-fold symmetric dimer, stabilized by a closely packed N-terminal PAS domain and a non-canonical EAL dimer. The auto-inhibitory switch is formed by an alpha helix (S-helix) immediately N-terminal to the GGDEF domain that interacts with the EAL dimerization helix (α6-E ) of the other EAL monomer and maintains the protein in a locked conformation. We propose that local conformational changes in cRbdA upon GTP binding lead to a structure with the PAS domain and S-helix shifted away from the GGDEF-EAL domains, as suggested by SAXS experiments. Domain reorientation should be facilitated by the presence of a α-helical lever (H-helix) that tethers the GGDEF and EAL regions, allowing the EAL domain to rearrange into an active dimeric conformation. IMPORTANCE Biofilm formation by bacterial pathogens increases resistance to antibiotics. RbdA positively regulates biofilm dispersal of Pseudomonas aeruginosa The crystal structures of the cytoplasmic region of RbdA protein presented here reveal that two evolutionary-conserved helices play an important role in regulating the activity of RbdA, with implications for other dual GGDEF-EAL domains that are abundant in the proteomes of several bacterial pathogens. Thus, this work could assist the development of small molecules that would promote bacterial biofilm dispersal.