Qian, H.; Bassler, B. L.; Liu, M.; Yang, Y.; Valastyan, J. S.; Zhang, Y.; Beggs, G. A.; Liu, Y.; Wang, Z.

Vibrio bacteria possess multiple quorum-sensing systems, each of which conveys information into the cell concerning the cell density and species composition of the vicinal community. Germane to the present work is the CqsA-CqsS quorum-sensing pathway that vibrios use for intra-genus communication. CqsA is the synthase for the autoinducer (S)-3-hydroxytridecan-4-one (CAI-1), and CqsS, a bi-functional two-component kinase-phosphatase, is the CAI-1 receptor. Here, we determine the cryo-EM structures of the full-length Vibrio harveyi CqsS dimer in the apo- and ATP{gamma}S-bound states. Our structures reveal the molecular basis for the initiation of the phosphorylation cascade, which we support through biochemical and genetic analyses. Autophosphorylation of CqsS occurs in cis at the conserved histidine residue (H1), whereas phospho-transfer to the conserved aspartate residue (D1) in the C-terminal receiver domain occurs in trans. By co-expressing CqsS and CqsA to capture CqsS bound to CAI-1, we unexpectedly discover a tight interaction between the CqsS intracellular domain and CqsA. In the complex, the CqsS H1 residues are positioned away from the catalytic cavity, and CqsS kinase activity is inhibited. Cryo-EM analysis, focused on the transmembrane domain of CqsS in this complex, reveals the presence of two CAI-1 molecules. CAI-1 binding induces an obvious movement of transmembrane helix 6, disrupting its extension into the cytosol, providing a structural basis for how CAI-1 binding in the periplasm is coupled to signal transduction into the cytoplasm. Together, our findings advance the understanding of CqsS-CAI-1-mediated quorum-sensing signal transduction, an activity that underpins the coordination of group behaviors in vibrios.