Molecular basis of quorum-sensing signal transduction by CqsS and its inhibition by CqsA, the autoinducer synthase.

Cryo-EM structures of full-length Vibrio harveyi CqsS show CAI-1 binding positions transmembrane helix 6 to disrupt cytosolic extension, explaining how receptor activation couples to signaling.

• Why it matters: Quorum-sensing pathways must convert periplasmic autoinducer detection into intracellular phosphorylation cascades, but the molecular coupling for CqsA–CqsS signaling was unclear.
• What they did: The team determined cryo-EM structures of CqsS apo and ATPγS-bound states, then used biochemical and genetic analyses to support cis autophosphorylation at H1 and trans phospho-transfer to D1. They coexpressed CqsS with CqsA to capture CAI-1-bound complexes and observed two CAI-1 molecules by cryo-EM.
• The result: In the CqsA–CqsS complex, CqsA interaction positions CqsS H1 away from the catalytic cavity and inhibits kinase activity, while CAI-1 binding moves helix 6 to provide a structural basis for signal transduction.

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