Sensory domain contraction in histidine kinase CitA triggers transmembrane signaling in the membrane-bound sensor

Bacteria use membrane-integral sensor histidine kinases (HK) to perceive stimuli and transduce signals from the environment to the cytosol. Information on how the signal is transmitted across the membrane by HKs is still scarce. Combining both liquid-and solid-state NMR, we demonstrate that structural rearrangements in the extracytoplasmic, citrate-sensing Per-Arnt-Sim (PAS) domain of HK CitA are identical for the isolated domain in solution and in a longer construct containing the membrane-embedded HK and lacking only the kinase core. We show that upon citrate binding, the PAS domain contracts, resulting in a shortening of the C-terminal beta-strand. We demonstrate that this contraction of the PAS domain, which is well characterized for the isolated domain, is the signal transmitted to the transmembrane (TM) helices in a CitA construct in liposomes. Putting the extracytoplasmic PAS domain into context of the membrane-embedded CitA construct slows down citrate-binding kinetics by at least a factor of 60, confirming that TM helix motions are linked to the citrate-binding event. Our results are confirmation of a hallmark of the HK signal transduction mechanism with atomic resolution on a full-length construct lacking only the kinase core domain.