Sequential structural changes in the monomeric LOV histidine kinase EL346 revealed by transient infrared spectroscopy.

Light-oxygen-voltage (LOV) domains are flavin-binding photoreceptors that mediate cellular responses to blue light. They often act as a structural switch in multidomain proteins to facilitate signaling, where the precise mechanism is dictated by the specific multidomain architecture. A few different LOV signaling models have been categorized, but an understanding of LOV signaling remains a challenge in the field. We studied the multidomain photoreceptor EL346, which couples a LOV domain to a histidine kinase effector domain. Its unique monomeric structure sets it apart from classical, dimeric LOV histidine kinases that act through a structurally entirely different pathway. We present transient infrared spectroscopy data that reveal the timescales on which activation takes place. By studying the isolated core LOV domain, we were able to isolate signal contributions originating from the histidine kinase output domain. The data reveal α-helical structural rearrangements that appear concomitantly with adduct formation at ∼10 µs. We identified a second kinetic step at ∼3 ms, where α-helical elements undergo a final structural adaptation. Our data put the sequence of structural events during the activation of EL346 into context with other LOV multidomain signaling modes, supporting a mechanism similar to