Signaling States of a Short Blue-Light Photoreceptor Protein PpSB1-LOV Revealed from Crystal Structures and Solution NMR Spectroscopy

•Comparison of crystal structures of PpSB1-LOV in dark and light states•Dimer interface and the C-terminal Jα-helix show major structural rearrangements.•A ~ 29° rotation between the two protein chains gated by light•Extensive NMR solution studies reveal light-induced conformational changes.•We propose a rotary switch mechanism for the activation.

Light-Oxygen-Voltage (LOV) domains represent the photo-responsive domains of various blue-light photoreceptor proteins and are widely distributed in plants, algae, fungi, and bacteria. Here, we report the dark-state crystal structure of PpSB1-LOV, a slow-reverting short LOV protein from Pseudomonas putida that is remarkably different from our previously published “fully light-adapted” structure [1]. A direct comparison of the two structures provides insight into the light-activated signaling mechanism. Major structural differences involve a ~ 11 Å movement of the C terminus in helix Jα, ~ 4 Å movement of Hβ-Iβ loop, disruption of hydrogen bonds in the dimer interface, and a ~ 29° rotation of chain-B relative to chain-A as compared to the light-state dimer. Both crystal structures and solution NMR data are suggestive of the key roles of a conserved glutamine Q116 and the N-cap region consisting of A′α-Aβ loop and the A′α helix in controlling the light-activated conformational changes. The activation mechanism proposed here for the PpSB1-LOV supports a rotary switch mechanism and provides insights into the signal propagation mechanism in naturally existing and artificial LOV-based, two-component systems and regulators.

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