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

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 similar to 11 angstrom movement of the C terminus in helix J alpha, similar to 4 angstrom movement of 11 beta-1 beta loop, disruption of hydrogen bonds in the dimer interface, and a similar to 29 degrees 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'alpha-A beta loop and the A'alpha 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. (C) 2016 Elsevier Ltd. All rights reserved.