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

Abstract 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 A movement of the C terminus in helix Jα, ~ 4 A 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.