Naturally cysteine-less LOV domains from halophilic archaea exhibit magnetic field effects on their fluorescence

Magnetic fields can modulate the function of certain flavoproteins through the radical pair mechanism, in which they influence the spin evolution of a coherent pair of radicals. This process can result in magneto-fluorescence, in which magnetic fields modulate the intensity of fluorescence emitted from the flavin cofactor. The prevalence of this property across protein families and domains of life, however, remains poorly characterized. In canonical Light-Oxygen-Voltage (LOV) domains, a conserved cysteine forms an adduct with the flavin that leads to downstream signaling. Mutating this cysteine instead yields signaling through the neutral semiquinone radical form, and the same mutation was crucial for enhancing magneto-fluorescence in the engineered protein MagLOV2. We therefore hypothesized that natural LOV domains that lack this conserved cysteine may exhibit magneto-fluorescence. Using a custom magneto-fluorescence imaging platform, we measured the fluorescence of E. coli colonies expressing three such domains, as well as a single mutant of one of them, under a switched external field. HsuLOV, from a halophilic archaeon, exhibited magneto-fluorescence, as did a single mutant of BAT-LOV, from a second halophilic archaeon. The domain amb2291 from magnetotactic bacteria, on the other hand, showed no detectable response under our illumination conditions. Magneto-fluorescence is therefore not exclusive to engineered proteins but is present in natural cysteine-less LOV domain sequences. The proteins reported here are, to our knowledge, the first magnetosensitive proteins reported from archaea. This suggests that radical-pair magnetosensitivity may be more widespread across LOV domains than previously appreciated.