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The acronym LOV stands for light, oxygen, or voltage – these are stimuli that can act on a LOV protein domain to produce a physiological response; this is one form of signal transduction. In the phototropic response of plants, for example, the amount of blue light shining on a leaf regulates the size of its stomatal apertures (stomata are vent holes for gas/water vapor). Another example is found in bacteria, where oxygen sensors containing LOV domains have been shown to regulate nitrogen fixation. LOV domains are structural homologues of the PAS superfamily, a group of diverse sensory proteins. Each LOV domain contains a non-covalently bound flavin mononucleotide (FMN) that performs reversible photochemistry [1]. The first X-ray crystal structure of a plant LOV domain was solved more than a decade ago by the Moffat group. That study answered many important questions regarding the binding geometry of the FMN cofactor and the possible course of the photochemical reaction [2]. In a more recent study, Cornell University professor Brian Crane and colleagues dissected the mechanism of light-induced subunit dissociation of RsLOV [3], a LOV domain photoreceptor from the photosynthetic Gram-negative bacteria Rhodobacter sphaeroides (Figure 1).

Figure 1 Figure 1 Part 2

FIGURE 1. Diversity of enzymes containing the LOV domain. A) LOV subunit structures. RsLOV structure in the dark state; dark-state structures of A. sativa LOV2, E. litoralis EL222, P. putida PpSB1 LOV, B. subtilis YtvA LOV, C. reinhardtii LOV1, and N. crassa VVD; N-terminal extensions are colored orange and C-terminal extensions light green. B) Domain arrangement of sequence-aligned LOV domain proteins. Abbreviations: HisK, histidine kinase; Ser/ThrK, serine/threonine kinase; STAS, sulfate transporter anti-s antagonist; HTH, helix−turn−helix.

In this study, the Crane group explored the “photodissociation” mechanism of RsLOV via the avenues of site-directed mutagenesis, size-exclusion chromatography, X-ray crystallography, SAXS, multi-angle light scattering, and time-resolved absorption spectroscopy. This multifaceted approach enabled them to identify key residues involved in the process, as well as to obtain the decay kinetics for the covalent cysteine-FMN adduct that triggers the conformational change. Utilizing small-angle X-ray scattering, they were able to not only validate the transition between the dark-state dimer and the light-induced monomeric form of RsLOV, but also provide visual proof of that switch in the form of a scattering envelope model (Figure 2).

Figure 2

FIGURE 2. SAXS of RsLOV in dark and light states. Fits of theoretical monomer and dimer scattering curves for dark-state and (A) light-state (B). The best fit to the light-state experimental data was obtained using the ensemble optimization method (lime). C) Scattering envelope models of dark-state RsLOV (top) and light-state (bottom) RsLOV, superimposed with the crystal structures as a complete dimer or truncated to a monomer. D) Schematic of light-induced cysteinyl-FMN (C4a) covalent adduct, the trigger for LOV domain monomerization.

X-ray diffraction and SAXS data were collected at CHESS on the A1 and F2 beamlines, respectively.


  1. Christie J. M., Salomon M., Nozue K., Wada M., Briggs W.R. (1999) LOV (light, oxygen, or voltage) domains of the blue-light photoreceptor phototropin (nph1): binding sites for the chromophore flavin mononucleotide. Proc Natl Acad Sci U S A. 96(15), 8779-83.
  2. Crosson S, Moffat K. (2001) Structure of a flavin-binding plant photoreceptor domain: insights into light-mediated signal transduction. Proc Natl Acad Sci U S A. 98(6), 2995-3000.
  3. Conrad K.S., Bilwes A.M., Crane B.R. (2013) Light-induced subunit dissociation by a light-oxygen-voltage domain photoreceptor from Rhodobacter sphaeroides. Biochemistry 52(2), 378-91.



Submitted by: Tiit Lukk, MacCHESS, Cornell University