The first LOV full structure: a competition story.
It´s very good news: thanks to the combined efforts of Hartmut Luecke, Kevin Gardner and collaborators, we have the first crystal structure of a naturally occurring full-length LOV protein (1) and a very good idea of what happens when light jumps on it forming the so-called photoadduct. To my delight the structure is from a bacterial protein, specifically from Erythrobacter litoralis, combining a photosensing LOV domain with a helix-turn-helix (HtH) DNA-binding motif.
ElLOV-HtH is able to bind DNA only after being photoexcited and having undergone a quite dramatic conformational change: this is, a posteriori, quite understandable, given that the "dark" LOV domain wants to bind where DNA should and prevent the necessary HtH-HtH dimerization(Fig. 1a and see ref. 2). Meaning that it efficiently competes with DNA-binding.
Dimerization in a definite configuration is required for HtH to bind to DNA. In the dark-state ElLOV-HtH structure, the helix involved in dimerization (4α) is "sequestered" into interactions with the LOV domain, precisely with its β-sheet surface: no wonder if one needs a trigger to liberate this structural element; in this case this trigger is blue light.
And the famous Jα-linker? that´s another nice story. Different from LOV2 of phototropin, where the linker is mostly attached to the β-sheet in the dark and mostly detached in the light, as part of its "transmitter" job, here it is not so: the linker cannot compete with the direct LOV-HtH interactions and assumes a different orientation and this place is taken by the dimerization 4α-helix. Things are different in the chimeric LOV2-Rac1 protein (Fig 1b), an enginereed light-activated GTPase (3). Whatever role might the Jα-linker have in light activation of LOV-HtH, one thing is certain: signaling occurs, also here, via the LOV β-sheet surface (4).
Figure 1:a) PDB code 3P7N (chain a), structure of the ElLOV-HtH protein in the dark state. The dimerization helix, 4α interacts directly with the LOV-domain b-sheet surface; the 3α-helix, devoted to DNA-binding, is also spatially blocked. Light activation removes the inhibition of HtH-dimerization/DNA-binding exerted by the LOV domain; b) structure of an engineered light-regulated LOV2-GTPase (a Rac protein, PDB code 2WKP). Light activation detaches the Jα-linker and unmasks binding sites for effector proteins.
Re: (1) Structural basis of photosensitivity in a bacterial light-oxygen-voltage/helix-turn-helix (LOV-HTH) DNA-binding protein, PNAS, 2011, 108: 9449-9454, by A. I. Nash, R. McNulty, M. E. Shillito, T. E. Swartz, R. A. Bogomolni, H. Luecke and K. H. Gardner
(2) A. Losi, W. Gartner, Shedding (blue) light on algal gene expression, Proc. Nat. Acad. Sci. USA, 2008, 105, 7-8.
(3) Y. I. Wu, D. Frey, O. I. Lungu, A. Jaehrig, I. Schlichting, B. Kuhlman and K. M. Hahn, A genetically encoded photoactivatable Rac controls the motility of living cells, Nature, 2009, 461, 104-108.
(4) A. Losi, A. and W. Gärtner, Old chromophores, new photoactivation paradigms, trendy applications: flavins in LOV and BLUF photoreceptors, Photochem. Photobiol., 2011, 87, 491–510