Blue-light photoreceptors: from activation to advanced applications

Blue-light effects on living organisms have been observed and reported for more than a century, but it was only during the last two decades that the underlying sensory photoreceptors, their structure and their activation mechanisms have been uncovered and studied in detail. Sensory photoreceptors are integrated protein–chromophore systems, where photochemical reactions induce conformational changes that trigger a signaling cascade, ultimately leading to a given response, e.g. phototropism, photoperiodism, photo-induced growth patterns and regulation of circadian rhythms. Non-opsin blue-light photoreceptors include proteins of the LOV (Light, Oxygen, Voltage), BLUF (Blue-Light sensing Using Flavins), Cry (cryptochromes) and xanthopsin superfamilies, as well as cyanobacterial OCP (Orange Carotenoid Protein). Very recently some phytochrome-related, bilin-binding GAF (cGMP-specific phosphodiesterases, cyanobacterial adenylate cyclases, and transcription activator FhlA) domains from cyanobacteria have been shown to be activable in the blue-light range. Given their spectral properties and their modular architecture, blue-light photoreceptors are also receiving great attention for advanced biotechnological applications, such as superresolution microscopy and optogenetics.

This part-themed issue on blue-light photoreceptors deals with some peculiar and novel aspects of flavin-binding blue-light LOV and BLUF, bilinGAF and OCP proteins. A paper from Carmen Mandalari et al. focuses on evolution, co-evolution and spreading of LOV, BLUF and bilinGAF proteins in the prokaryotic world. These soluble light-sensors appear to be spread throughout the three superkingdoms of life, including a notable percentage of Bacteria and Archaea.

Computational work by Peter Freddolino and co-authors offers new insights into the mechanisms of signal propagation within LOV domains, from the flavin-binding site to the domain surface, via a shift in the dynamics of three protein regions triggered by the light-driven conformational transition of a conserved glutamine. Extending the analysis from the fast pathways of intradomain signal propagation to the next signal transduction steps, Yusuke Nakasone et al. employed a photothermal method, uniquely able to measure diffusion coefficients, to investigate the light-induced changes in the oligomerization state of a LOV domain and their possible role during signaling. A similar methodology, together with other biophysical techniques, has been exploited by Kunisato Kuroi and co-authors to investigate light-driven dissociation of a BLUF-protein decamer into a pentameric state, suggested to be able to bind to a protein partner of the signaling cascade.

The perspective by Thomas Drepper et al. highlights the biotechnological potential of LOV proteins due to their intrinsic fluorescence in the cyan–green spectral region. Their properties as oxygen-independent fluorescent labels are discussed, as well as real-time in vivo reporters for quantitative biotechnological approaches, e.g. as markers for protein synthesis.

Last but not least, a welcome perspective by Diana Kirilovsky and Cheryl Kerfeld addresses the issue of OCP, namely the mechanism of its photoactivation, in vivo roles and distribution among bacteria. OCP, whose structure was solved in 2003, has emerged during the last few years as a novel photosensor involved in a photoprotection mechanism in cyanobacteria. OCP is the first photosensor known to bind a carotenoid as the active chromophore.

We are deeply grateful to all authors for their excellent contributions and we hope the readers find this part-themed issue on blue-light photoreceptors enjoyable and stimulating.

 

Aba Losi

University of Parma, Italy, http://www.unipr.it

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