Phytochrome-Activated Diguanylyl Cyclases: The Functional Relevance of Asymmetry

Photoreceptor proteins enable perception of light using intrinsic or bound cofactors, allowing for adaptation to changing environmental conditions. These chromophores interact with photons of different wavelengths, eventually resulting in structural rearrangements of the photoreceptor. This facilitates allosteric regulation of biomolecular interactions or enzymatic activities.
One family of such systems are the phytochrome-activated diguanylyl cyclases, short PadCs. Red light-triggered isomerisation of the biliverdin cofactor has a profound effect on the diguanylyl cyclase (DGC) activity of the effector domain. Enzymatic turnover of two molecules of GTP yields the bacterial second messenger c-di-GMP, which is formed at the active site generated at the interface of two cyclase protomers. While homodimeric under dark conditions, nine out of ten PadC homologs investigated show indications of a structural heterodimer upon red light illumination. This asymmetric activation leads to a significant increase in DGC activity, as confirmed by various PadCs examined in our group [1]. In contrast, symmetric activation and no increase in c-di-GMP production upon illumination are observed in other homologs. In an attempt to address the functional relevance of individual structural elements, a number of chimeras between symmetrically and asymmetrically activated PadCs were created. Spectral data and activity measurements showed that asymmetric phytochrome activation, while potentially advantageous for substrate binding and product formation [2], does not appear to be an absolute requirement for red light induced increase in GTP turnover in bacteriophytochromes.
Biliverdin-based bacterial photoreceptors have recently attracted interest due to the deep tissue penetration properties of red light, thus making them promising building blocks for rationally designed optogenetic systems. Better understanding of the complexity behind the mechanisms of activity and signal integration is a prerequisite for improving future application of bacteriophytochrome-based optogenetic tools.

References:
[1] Gourinchas, G., Heintz, U., Winkler, A.: Asymmetric activation mechanism of a homodimeric red light-regulated photoreceptor, eLife (2018) 7, 1-25
[2] Gourinchas, G., Vide, U., Winkler, A.: Influence of the N-terminal segment and the PHY-tongue element on light-regulation in bacteriophytochromes, JBC (2019) 294, 4498-4510