How bristle worms use a protein to distinguish between sunlight and moonlight

In close cooperation with scientists from the University of Mainz and the University of Oldenburg, researchers from the Institute of Biochemistry at the University of Cologne have discovered how a special protein of the marine bristle worm Platynereis dumerilii allows it to distinguish between the day-night cycle and the moon phase. So-called Cryptochrome proteins (CRYs) are found in a variety of organisms and are often involved in light-controlled biological processes. However, in the special protein of the bristle worm, L-Cry, the reaction to incident light is very unusual. The study was published in the journal Nature Communications under the title ‘A marine cryptochrome with an inverse photo-oligomerization mechanism”.

In their work, the researchers used the cryo-electron microscopy platform of the Department of Chemistry and Biochemistry, which was commissioned only in 2021, to make L-Cry visible under different light conditions. The investigation revealed that L-Cry disassembles into its two subunits (monomers) under intense, sunlight-like illumination. In the dark, however, L-Cry forms a stable connection of both subunits (dimer).

It is not only the spatial arrangement of the two subunits in the dark that is unusual and corresponds to an arrangement not previously observed in other Cry proteins. The direction of separation is also unusual, since for other Cry proteins only the reverse process has been described: from monomer arrangements in the dark to dimer or higher oligomer arrangements in the light. In their study, the University of Cologne (UoC) scientists were able to further identify the structural features in the protein that are important for this unusual behaviour. These could explain how L-Cry manages to distinguish between sunlight and moonlight: Intense sunlight always activates both subunits in the protein simultaneously, which initiates the breakdown into the individual subunits. In the significantly weaker moonlight, on the other hand, only one subunit is statistically activated at a time, so that this subunit can pass on the light signal to the cell.

Cry proteins are highly diverse, and among other functions are also thought to be sensor proteins in the perception of the earth's magnetic field in birds.  The results of the study underline the uniqueness of L-Cry. “Working with sensor proteins is always a challenge. L-Cry has proven to be incredibly sensitive to light in the laboratory, so that we could only carry out large parts of the experimental work under defined far-red light illumination in order not to inadvertently activate the protein in advance,” said Dr Heide Behrmann, one of the first authors of the study. “This showed the enormous specialization of L-Cry, which enables the protein to react robustly even to dim moonlight.”

The publication not only provides new insights into the unusual sensor protein. Professor Dr Elmar Behrmann from the Institute of Biochemistry, who led the study on the UoC team, adds: “Our work shows how flexibly nature adapts existing building blocks to develop new, fascinating mechanisms. In addition, our results are a strong proof of the possibilities of cryo-electron microscopy in the UoC: Without direct access to a high-end electron microscope in the UoC, we would never have obtained our results so quickly.”