Skip to main content

UV-B perception and signalling by the UVR8 photoreceptor

Final Report Summary - UV-B PERCEPTION (UV-B perception and signalling by the UVR8 photoreceptor)

Plants are sessile photoautotrophic organisms, and as such dependent on sunlight. Part of the sunlight reaching the Earth’s surface includes potentially damaging ultraviolet-B radiation (UV-B), notorious for causing sunburns in humans. Plants are, however, rather well protected and usually not sunburned. This can be appreciated during a hike in summer through green fields. The plant-specific UV-B photoreceptor UVR8 perceives UV-B photons and initiates acclimatory responses resulting in high levels of UV-B protection contributing to survival. The UVR8 photoreceptor is unique in its ability to perceive UV-B and in its photochemistry. Homodimeric UVR8 monomerizes in response to UV-B photon absorption through intrinsic tryptophan aromatic amino acids. Active UVR8 monomers interact with the E3 ubiquitin ligase COP1, a key player in UV-B signaling. Ground state reversion of UVR8 is mediated through its redimerization, which is facilitated by RUP1 and RUP2 proteins. In frame of the ERC project, we were investigating the unique UVR8 signaling pathway, including structure-function relationship of the key players in the model flowering plant Arabidopsis thaliana, and its evolutionary conservation in the single-cell algae Chlamydomonas reinhardtii.
Our work provided a mechanistic and functional understanding of the UVR8 photocycle, including the UV-B-dependent interaction of UVR8 with COP1 and the negative feed-back regulation by RUP1 and RUP2, as well as its physiological function. UVR8 contains a C-terminal domain that mimics the interaction domains of COP1 substrates and that is released from structural restraints that prevent its interaction with COP1 in the absence of UV-B. Interestingly, the same UVR8 C-terminal domain is also mediating the interaction with the negative regulatory proteins RUP1 and RUP2, indicating a regulatory network of competing interactions.
Although UVR8-like sequences were previously identified in all sequenced plant genomes, understanding of UV-B photoreceptor-mediated signaling has been largely based on the Arabidopsis UVR8 pathway. Within this project, we could demonstrate the functional conservation of the UVR8 pathway in the single-cell green algae Chlamydomonas reinhardtii, indicating evolutionary conservation of UV-B photoreceptor signaling throughout the green lineage. Finally, this work led to the discovery that UVR8 regulates a key chloroplastic photoprotection pathway (non-photochemical quenching) in Chlamydomonas, important in establishing photoprotection of the photosynthetic machinery.