The following is my summary of a paper (Yin et al., 2016) recently published on Proceedings of the National Academy of Sciences. It reveals the interaction between two important proteins in UV-B radiation signaling and provides insight on the precise molecular mechanism of UV-B signal transduction.
Ultraviolet-B (UV-B) radiation is crucial in the regulation of plant photomorphogenesis and secondary metabolism. The photoreceptor for UV-B radiation in Arabidopsis thaliana is UV RESISTANCE LOCUS 8 (UVR8). Upon exposure to UV-B radiation, the inactive homodimeric UVR8 protein is converted to the biologically activemonomeric conformer. After UVR8 is activated by UV-B radiation, it can interact with the CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) protein, an E3 ubiquitin ligase, and then upregulate the proteins involved in photomorphogenesis, such as the transcription factor ELONGATED HYPOCOTYL 5 (HY5) which subsequently activates target genes. COP1 has contrasting roles in mediating photomorphogenesis in that COP1 represses photomorphogenesis under white light but facilitates the promotion of photomorphogenesis when interacting with UVR8.
The lack of information on the mechanism by which UV-B-activated UVR8 undergoes nuclear localization and the role of COP1 in this process necessitated the present study. The authors postulated that UVR8 needed to be accumulated in nucleus and interacted with COP1, which would help with signaling upon UV-B radiation and nuclear localization, in order toexert its regulatory role of downstream target genes important in photomorphogenesis.
First of all, when UV-B radiation is present, COP1 is indispensable for the nuclear localization or accumulation of UVR8. Without COP1, UVR8 in the cytosol cannot relocate to the nucleus. In the peptide sequence of UVR8, the N-terminal 23 amino acids are of most significance on the nuclear accumulation of UVR8. Without this functional fragment, UVR8 cannot interact properly with COP1.
Secondly, UVR8 needs to be in the nucleus to regulate photomorphogenic responses. The authors used GR-tagged UVR8 and found that GR-UVR8 can be activated by UV-B radiation to complement uvr8 mutants irrespective of its subcellular location. It is only in the nucleus, not the cytosol, that the monomerization of GR-UVR8 functionally induces suppression of hypocotyl elongation. The same conclusions can be drawn for UVR8.
Last but not the least, COP1 not only interacts with UVR8 for
the latter’s nuclear localization, but also acts in signaling in response to UV-B radiation. Furthermore, UVR8 activated by UV-B radiation in the cytosol can be coimported by COP1 into the nucleus through the interaction between UVR8 and COP1.
The present study tested the proposed hypothesis and concluded that COP1 interacts with UVR8 to facilitate the nuclear accumulation of UVR8 and also serves in UV-B signaling in regulation of photomorphogenesis.
Source: Yin, R., Skvortsova, M.Y., Loubéry, S. and Ulm, R., 2016. COP1 is required for UV-B–induced nuclear accumulation of the UVR8 photoreceptor. Proceedings of the National Academy of Sciences, 113(30), pp.E4415-E4422.