The UVR8 UV-B-specific signalling pathway

Lead Research Organisation: University of Glasgow
Department Name: School of Life Sciences

Abstract

UV-B wavelengths (280-320 nm) are an integral component of the daylight spectrum. UV-B induces physiological responses in plants, including changes in morphology and development and the regulation of various genes involved in secondary metabolism, chloroplast function and protection of the plant from damage by UV-B. Remarkably, very little is known about how UV-B is perceived by plants and how it initiates responses. However, the first component involved in these processes has recently been identified, a protein called UVR8. Moreover, UV-B exposure stimulates movement of UVR8 into the cell nucleus. The aim of this project is to define regions of the UVR8 protein involved in movement into the nucleus and the regulation of transcription and to identify proteins that interact with UVR8 to mediate these processes. An additional objective is to investigate how UVR8 associates with DNA in the region of a gene it regulates. Establishing how UVR8 is regulated and how it functions to control gene expresion will help us to understand how UV-B regulates aspects of plant growth and development and how plants survive in sunlight.

Technical Summary

UV-B wavelengths (280-320 nm) induce physiological responses in plants, but very little is known about the processes of UV-B perception and signal transduction. We have recently discovered that the Arabidopsis protein UVR8 is a UV-B-specific signalling component that mediates a range of gene expression responses involved in secondary metabolism, anti-oxidant defence and UV-protection and have shown that UVR8 associates with chromatin to regulate the expression of a key target gene, encoding the transcription factor HY5. In addition, we have found that UV-B stimulates the rapid nuclear translocation of UVR8. The first objectives of this project are to define sequences in the N-terminal region of UVR8 that are involved in nuclear translocation and to identify proteins that mediate this process. Second, we will define sequences in the C-terminal region of UVR8 required for function in transcriptional regulation and identify proteins that act with UVR8 in this process. Finally we will start to map sites of UVR8 interaction and UV-B-induced histone modification in the HY5 genomic region.

Publications

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Cloix C (2012) C-terminal region of the UV-B photoreceptor UVR8 initiates signaling through interaction with the COP1 protein. in Proceedings of the National Academy of Sciences of the United States of America

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Rizzini L (2011) Perception of UV-B by the Arabidopsis UVR8 protein. in Science (New York, N.Y.)

 
Description How does UV-B control UVR8 localisation? As proposed, we made several N-terminal deletions of UVR8 to test their involvement in nuclear accumulation. The fusions were transformed into Arabidopsis uvr8 mutant. In addition, we tested the involvement of the N-terminal sequences in nuclear accumulation using transient expression in Nicotiana, which shows similar UV-B-stimulated nuclear accumulation to Arabidopsis plants. The combined results from transgenic plants and transient expression indicate that a region in the N-terminus of UVR8, between amino acids 12 to 20, is required for nuclear accumulation. This is consistent with our published findings for the N-terminal 23 amino acid deletion (Kaiserli & Jenkins, 2007).

How does UVR8 function to control transcription? We showed that a 27 amino acid region towards the C-terminus of UVR8 (C27) is required for its function. The C27 region is required for interaction with COP1 in plants and UVR8 lacking this region is not functional in regulating transcription. Nevertheless, UVR8 lacking the C27 region behaves like wild-type UVR8 in all other respects, including the UV-B dependent conversion of UVR8 from the dimeric form to the monomer, nuclear accumulation following UV-B exposure and interaction with chromatin. We made 6 single amino acid mutations in the C27 region of UVR8 to test their effect on function in UV-B responses. The interaction of these UVR8 mutants with COP1 and RUP proteins was assayed in yeast. These observations were published (Cloix et al. 2012).
We discovered that UVR8 interacts with the COP1 protein in a UV-B dependent manner in yeast. We found that the interaction in yeast is specific to UV-B wavelengths and is not mediated by the yeast DNA damage signalling pathway. After we made this discovery we focused attention on defining sequences involved in the interaction and investigating whether UVR8 could function as a UV-B photoreceptor. We found that intact UVR8 is able to mediate a UV-B-dependent interaction with the WD40 domain of COP1. However, intact COP1 is not able to mediate a UV-B-dependent interaction with any modified version of UVR8. We showed that the C27 region of UVR8 is both necessary and sufficient for interaction with the WD40 region of COP1. The interaction between C27 alone and the WD40 domain of COP1 is not UV-B dependent. Together our results show that UV-B dependence of the interaction with COP1 resides with intact UVR8, indicating that it is a UV-B photoreceptor. This novel finding was published with collaborators who had independently come to the same conclusion (Rizzini et al. 2011).
Exploitation Route The findings provide a basis for further studies of the relationship between UVR8 structure and function. We are pursuing several aspects of this research. Since UV-B light has a wide ranging impact on plant development, metabolism, defence and responses to abiotic stimuli, knowledge of how UVR8 functions has potential applications in the agricultural sector. In addition, fundamental knowledge of how UVR8 protein functions has application in providing ontogenetic tools for biotechnological applications.
Sectors Agriculture, Food and Drink