C-terminal region of the UV-B photoreceptor UVR8 initiates signaling through interaction with the COP1 protein

Abstract

UV-B light initiates photomorphogenic responses in plants. Arabidopsis UV RESISTANCE LOCUS8 (UVR8) specifically mediates these responses by functioning as a UV-B photoreceptor. UV-B exposure converts UVR8 from a dimer to a monomer, stimulates the rapid accumulation of UVR8 in the nucleus, where it binds to chromatin, and induces interaction of UVR8 with CONSTITUTIVELY PHOTO-MORPHOGENIC1 (COP1), which functions with UVR8 to control photomorphogenic UV-B responses. Although the crystal structure of UVR8 reveals the basis of photoreception, it does not show how UVR8 initiates signaling through interaction with COP1. Here we report that a region of 27 amino acids from the C terminus of UVR8 (C27) mediates the interaction with COP1. The C27 region is necessary for UVR8 function in the regulation of gene expression and hypocotyl growth suppression in Arabidopsis. However, UVR8 lacking C27 still undergoes UV-B-induced monomerization in both yeast and plant protein extracts, accumulates in the nucleus in response to UV-B, and interacts with chromatin at the UVR8-regulated ELONGATED HYPOCOTYL5 (HY5) gene. The UV-B-dependent interaction of UVR8 and COP1 is reproduced in yeast cells and we show that C27 is both necessary and sufficient for the interaction of UVR8 with the WD40 domain of COP1. Furthermore, we show that C27 interacts in yeast with the REPRESSOR OF UV-B PHOTOMORPHO-GENESIS proteins, RUP1 and RUP2, which are negative regulators of UVR8 function. Hence the C27 region has a key role in UVR8 function.U V-B wavelengths (280-315 nm) are a minor component of sunlight but have a major impact on living organisms. The damaging effects of UV-B are well documented, but plants rarely show signs of UV-damage despite constant exposure to sunlight. This is because plants have evolved effective means of protection against UV-B, including the deposition of UV-absorbing phenolic compounds in the outer tissues and the production of efficient antioxidant and DNA repair systems (1-4). These UV-protective mechanisms are stimulated by low doses of UV-B through differential gene expression. Moreover, low levels of UV-B regulate other responses in plants, including the suppression of hypocotyl extension (5). Thus, in plants, UV-B acts as a key regulatory signal that initiates photomorphogenic responses and promotes survival.The low dose, photomorphogenic responses to UV-B are mediated by the photoreceptor UVR8 (3-7). UVR8 acts specifically in UV-B to regulate over 100 genes, many of which are involved in UV protection (5, 6). Arabidopsis uvr8 mutant plants are highly sensitive to UV-B because they fail to express UVprotective genes (6,8). Among the genes regulated by UVR8 is that encoding the ELONGATED HYPOCOTYL 5 (HY5) transcription factor, which mediates most, if not all, gene expression responses initiated by UVR8 (6, 7). UVR8 interacts with chromatin via histones, in particular H2B (9) at the HY5 gene (6, 9) and a number of other UVR8-regulated genes (9), which raises the possibility that UVR8 promotes recruitment or activation of...

UV-B Perception and Acclimation in Chlamydomonas reinhardtii

“…In a yeast two-hybrid assay, Cr-UVR8 interacted with Cr-COP1 and At-COP1 specifically under UV-B ( Figure 2B), mimicking the UV-B-dependent interaction previously observed between At-UVR8 and At-COP1 (Rizzini et al, 2011;Cloix et al, 2012;O'Hara and Jenkins, 2012;Huang et al, 2014;Yin et al, 2015). This suggests that a UVR8 photocycle exists in Chlamydomonas ( Figure 2C) that is very similar to what has been demonstrated in Arabidopsis (Tilbrook et al, 2013).…”

“…Briefly, the UVR8 homodimer monomerizes upon UV-B exposure and interacts with CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) (Oravecz et al, 2006;Favory et al, 2009;Rizzini et al, 2011). Two separate domains of UVR8 participate in the interaction with COP1: The b-propeller core of UVR8 mediates UV-Bdependent interaction with COP1 and the UVR8 C-terminal C27 domain further stabilizes the interaction and regulates COP1 activity (Cloix et al, 2012;Yin et al, 2015). UVR8-COP1 interaction induces UV-B signaling governed by the bZIP transcription factor ELONGATED HYPOCOTYL5 (HY5) and its homolog HYH (Ulm et al, 2004;Brown and Jenkins, 2008;Stracke et al, 2010;Binkert et al, 2014), which leads to UV-B acclimation (Brown et al, 2005;Oravecz et al, 2006;Favory et al, 2009).…”

Reversion of the Arabidopsis UV-B photoreceptor UVR8 to the homodimeric ground state

“…The RUP1 and RUP2 mechanism of action may be by preventing UVR8 monomerization and/or facilitating UVR8 redimerization post UV-B exposure. Consistent with both possibilities and in contrast to the UV-B-specific UVR8-COP1 interaction, RUP1 and RUP2 can interact with UVR8 under conditions with and without UV-B and thus with both UVR8 monomers and homodimers (22,24). To begin with, we tested whether RUP1 and RUP2 influence the UVR8 dimer-monomer ratio upon UV-B exposure, which would imply that RUP1 and RUP2 act upstream of the UVR8-COP1 interaction.…”

UV-B-Responsive Association of the Arabidopsis bZIP Transcription Factor ELONGATED HYPOCOTYL5 with Target Genes, Including Its Own Promoter

“…Photoactivated UVR8 then interacts with the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) (Favory et al, 2009;Rizzini et al, 2011;Cloix et al, 2012;Huang et al, 2014), which is a well-known repressor of photomorphogenesis and also plays an important role in UV-B signaling (Oravecz et al, 2006). As part of the UVR8 photocycle, regeneration of reactive UVR8 occurs by rapid reversion from the monomer to the dimer Heilmann and Jenkins, 2013).…”