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

Abstract

Plants require the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8) for acclimation and survival in sunlight. Upon UV-B perception, UVR8 switches instantaneously from a homodimeric to monomeric configuration, which leads to interaction with the key signaling protein CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and induction of UV-B-protective responses. Here, we show that UVR8 monomerization is reversible in vivo, restoring the homodimeric ground state. We also demonstrate that the UVR8-interacting proteins REPRESSOR OF UV-B PHOTOMORPHOGENESIS (RUP)1 and RUP2 mediate UVR8 redimerization independently of COP1. UVR8 redimerization consequently disrupts the UVR8-COP1 interaction, which halts signaling. Our results identify a key role of RUP1-and RUP2-mediated UVR8 redimerization in photoreceptor inactivation, a crucial process that regenerates reactivatable UVR8 homodimers.light signaling | photobiology | signal transduction U V-B radiation (UV-B; 280-315 nm) is an integral part of sunlight with a strong impact on terrestrial ecosystems (1-3). In plants, UV-B perception is necessary for UV-B acclimation and UV-B stress tolerance (4-6). Specific UV-B perception is facilitated by the photoreceptor UV RESISTANCE LOCUS 8 (UVR8) identified only recently in Arabidopsis (7). In agreement with its photoreceptor function, uvr8 null mutants show a strongly reduced response to UV-B (8-11), which even is absent under conditions specifically activating UV-B photoreceptor responses (4). In contrast, UV-B stress responses are not affected per se in uvr8 mutants (12).Upon UV-B irradiation, UVR8 homodimers monomerize instantaneously to active monomers (7). The UVR8 monomer then interacts with the WD40-repeat domain of the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) (4), a central regulator of light-dependent plant photomorphogenesis and also of utmost importance in UV-B signaling (13,14). COP1-UVR8 interaction is an early event in the UV-B perception and signaling pathway and essential for UV-B-dependent photomorphogenesis and acclimation (4). One of the main molecular outcomes of this interaction is an increase in protein level of the bZIP transcription factor ELONGATED HYPOCOTYL 5 (HY5), which may be the result of reduced HY5 ubiquitination by COP1 (4). HY5 together with its homolog HYH induce expression of the majority but not all genes included in the UVR8-dependent UV-B response (15-18). In a negative feedback loop, the light-regulated SALT TOLERANCE/B-BOX DOMAIN PROTEIN 24 (STO/BBX24) was shown to fine-tune the UV-B response by impinging on HY5 (19).UVR8 is a seven-bladed β-propeller protein that makes use of tryptophan residues intrinsic to the protein as chromophores for UV-B absorption, with a primary role established for tryptophan-285 (7,20,21). In agreement with the major role that Trp-285 plays in UV-B-mediated monomerization of UVR8 (7), it was proposed that UV-B absorption by specific tryptophans, namely Trp-285 and Trp-233, leads to disruption of cross-dimer salt bridges involving crucial arginins (20,21...

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

“…The WD40-repeat protein RUP2 acts as repressor of the UV-B response by facilitating UVR8 redimerization and thus ground state reversion Heijde and Ulm, 2013). In agreement with this activity, RUP2 overexpression resulted in a reduced UV-B-specific response , and this was associated with the absence of UV-B-induced enhancement of HY5 chromatin association at target genes ( Figure 2B).…”

“…In agreement, the UV-Bresponsive increased binding is also affected by overexpression of the negative regulatory RUP2 gene. In RUP2 overexpression lines, UVR8 remains in its inactive homodimeric form, which blocks signaling Heijde and Ulm, 2013). Thus, enhanced chromatin association of HY5 is a response closely associated with UVR8-mediated gene expression changes.…”

“…For example, HY5 binds and regulates COP1 as well as RUP1 and RUP2. This transcriptional modulation appears to increase the abundance of active UVR8-COP1-HY5 core pathway components but, in parallel, also to induce the negative feedback regulators RUP1 and RUP2 (Oravecz et al, 2006;Favory et al, 2009;Gruber et al, 2010;Heijde and Ulm, 2013). The induction of COP1 by UV-B was shown to be modulated by combinatorial regulation of FHY3 and HY5 .…”

Ultraviolet-B-mediated induction of protein–protein interactions in mammalian cells

“…However, in Arabidopsis we have recently demonstrated that the RUP1 and RUP2 proteins disrupt the interaction between UVR8 and COP1 (ref. 19), raising the possibility that in the future the optogenetic system described here may be adapted for reversible protein-protein interactions. In certain settings, the irreversible nature of the UVR8-COP1 interaction may be an advantage, as continuous exposure to ultraviolet-B light is not required to maintain the interaction.…”

Grapevine Biotechnology: Molecular Approaches Underlying Abiotic and Biotic Stress Responses

“…Other components of the UVR8 signalling pathway are repressor of UV-B photomorphogenesis 1 (RUP1) and RUP2. Both RUP1 and RUP2 act as feedback inhibitors of UVR8 signalling by facilitating UVR8 redimerization after exposure to UV-B and thus preserve responsiveness to changing levels of the input signal [149,150].…”