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Organization of protein complexes under photomorphogenic UV-B in Arabidopsis a
Xi Huang & Xing Wang Deng
a
a
Peking-Yale Joint Center for Plant Molecular Genetics and Agro-Biotechnology; National Laboratory of Protein Engineering and Plant Genetic Engineering; College of Life Sciences; Peking University; Beijing, PR China Published online: 04 Dec 2013.
To cite this article: Xi Huang & Xing Wang Deng (2013) Organization of protein complexes under photomorphogenic UV-B in Arabidopsis, Plant Signaling & Behavior, 8:11, e27206, DOI: 10.4161/psb.27206 To link to this article: http://dx.doi.org/10.4161/psb.27206
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Plant Signaling & Behavior 8, e27206; November; © 2013 Landes Bioscience
Organization of protein complexes under photomorphogenic UV-B in Arabidopsis Xi Huang* and Xing Wang Deng
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Peking-Yale Joint Center for Plant Molecular Genetics and Agro-Biotechnology; National Laboratory of Protein Engineering and Plant Genetic Engineering; College of Life Sciences; Peking University; Beijing, PR China
L
Keywords: UV-B, protein complex, light signaling *Correspondence to: Xi Huang; Email: [email protected] Submitted: 11/06/2013 Accepted: 11/14/2013 Citation: Huang X, Deng XW. Organization of protein complexes under photomorphogenic UV-B in Arabidopsis. Plant Signaling & Behavior 2013; 8:e27206; PMID: 24304604; http://dx.doi.org/10.4161/psb.27206 Huang X, Ouyang X, Yang P, Lau OS, Chen L, Wei N, Deng XW. Conversion from CUL4-based COP1-SPA E3 apparatus to UVR8-COP1-SPA complexes underlies a distinct biochemical function of COP1 under UV-B. Proc Natl Acad Sci U S A 2013; 110:16669-74; http://www.pnas.org/content/110/41/16669.long; PMID:24067658; http://dx.doi.org/10.1073/pnas.1316622110
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ow-fluence and long-wavelength UV-B light promotes photomorphogenic development in Arabidopsis. CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is a positive regulator in this pathway while it is a negative regulator of the traditional photomorphogenesis triggered by far-red and visible light. We have recently reported the mechanism by which the switch of COP1 function is accomplished in distinct light contexts. In response to photomorphogenic UV-B, the photoactivated UV RESISTANCE LOCUS 8 (UVR8) associates with the COP1 – SUPRESSOR OF PHYA (SPA) core complexes, resulting in the physical and functional disassociation of COP1SPA from the CULLIN4-DAMAGED DNA BINDING PROTEIN 1 (CUL4DDB1) E3 scaffold. These UV-B dependent UVR8-COP1-SPA complexes promote the stability and activity of ELONGATED HYPOCOTYL 5 (HY5), and eventually cause COP1 to switch from repressing to promoting photomorphogenesis. In addition, it is possible that CUL4-DDB1 might simultaneously recruit alternative DDB1 BINDING WD40 (DWD) proteins to repress this UV-B-specific signaling. Further investigation is required, however, to verify this hypothesis. Overall, the coordinated organization of various protein complexes facilitates an efficient and balanced UV-B signaling. To cope with the changing light environment, higher plants have evolved the capacity to sense and interpret diverse light signals, in order to optimize their
growth. UV-B light (280–315 nm) comprises a minor proportion of sunlight that reaches the earth surface, and is processed by plants not only as a damaging stimulus, but also as an informational signal. Specifically, high-fluence and short-wavelength UV-B light induces stress-related physiological responses, while low-fluence and long-wavelength UV-B light promotes photomorphogenic development.1 UV RESISTANCE LOCUS 8 (UVR8) is a UV-B light receptor that has been recently identified in Arabidopsis thaliana. Upon irradiation with photomorphogenic UV-B, the dark-state dimeric UVR8 perceives UV-B light via its own tryptophan residues, which acts as internal chromophores, and monomerizes by disrupting the critical intermolecular bonds shaped by its own arginine residues.2,3 The litstate monomeric UVR8 then rapidly interacts with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) in order to trigger downstream signaling.4,5 COP1 is an evolutionally conserved and multifunctional RING E3 ubiquitin ligase. It was originally identified by screening Arabidopsis seedlings that exhibit constitutive photomorphogenesis in darkness.6 Molecular and biochemical studies have revealed that COP1 functions in COP1-SUPRESSOR OF PHYA (SPA) complexes.7 They serve as substrate receptors for the CULLIN4 – DAMAGED DNA BINDING PROTEIN 1 (CUL4-DDB1) E3 apparatus, to target photomorphogenesis promoting transcription factors including ELONGATED HYPOCOTYL 5 (HY5)
Plant Signaling & Behavior e27206-1
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Figure 1. A model for the organization of protein complexes under photomorphogenic UV-B. Upon UV-B irradiation, UVR8 switches from dimer to monomer, and directly interacts with COP1 to form UVR8-COP1-SPA complexes, which promote the stability and activity of HY5. HY5 positively regulates downstream transcriptional responses so as to facilitate UV-B-induced photomorphogenesis and acclimation. On the other hand, the CUL4-DDB1 E3 apparatus, which releases the COP1-SPA core complexes sequestered by UVR8 monomers, might recruit alternative DWD proteins such as RUP1/RUP2 to repress UV-B-induced photomorphogenesis. Once the UV-B irradiation is removed, RUP1 and RUP2 mediate the redimerization of UVR8. Solid lines indicate direct effects, while dotted lines represent indirect regulation. Question marks denote the hypothesis, which awaits further investigation.
for the ubiquitin-proteasome system (UPS)-mediated degradation.8 Thus, COP1 is a central repressor of the traditional photomorphogenesis triggered by far-red and visible light.9 Intriguingly, COP1 has also been shown to be a positive regulator of UV-Binduced photomorphogenesis,4,10 and to no longer act antagonistically against HY5 under UV-B.11,12 The loss-of-function mutation of either COP1 or HY5 suffers from abolished transcriptomic responses to photomorphogenic UV-B, and consequently results in defective UV-B-induced photomorphogenesis and UV-B acclimation.10 In addition to these discoveries, several additional observations listed below prompted us to investigate the mechanism of COP1’s role in this UV-B specific signaling, particularly with regard to the changes induced in the biochemical relevance between COP1 and HY5. Both COP1 and HY5 show nuclear localization under UV-B.10 COP1 is required for
2
the UV-B-mediated HY5 induction,4 and HY5 activates COP1 in a positive feedback by targeting the COP1 promoter.12 We found that under photomorphogenic UV-B, monomerized UVR8 sequestered COP1 from DDB1, which in turn caused COP1-SPA to disassociate from CUL4-DDB1. In agreement with this physical segregation, COP1SPA also displayed functional disassociation from CUL4-DDB1, and CUL4 was identified as a repressor of UV-B-induced photomorphogenesis.11 In contrast, we showed that COP1 and SPA proteins remained tightly associated with one another regardless of the presence or absence of UV-B. Under photomorphogenic UV-B, UVR8 was observed to associate with the COP1-SPA core to constitute novel complexes. We then verified that SPA proteins, like COP1 and UVR8, played positive regulatory roles in UV-B-induced photomorphogenesis. More importantly, we demonstrated that
Plant Signaling & Behavior
the UV-B-dependent UVR8-COP1-SPA complexes promoted the stability and activity of HY5, which enabled a functional switch of COP1 from repressing to promoting photomorphogenesis. In YFPUVR8W285A transgenic plants, the constitutive UVR8-COP1-SPA complexes were found to continuously facilitate HY5 stability and activity, and to eventually drive the Arabidopsis seedlings to develop constitutive photomorphogenesis in darkness.11 However, how HY5 is stabilized is a subject that awaits further investigation. Our results have indicated that an alternative E3 ubiquitin ligase might be responsible for the turnover of HY5 protein, and UVR8-COP1-SPA complexes might deactivate this E3 ligase. Also, it has been pointed out that the stability and activity of HY5 is modulated by its phosphorylation.13 Thus a close examination of the phosphorylation or/and other modifications of HY5 might reveal that how HY5 is regulated post-transcriptionally by photomorphogenic UV-B. As it does under other light conditions, CUL4 represses photomorphogenesis under UV-B.11,14 Nevertheless, it remains unknown, regarding the mechanism by which CUL4 plays a repressive role in UV-B-induced photomorphogenesis. It has been extensively studied that DDB1 is an adaptor protein that recruits a group of proteins named DDB1-CUL4 ASSOCIATED FACTOR (DCAF) as substrate receptors for CUL4-based E3 ligase in both animals and plants.15-19 In Arabidopsis, DCAFs, also designated as DWD proteins, are functionally involved in a wide variety of developmental processes.19 Therefore, it is reasonable that one or more unknown DWD proteins might join the CUL4-DDB1 E3 scaffold to repress UV-B-induced photomorphogenesis. Two previously identified negative regulators in this pathway, REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2,20 are potential DWD proteins as each of them possesses one typical WDXR motif that might mediate their direct interaction with DDB1. Furthermore, multiple lines of evidence indicate that CUL4-DDB1 and RUP1/ RUP2 might function in concert. For example, a reduction in CUL4 or RUP1/
Volume 8 Issue 11
RUP2 function was observed to result in hypersensitivity to photomorphogenic UV-B in Arabidopsis seedlings, which were found to display enhanced induction of UV-B-responsive genes and increased accumulation of HY5 protein, the known UV-B signaling hub.11,20 Thus, further analyses of the genetic and biochemical interaction between CUL4 and RUP1/ RUP2 might assist substantiating this hypothesis. Since RUP1 and RUP2 are known to facilitate the conformational reversion of UVR8,21,22 it is also of interest and significance to investigate whether or not CUL4 participates in this process.
Taken together, we propose a model for the organization of protein complexes under photomorphogenic UV-B (Fig. 1). Following UVR8-mediated UV-B light perception, monomerized UVR8 directly interacts with COP1 so as to form UVR8-COP1-SPA complexes, which, in turn, promote the stability and activity of HY5. HY5 is then critically involved in downstream transcriptional regulation. On the other hand, when sequestered by UVR8 monomers, COP1SPA core complexes disassociate from the CUL4-DDB1 E3 apparatus which might then recruit alternative DWD proteins to
repress UV-B-induced photomorphogenesis. Overall, these complexes are structurally and functionally coordinated to ensure an efficient and balanced photomorphogenic UV-B signaling.
References
9. Lau OS, Deng XW. The photomorphogenic repressors COP1 and DET1: 20 years later. Trends Plant Sci 2012; 17:584-93; PMID:22705257; http://dx.doi. org/10.1016/j.tplants.2012.05.004 10. Oravecz A, Baumann A, Máté Z, Brzezinska A, Molinier J, Oakeley EJ, Adám E, Schäfer E, Nagy F, Ulm R. CONSTITUTIVELY PHOTOMORPHOGENIC1 is required for the UV-B response in Arabidopsis. Plant Cell 2006; 18:1975-90; PMID:16829591; http:// dx.doi.org/10.1105/tpc.105.040097 11. Huang X, Ouyang X, Yang P, Lau OS, Chen L, Wei N, Deng XW. Conversion from CUL4-based COP1-SPA E3 apparatus to UVR8-COP1-SPA complexes underlies a distinct biochemical function of COP1 under UV-B. Proc Natl Acad Sci U S A 2013; 110:1666974; PMID:24067658; http://dx.doi.org/10.1073/ pnas.1316622110 12. Huang X, Ouyang X, Yang P, Lau OS, Li G, Li J, Chen H, Deng XW. Arabidopsis FHY3 and HY5 positively mediate induction of COP1 transcription in response to photomorphogenic UV-B light. Plant Cell 2012; 24:4590-606; PMID:23150635; http://dx.doi. org/10.1105/tpc.112.103994 13. Hardtke CS, Gohda K, Osterlund MT, Oyama T, Okada K, Deng XW. HY5 stability and activity in arabidopsis is regulated by phosphorylation in its COP1 binding domain. EMBO J 2000; 19:49975006; PMID:10990463; http://dx.doi.org/10.1093/ emboj/19.18.4997 14. Chen H, Shen Y, Tang X, Yu L, Wang J, Guo L, Zhang Y, Zhang H, Feng S, Strickland E, et al. Arabidopsis CULLIN4 Forms an E3 Ubiquitin Ligase with RBX1 and the CDD Complex in Mediating Light Control of Development. Plant Cell 2006; 18:19912004; PMID:16844902; http://dx.doi.org/10.1105/ tpc.106.043224 15. He YJ, McCall CM, Hu J, Zeng Y, Xiong Y. DDB1 functions as a linker to recruit receptor WD40 proteins to CUL4-ROC1 ubiquitin ligases. Genes Dev 2006; 20:2949-54; PMID:17079684; http://dx.doi. org/10.1101/gad.1483206 16. Angers S, Li T, Yi X, MacCoss MJ, Moon RT, Zheng N. Molecular architecture and assembly of the DDB1CUL4A ubiquitin ligase machinery. Nature 2006; 443:590-3; PMID:16964240
17. Jin J, Arias EE, Chen J, Harper JW, Walter JC. A family of diverse Cul4-Ddb1-interacting proteins includes Cdt2, which is required for S phase destruction of the replication factor Cdt1. Mol Cell 2006; 23:709-21; PMID:16949367; http://dx.doi. org/10.1016/j.molcel.2006.08.010 18. Higa LA, Banks D, Wu M, Kobayashi R, Sun H, Zhang H. L2DTL/CDT2 interacts with the CUL4/ DDB1 complex and PCNA and regulates CDT1 proteolysis in response to DNA damage. Cell Cycle 2006; 5:1675-80; PMID:16861906; http://dx.doi. org/10.4161/cc.5.15.3149 19. Lee JH, Terzaghi W, Gusmaroli G, Charron JB, Yoon HJ, Chen H, He YJ, Xiong Y, Deng XW. Characterization of Arabidopsis and rice DWD proteins and their roles as substrate receptors for CUL4-RING E3 ubiquitin ligases. Plant Cell 2008; 20:152-67; PMID:18223036; http://dx.doi. org/10.1105/tpc.107.055418 20. Gruber H, Heijde M, Heller W, Albert A, Seidlitz HK, Ulm R. Negative feedback regulation of UV-Binduced photomorphogenesis and stress acclimation in Arabidopsis. Proc Natl Acad Sci U S A 2010; 107:20132-7; PMID:21041653; http://dx.doi. org/10.1073/pnas.0914532107 21. Heijde M, Ulm R. Reversion of the Arabidopsis UV-B photoreceptor UVR8 to the homodimeric ground state. Proc Natl Acad Sci U S A 2013; 110:1113-8; PMID:23277547; http://dx.doi. org/10.1073/pnas.1214237110 22. Heilmann M, Jenkins GI. Rapid reversion from monomer to dimer regenerates the ultraviolet-B photoreceptor UV RESISTANCE LOCUS8 in intact Arabidopsis plants. Plant Physiol 2013; 161:54755; PMID:23129206; http://dx.doi.org/10.1104/ pp.112.206805
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Jenkins GI. Signal transduction in responses to UV-B radiation. Annu Rev Plant Biol 2009; 60:407-31; PMID:19400728; http://dx.doi.org/10.1146/annurev. arplant.59.032607.092953 Christie JM, Arvai AS, Baxter KJ, Heilmann M, Pratt AJ, O’Hara A, Kelly SM, Hothorn M, Smith BO, Hitomi K, et al. Plant UVR8 photoreceptor senses UV-B by tryptophan-mediated disruption of cross-dimer salt bridges. Science 2012; 335:14926; PMID:22323738; http://dx.doi.org/10.1126/ science.1218091 Wu D, Hu Q, Yan Z, Chen W, Yan C, Huang X, Zhang J, Yang P, Deng H, Wang J, et al. Structural basis of ultraviolet-B perception by UVR8. Nature 2012; 484:214-9; PMID:22388820; http://dx.doi. org/10.1038/nature10931 Favory JJ, Stec A, Gruber H, Rizzini L, Oravecz A, Funk M, Albert A, Cloix C, Jenkins GI, Oakeley EJ, et al. Interaction of COP1 and UVR8 regulates UV-B-induced photomorphogenesis and stress acclimation in Arabidopsis. EMBO J 2009; 28:591601; PMID:19165148; http://dx.doi.org/10.1038/ emboj.2009.4 Rizzini L, Favory JJ, Cloix C, Faggionato D, O’Hara A, Kaiserli E, Baumeister R, Schäfer E, Nagy F, Jenkins GI, et al. Perception of UV-B by the Arabidopsis UVR8 protein. Science 2011; 332:103-6; PMID:21454788; http://dx.doi.org/10.1126/science.1200660 Yi C, Deng XW. COP1 – from plant photomorphogenesis to mammalian tumorigenesis. Trends Cell Biol 2005; 15:618-25; PMID:16198569; http://dx.doi. org/10.1016/j.tcb.2005.09.007 Zhu D, Maier A, Lee JH, Laubinger S, Saijo Y, Wang H, Qu LJ, Hoecker U, Deng XW. Biochemical characterization of Arabidopsis complexes containing CONSTITUTIVELY PHOTOMORPHOGENIC1 and SUPPRESSOR OF PHYA proteins in light control of plant development. Plant Cell 2008; 20:230723; PMID:18812498; http://dx.doi.org/10.1105/ tpc.107.056580 Chen H, Huang X, Gusmaroli G, Terzaghi W, Lau OS, Yanagawa Y, Zhang Y, Li J, Lee JH, Zhu D, et al. Arabidopsis CULLIN4-damaged DNA binding protein 1 interacts with CONSTITUTIVELY PHOTOMORPHOGENIC1-SUPPRESSOR OF PHYA complexes to regulate photomorphogenesis and flowering time. Plant Cell 2010; 22:10823; PMID:20061554; http://dx.doi.org/10.1105/ tpc.109.065490
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Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed. Acknowledgments
We thank Abigail Coplin for her critical reading and commenting on the manuscript.
Plant Signaling & Behavior e27206-3
Plant Signaling & Behavior Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/kpsb20
Organization of protein complexes under photomorphogenic UV-B in Arabidopsis a
Xi Huang & Xing Wang Deng
a
a
Peking-Yale Joint Center for Plant Molecular Genetics and Agro-Biotechnology; National Laboratory of Protein Engineering and Plant Genetic Engineering; College of Life Sciences; Peking University; Beijing, PR China Published online: 04 Dec 2013.
To cite this article: Xi Huang & Xing Wang Deng (2013) Organization of protein complexes under photomorphogenic UV-B in Arabidopsis, Plant Signaling & Behavior, 8:11, e27206, DOI: 10.4161/psb.27206 To link to this article: http://dx.doi.org/10.4161/psb.27206
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
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Article Addendum
Plant Signaling & Behavior 8, e27206; November; © 2013 Landes Bioscience
Organization of protein complexes under photomorphogenic UV-B in Arabidopsis Xi Huang* and Xing Wang Deng
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Peking-Yale Joint Center for Plant Molecular Genetics and Agro-Biotechnology; National Laboratory of Protein Engineering and Plant Genetic Engineering; College of Life Sciences; Peking University; Beijing, PR China
L
Keywords: UV-B, protein complex, light signaling *Correspondence to: Xi Huang; Email: [email protected] Submitted: 11/06/2013 Accepted: 11/14/2013 Citation: Huang X, Deng XW. Organization of protein complexes under photomorphogenic UV-B in Arabidopsis. Plant Signaling & Behavior 2013; 8:e27206; PMID: 24304604; http://dx.doi.org/10.4161/psb.27206 Huang X, Ouyang X, Yang P, Lau OS, Chen L, Wei N, Deng XW. Conversion from CUL4-based COP1-SPA E3 apparatus to UVR8-COP1-SPA complexes underlies a distinct biochemical function of COP1 under UV-B. Proc Natl Acad Sci U S A 2013; 110:16669-74; http://www.pnas.org/content/110/41/16669.long; PMID:24067658; http://dx.doi.org/10.1073/pnas.1316622110
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ow-fluence and long-wavelength UV-B light promotes photomorphogenic development in Arabidopsis. CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is a positive regulator in this pathway while it is a negative regulator of the traditional photomorphogenesis triggered by far-red and visible light. We have recently reported the mechanism by which the switch of COP1 function is accomplished in distinct light contexts. In response to photomorphogenic UV-B, the photoactivated UV RESISTANCE LOCUS 8 (UVR8) associates with the COP1 – SUPRESSOR OF PHYA (SPA) core complexes, resulting in the physical and functional disassociation of COP1SPA from the CULLIN4-DAMAGED DNA BINDING PROTEIN 1 (CUL4DDB1) E3 scaffold. These UV-B dependent UVR8-COP1-SPA complexes promote the stability and activity of ELONGATED HYPOCOTYL 5 (HY5), and eventually cause COP1 to switch from repressing to promoting photomorphogenesis. In addition, it is possible that CUL4-DDB1 might simultaneously recruit alternative DDB1 BINDING WD40 (DWD) proteins to repress this UV-B-specific signaling. Further investigation is required, however, to verify this hypothesis. Overall, the coordinated organization of various protein complexes facilitates an efficient and balanced UV-B signaling. To cope with the changing light environment, higher plants have evolved the capacity to sense and interpret diverse light signals, in order to optimize their
growth. UV-B light (280–315 nm) comprises a minor proportion of sunlight that reaches the earth surface, and is processed by plants not only as a damaging stimulus, but also as an informational signal. Specifically, high-fluence and short-wavelength UV-B light induces stress-related physiological responses, while low-fluence and long-wavelength UV-B light promotes photomorphogenic development.1 UV RESISTANCE LOCUS 8 (UVR8) is a UV-B light receptor that has been recently identified in Arabidopsis thaliana. Upon irradiation with photomorphogenic UV-B, the dark-state dimeric UVR8 perceives UV-B light via its own tryptophan residues, which acts as internal chromophores, and monomerizes by disrupting the critical intermolecular bonds shaped by its own arginine residues.2,3 The litstate monomeric UVR8 then rapidly interacts with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) in order to trigger downstream signaling.4,5 COP1 is an evolutionally conserved and multifunctional RING E3 ubiquitin ligase. It was originally identified by screening Arabidopsis seedlings that exhibit constitutive photomorphogenesis in darkness.6 Molecular and biochemical studies have revealed that COP1 functions in COP1-SUPRESSOR OF PHYA (SPA) complexes.7 They serve as substrate receptors for the CULLIN4 – DAMAGED DNA BINDING PROTEIN 1 (CUL4-DDB1) E3 apparatus, to target photomorphogenesis promoting transcription factors including ELONGATED HYPOCOTYL 5 (HY5)
Plant Signaling & Behavior e27206-1
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Figure 1. A model for the organization of protein complexes under photomorphogenic UV-B. Upon UV-B irradiation, UVR8 switches from dimer to monomer, and directly interacts with COP1 to form UVR8-COP1-SPA complexes, which promote the stability and activity of HY5. HY5 positively regulates downstream transcriptional responses so as to facilitate UV-B-induced photomorphogenesis and acclimation. On the other hand, the CUL4-DDB1 E3 apparatus, which releases the COP1-SPA core complexes sequestered by UVR8 monomers, might recruit alternative DWD proteins such as RUP1/RUP2 to repress UV-B-induced photomorphogenesis. Once the UV-B irradiation is removed, RUP1 and RUP2 mediate the redimerization of UVR8. Solid lines indicate direct effects, while dotted lines represent indirect regulation. Question marks denote the hypothesis, which awaits further investigation.
for the ubiquitin-proteasome system (UPS)-mediated degradation.8 Thus, COP1 is a central repressor of the traditional photomorphogenesis triggered by far-red and visible light.9 Intriguingly, COP1 has also been shown to be a positive regulator of UV-Binduced photomorphogenesis,4,10 and to no longer act antagonistically against HY5 under UV-B.11,12 The loss-of-function mutation of either COP1 or HY5 suffers from abolished transcriptomic responses to photomorphogenic UV-B, and consequently results in defective UV-B-induced photomorphogenesis and UV-B acclimation.10 In addition to these discoveries, several additional observations listed below prompted us to investigate the mechanism of COP1’s role in this UV-B specific signaling, particularly with regard to the changes induced in the biochemical relevance between COP1 and HY5. Both COP1 and HY5 show nuclear localization under UV-B.10 COP1 is required for
2
the UV-B-mediated HY5 induction,4 and HY5 activates COP1 in a positive feedback by targeting the COP1 promoter.12 We found that under photomorphogenic UV-B, monomerized UVR8 sequestered COP1 from DDB1, which in turn caused COP1-SPA to disassociate from CUL4-DDB1. In agreement with this physical segregation, COP1SPA also displayed functional disassociation from CUL4-DDB1, and CUL4 was identified as a repressor of UV-B-induced photomorphogenesis.11 In contrast, we showed that COP1 and SPA proteins remained tightly associated with one another regardless of the presence or absence of UV-B. Under photomorphogenic UV-B, UVR8 was observed to associate with the COP1-SPA core to constitute novel complexes. We then verified that SPA proteins, like COP1 and UVR8, played positive regulatory roles in UV-B-induced photomorphogenesis. More importantly, we demonstrated that
Plant Signaling & Behavior
the UV-B-dependent UVR8-COP1-SPA complexes promoted the stability and activity of HY5, which enabled a functional switch of COP1 from repressing to promoting photomorphogenesis. In YFPUVR8W285A transgenic plants, the constitutive UVR8-COP1-SPA complexes were found to continuously facilitate HY5 stability and activity, and to eventually drive the Arabidopsis seedlings to develop constitutive photomorphogenesis in darkness.11 However, how HY5 is stabilized is a subject that awaits further investigation. Our results have indicated that an alternative E3 ubiquitin ligase might be responsible for the turnover of HY5 protein, and UVR8-COP1-SPA complexes might deactivate this E3 ligase. Also, it has been pointed out that the stability and activity of HY5 is modulated by its phosphorylation.13 Thus a close examination of the phosphorylation or/and other modifications of HY5 might reveal that how HY5 is regulated post-transcriptionally by photomorphogenic UV-B. As it does under other light conditions, CUL4 represses photomorphogenesis under UV-B.11,14 Nevertheless, it remains unknown, regarding the mechanism by which CUL4 plays a repressive role in UV-B-induced photomorphogenesis. It has been extensively studied that DDB1 is an adaptor protein that recruits a group of proteins named DDB1-CUL4 ASSOCIATED FACTOR (DCAF) as substrate receptors for CUL4-based E3 ligase in both animals and plants.15-19 In Arabidopsis, DCAFs, also designated as DWD proteins, are functionally involved in a wide variety of developmental processes.19 Therefore, it is reasonable that one or more unknown DWD proteins might join the CUL4-DDB1 E3 scaffold to repress UV-B-induced photomorphogenesis. Two previously identified negative regulators in this pathway, REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2,20 are potential DWD proteins as each of them possesses one typical WDXR motif that might mediate their direct interaction with DDB1. Furthermore, multiple lines of evidence indicate that CUL4-DDB1 and RUP1/ RUP2 might function in concert. For example, a reduction in CUL4 or RUP1/
Volume 8 Issue 11
RUP2 function was observed to result in hypersensitivity to photomorphogenic UV-B in Arabidopsis seedlings, which were found to display enhanced induction of UV-B-responsive genes and increased accumulation of HY5 protein, the known UV-B signaling hub.11,20 Thus, further analyses of the genetic and biochemical interaction between CUL4 and RUP1/ RUP2 might assist substantiating this hypothesis. Since RUP1 and RUP2 are known to facilitate the conformational reversion of UVR8,21,22 it is also of interest and significance to investigate whether or not CUL4 participates in this process.
Taken together, we propose a model for the organization of protein complexes under photomorphogenic UV-B (Fig. 1). Following UVR8-mediated UV-B light perception, monomerized UVR8 directly interacts with COP1 so as to form UVR8-COP1-SPA complexes, which, in turn, promote the stability and activity of HY5. HY5 is then critically involved in downstream transcriptional regulation. On the other hand, when sequestered by UVR8 monomers, COP1SPA core complexes disassociate from the CUL4-DDB1 E3 apparatus which might then recruit alternative DWD proteins to
repress UV-B-induced photomorphogenesis. Overall, these complexes are structurally and functionally coordinated to ensure an efficient and balanced photomorphogenic UV-B signaling.
References
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Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed. Acknowledgments
We thank Abigail Coplin for her critical reading and commenting on the manuscript.
Plant Signaling & Behavior e27206-3
