Light Regulates COP1-Mediated Degradation of HFR1, a Transcription Factor Essential for Light Signaling in Arabidopsis

Abstract

Arabidopsis thaliana seedlings undergo photomorphogenesis in the light and etiolation in the dark. Long Hypocotyl in FarRed 1 (HFR1), a basic helix-loop-helix transcription factor, is required for both phytochrome A-mediated far-red and cryptochrome 1-mediated blue light signaling. Here, we report that HFR1 is a short-lived protein in darkness and is degraded through a 26S proteasome-dependent pathway. Light, irrespective of its quality, enhances HFR1 protein accumulation via promoting its stabilization. We demonstrate that HFR1 physically interacts with Constitutive Photomorphogenesis 1 (COP1) and that COP1 exhibits ubiquitin ligase activity toward HFR1 in vitro. In addition, we show that COP1 is required for degradation of HFR1 in vivo. Furthermore, plants overexpressing a C-terminal 161-amino acid fragment of HFR1 (CT161) display enhanced photomorphogenesis, suggesting an autonomous function of CT161 in promoting light signaling. This truncated HFR1 gene product is more stable than the full-length HFR1 protein in darkness, indicating that the COP1-interacting N-terminal portion of HFR1 is essential for COP1-mediated destabilization of HFR1. These results suggest that light enhances HFR1 protein accumulation by abrogating COP1-mediated degradation of HFR1, which is necessary and sufficient for promoting light signaling. Additionally, our results substantiate the E3 ligase activity of COP1 and its critical role in desensitizing light signaling.

PHYTOCHROME INTERACTING FACTOR1 Enhances the E3 Ligase Activity of CONSTITUTIVE PHOTOMORPHOGENIC1 to Synergistically Repress Photomorphogenesis in Arabidopsis

“…PIFs Promote COP1/SPA-Mediated Degradation of HY5 Posttranslationally in the Dark Previous studies have shown that COP1-SPA complexes interact with the positively acting transcription factors (e.g., HY5, HFR1, LAF1, and others) and induce their degradation through the ubiquitin/26S proteasome pathway in the dark Saijo et al, 2003;Seo et al, 2003;Jang et al, 2005;Yang et al, 2005;Lau and Deng, 2012). HY5 is a bZIP transcription factor whose abundance directly correlates with the level of photomorphogenesis .…”

“…COP1 showed both autoubiquitylation and transubiquitylation to HY5, HFR1, LAF1, and others in vitro (Saijo et al, 2003;Seo et al, 2003;Yang et al, 2005;Lau and Deng, 2012). To examine the autoubiquitylation of COP1, we performed in vitro ubiquitylation assays in the absence and presence of increasing concentrations of PIF1.…”

“…COP1 plays a central role in repressing photomorphogenesis in the dark by forming multiple complexes with SUPPRESSOR OF PHYTOCHROME A-105 (SPA1-4) family members in a tissue-and developmental stage-specific manner (Deng et al, 1992;Hoecker et al, 1999;Osterlund et al, 2000;Laubinger et al, 2004;Zhu et al, 2008;Lau and Deng, 2012). These complexes ubiquitylate and degrade positively acting transcription factors such as LONG HYPOCOTYL5 (HY5)/ LONG AFTER FAR-RED LIGHT1 (LAF1)/LONG HYPOCOTYL IN FAR-RED1 (HFR1) to repress photomorphogenesis in the dark (Hoecker et al, 1999;Saijo et al, 2003;Seo et al, 2003;Jang et al, 2005;Yang et al, 2005;Zhu et al, 2008). COP1-SPA complexes also associate with Cullin4, COP9 signalosome (CSN), and the CDD (for COP10, DDB1a, and DET1) complexes, and all three subcomplexes synergistically repress photomorphogenesis in the dark (H.D.…”

Biochemical Characterization ofArabidopsisComplexes Containing CONSTITUTIVELY PHOTOMORPHOGENIC1 and SUPPRESSOR OF PHYA Proteins in Light Control of Plant Development

“…However, COP1 also acts as a positive regulator for several specific responses in tolerance toward low levels of UV-B light and possibly in a response mediated by PHYB (Boccalandro et al, 2004;Oravecz et al, 2006). Molecularly, COP1 acts as an E3 ligase to target several photomorphogenesis-promoting transcription factors, such as ELONGATED HYPOCOTYL5 (HY5) (Osterlund et al, 2000b), LONG AFTER FAR-RED LIGHT1 (Seo et al, 2003), and LONG HYPOCOTYL IN FAR-RED1 (HFR1) (Duek et al, 2004;Jang et al, 2005;Yang et al, 2005;Lin and Wang, 2007), for degradation via the 26S proteasome. In addition, the far-red light photoreceptor phyA (Seo et al, 2004) and the blue light photoreceptor cry2 (Wang et al, 2001;Shalitin et al, 2002) are also likely targets of COP1.…”

BBX32, an Arabidopsis B-Box Protein, Functions in Light Signaling by Suppressing HY5-Regulated Gene Expression and Interacting with STH2/BBX21

“…COP1 is a RING-finger-type ubiquitin E3 ligase involved in ubiquitin-mediated proteolysis of photomorphogenesis-promoting factors, such as HY5 (Osterlund et al, 2000;Saijo et al, 2003), HYH (Holm et al, 2002), a MYB transcription factor, LONG AFTER FAR-RED LIGHT1 (Seo et al, 2003), and a bHLH transcription factor, LONG HY-POCOTYL IN FAR-RED1 (Duek et al, 2004;Jang et al, 2005;Yang et al, 2005). Other proteins have been identified that physically interact with COP1, including COP1-INTERACTING PROTEINs (Matsui et al, 1995;Yamamoto et al, 1998;Yamamoto et al, 2001), and several proteins with a zinc-binding B-box motif, namely CONSTANS (CO; Liu et al, 2008), CO-LIKE3 (COL3;Datta et al, 2006), SALT TOLERANCE (STO), and STO HOMOLOG1 (STH1; Holm et al, 2001).…”