The Degradation of HFR1, a Putative bHLH Class Transcription Factor Involved in Light Signaling, Is Regulated by Phosphorylation and Requires COP1

• Published in: Current biology 2004-12, Vol.14 (24), p.2296-2301
• Main Authors: Duek, Paula D., Elmer, Mireille V., van Oosten, Vivian R., Fankhauser, Christian
• Format: Article
• Language: English
• Subjects: Agrobacterium tumefaciens; Arabidopsis; Arabidopsis - metabolism; arabidopsis cryptochromes; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; binding; Blotting, Western; branch pathway; Cryptochromes; DNA Primers; DNA, Complementary - genetics; DNA-Binding Proteins - genetics; DNA-Binding Proteins - metabolism; factor family; Flavoproteins - physiology; Genetic Vectors; hy5; Immunoprecipitation; Light; loop-helix protein; Nuclear Proteins - genetics; Nuclear Proteins - metabolism; Phosphorylation; photomorphogenesis; Phytochrome - physiology; phytochrome-a; Plants, Genetically Modified; Signal Transduction; thaliana; Transcription Factors - metabolism; Two-Hybrid System Techniques; ubiquitination
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2004.12.026

All developmental transitions throughout the life cycle of a plant are influenced by light. In Arabidopsis, multiple photoreceptors including the UV-A/blue-sensing cryptochromes (cry1-2) and the red/far-red responsive phytochromes (phyA-E) monitor the ambient light conditions [1, 2]. Light-regulated protein stability is a major control point of photomorphogenesis [3]. The ubiquitin E3 ligase COP1 (constitutively photomorphogenic 1) regulates the stability of several light-signaling components [4–6]. HFR1 (long hypocotyl in far-red light) is a putative transcription factor with a bHLH domain acting downstream of both phyA and the cryptochromes [7–9]. HFR1 is closely related to PIF1, PIF3, and PIF4 (phytochrome interacting factor 1, 3 and 4), but in contrast to the latter three, there is no evidence for a direct interaction between HFR1 and the phytochromes [7, 10–12]. Here, we show that the protein abundance of HFR1 is tightly controlled by light. HFR1 is an unstable phosphoprotein, particularly in the dark. The proteasome and COP1 are required in vivo to degrade phosphorylated HFR1. In addition, HFR1 can interact with COP1, consistent with the idea of COP1 directly mediating HFR1 degradation. We identify a domain, conserved among several bHLH class proteins involved in light signaling [13, 14], as a determinant of HFR1 stability. Our physiological experiments indicate that the control of HFR1 protein abundance is important for a normal de-etiolation response.