Minisymposium 7: Photomorphogenesis

Abs # 17002: Functional interactions of COP1 and SPA1 in repressing photomorphogenic developement of Arabidopsis

Presenter:	Saijo, Yusuke , yusuke.saijo@yale.edu
Authors	Saijo, Yusuke  (A)   Wang, Haiyang  (A) (B)  Sullivan, James A (A)   Ma, Ligeng  (A)   Rubio, Vicente  (A)   Shen, Yunping  (A)   Deng, Xing Wang  (A)
Affiliations:	(A): Dept of Molecular, Cellular and Developmental Biology, Yale University (B): Boyce Thompson Institute for Plant Research, Cornell University

Arabidopsis COP1 is a negative regulator of photomorphogenesis, and its activity is high in the dark but reduced by light. The WD40 domain of COP1 is responsible for the direct interaction with photomorphogenesis-promoting factors such as HY5 (a bZIP transcription factor) in the nucleus, and their subsequent proteasome-mediated degradation. The presence of the RING finger domain suggests that COP1 acts as a ubiqutin E3 ligase, however, the biochemical mode of action of COP1 still remains unclear. SPA1, a nuclear repressor of phytochrome A (phyA) signaling, has a WD40 domain highly homologous to that of COP1 and physically interacts with COP1 in vitro, suggesting the possibility that these two proteins work together in targeted destabilization of the substrate proteins. Here we show by gel filtration analyses of plant extracts that COP1 exists as a part of an approximately 700 kDa protein complex that is clearly different from the COP9 signalosome, a photomorphogenic-repressor protein complex. SPA1 interacts with HY5 in yeast two-hybrid assays. HY5 protein accumulates to high levels without significant increase in mRNA abundance in spa1-3 mutant seedlings under the continuous far-red light (cFR). In addition, genetic studies indicate a closer functional relationship than simply additive one between COP1 and SPA1. The spa1-3 mutation enhances the weak cop1-6 mutant phenotype under cFR as well as dark conditions, where no obvious phenotype is observed in dark-grown spa1-3 mutant plants. The genomic expression profiles obtained by microarray analyses focusing on the far-red light regulated genes are also consistent with these genetic observations. To further investigate the protein function of SPA1, we generated transgenic plants by introducing a fusion of the spa1 gene with the modified TAP (Tandem Affinity Purification) tag. This construct complemented the spa1-3 mutation. Using the TAP-tagged SPA1 transgenic plants, we found that SPA1 interacts with COP1 in vivo. Taken together, our data supports the hypothesis that SPA1 acts in concert with COP1 in repressing the phyA-mediated photomorphogenic responses, by helping COP1 to capture the substrates and/or modulating the proposed E3 ligase activity of COP1.