Role of the Arabidopsis DDB Protein in photomorphogenesis and regulated protein degradation

Elizabeth Strickland and Xing Wang Deng

Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, CT

The dramatic developmental transition that occurs when a seedling encounters light is called photomorphogenesis. Genetic screens have identified four biochemical entities that are critical for this process in Arabidopsis thaliana. These are COP1 (for constitutively photomorphogenic), COP10, the COP9 signalosome (CSN), and DET1 (for de-etiolated). COP1, COP10, and the CSN all act in the ubiquitin-proteasome pathway of regulated protein degradation. COP1 is an E3 ligase, while COP10 is similar, at least in amino acid sequence, to E2 ubiquitin conjugase variant proteins. The CSN, in part, regulates the activity of cullin-based E3 ligases by controlling the level of RUB1 modification of cullin molecules. These protein degradation-related activities of COP1, COP10, and the CSN result in changes in gene expression by controlling the abundance of key transcription factors. DET1, on the other hand, is hypothesized to be involved in chromatin regulation through its interaction with histones.

Interestingly, two groups have now reported that an unexpected protein -- DDB (for damaged DNA binding) -- interacts with several key players in photomorphogenesis. First isolated in a mammalian system, DDB is a heterodimer composed of DDB1 and DDB2. DDB1 shuttles between the cytoplasm and the nucleus, while DDB2 is constitutively nuclear. The evidence that DDB functions in the same pathway as genes that control photomorphogenesis is the following. First, DDB1 binds to DET1 (Shroeder, et al,. Curr. Biol. 2002). Second, in a mammalian system, a ubiquitin ligase activity attributed to the DDB1-DDB2 complex is regulated by the CSN (Groisman, et al., Cell 2003).

I am utilizing a combination of genetic and biochemical techniques to address the role of DDB in photomorphogenesis and in regulated protein degradation. Arabidopsis contains two copies of the DDB1 gene, DDB1a and DDB1b. While T-DNA knock-out lines of DDB1a have no phenotype, a lack of DDB1b is lethal and results in defective embryogenesis. The expectation is that these studies will yield insight into the role of the DDB protein in the life of the Arabidopsis plant as well as the more general question of how DDB functions in regulated protein degradation in eukaryotic cells.