Developmental Regulation and Light Signal Transduction in Plants: The Fus5 Subunit of the Cop9 Signalosome

Abstract

Plants adjust their growth and development in a manner optimal for the prevailing light conditions. The molecular mechanisms by which light signals are transduced and integrated with other environmental and developmental signals are an area of intense research. (Batschauer, 1999; Quail, 2002) One paradigm emerging from this work is the interconnectedness of discrete physiological responses at the biochemical level, for instance, between auxin and light signaling (Colon-Carmona et al., 2000; Schwechheimer and Deng, 2001; Tian and Reed, 1999) and between light signaling and plant pathogen interactions (Azevedo et al., 2002; Liu et al., 2002). The COP9 signalosome (CSN) protein complex has a central role in the light control of plant development. Arabidopsis mutants that lack this complex develop photomorphogenically even in the absence of light signals (reviewed in (Karniol and Chamovitz, 2000; Schwechheimer and Deng, 2001). Thus the CSN was hypothesized to be a master repressor of photomorphogenesis in darkness, and light acts to bypass or eliminate this repression. However, the CSN regulates more than just photomorphogenesis as all mutants lacking this complex die near the end of seedling development. Moreover, an essentially identical complex was subsequently discovered in animals and yeast, organisms whose development is not light responsive, exemplifying how plant science can lead the way to exciting discoveries in biomedical model species (Chamovitz and Deng, 1995; Freilich et al., 1999; Maytal-Kivity et al., 2002; Mundt et al., 1999; Seeger et al., 1998; Wei et al., 1998). Our long-term objective is to determine mechanistically how the CSN controls plant development. We previously that this complex contains eight subunits (Karniol et al., 1998; Serino et al., 1999) and that the 27 ilia subunit is encoded by the FUS5/CSN7 locus (Karniol et al., 1999). The CSN7 subunit also has a role extraneous to the COP9 signalosome, and differential kinase activity has been implicated in regulating CSN7 and the COP9 signalosome (Karniol et al., 1999). In the present research, we further analyzed CSN7, both in terms of interacting proteins and in terms of kinases that act on CSN7. Furthermore we completed our analysis of the CSN in Arabidopsis by analyzing the remaining subunits. Outline of Original Objectives and Subsequent Modifications The general goal of the proposed research was to study the CSN7 (FUS5) subunit of the COP9 signalosome. To this end we specifically intended to: 1. Identify the residues of CSN7 that are phosphorylated. 2. Monitor the phosphorylation of CSN7 under different environmental conditions and under different genetic backgrounds. 3. Generate transgenic plants with altered CSN7 phosphorylation sites. 4. Purify CSN7 kinase from cauliflower. 5. Clone the Arabidopsis cDNA encoding CSN7 kinase 6. Isolate and characterize additional CSN7 interacting proteins. 7. Characterize the interaction of CSN7 and the COP9 signalosome with the HY5-COP1 transcriptional complex. Throughout the course of the research, emphasis shifted from studying CSN7 phosphorylation (Goals 1-3), to studying the CSN7 kinase (Goal 4 and 5), an in depth analysis of CSN7 interactions (Goal 6), and the study of additional CSN subunits. Goal 7 was also abandoned as no data was found to support this interaction.