Molecular and functional analysis of the COP9 signalosome in Arabidopsis

The COP9 signalosome (CSN) is an evolutionary conserved multi-protein complex, which has been first identified in Arabidopsis thaliana as a repressor of light-induced growth and development. Null mutations in CSN subunits result in seedlings displaying a constitutive light-grown phenotype and lethality after seedling stage and can be categorized as members of the cop/det/fus class of mutants. My research project aimed at characterizing, both molecularly and functionally, the plant CSN. I first focused on establishing a novel purification procedure to isolate CSN from cauliflower to homogeneity. The purified CSN contained eight distinct subunits, four of which had not been isolated before. I next focused on the characterization of two novel CSN subunits and was able to prove their identity with two previously defined COP/ DET/FUS loci. The second part of my research aimed at investigating the cellular function of CSN. Comparison of the plant CSN subunits with their homologues from mammals suggested a possible role of CSN within the ubiquitin/proteasome pathway, one of the major cellular pathways for degradation of specific cellular proteins. I confirmed this hypothesis by showing that CSN, as part of its function, interacts with SCF-type ubiquitin ligases and regulate their activity, thus ensuring that proper protein degradation will be carried out in the cell and therefore achieving its central role in regulating diverse developmental processes.