Conservation of the COP9/signalosome in budding yeast

We show that disruption of four budding yeast genes, PCI8 and three previously uncharacterized ORFs, which encode proteins interacting with Rrr1p/Csn5p, each results in the accumulation of the cullin Cdc53p exclusively in the Rub1p-modified state. This phenotype, which resembles that of fission yeast csn mutants, is due to a biochemical defect in deneddylation that is complemented by wild-type cell lysate and by purified human CSN in vitro. Although three of the four genes encode proteins with PCI domains conserved in metazoan CSN proteins, their disruption does not confer the DNA damage sensitivity described in some fission yeast csn mutants.Our studies present unexpected evidence for the conservation of a functional homologue of the metazoan CSN, which mediates control of cullin neddylation in budding yeast.The COP9/signalosome (CSN) was first identified in Arabidopsis thaliana as an eight subunit complex involved in the suppression of light-dependent development [1]. Subsequent studies have led to the identification of similar complexes in other plant species, Drosophila melanogaster, human cells, and fission yeast [2-7], thus indicating a high degree of structural conservation during evolution. Cloning of CSN subunits revealed their structural similarities to the eight subunits of the lid complex of the 26S proteasome [3,8-10]. The similarity was most pronounced within the so-called MPN domains of CSN5 and 6 and the PCI domains of the remaining subunits [11].CSN has been implicated in multiple biological processes, many involving ubiquitin-mediated proteolysis (reviewed in [12,13]). For example, CSN is required for degradation of the plant transcription factor HY5 by the putative COP1 ubiquitin ligase [14]. In addition, CSN is involved in auxin-induced turn-over of the transcriptional repressor AUX/IAA [15]. This process is mediated by an ubiquitin ligase [15] related to SCF complexes first identified in budding yeast [16,17]. All SCF complexes share the core su