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The COP9 signalosome (CSN) is a highly conserved eukaryotic protein complex which regulates the cullin RING family of ubiquitin ligases and carries out a deneddylase activity that resides in subunit 5 (CSN5). Whereas CSN activity is essential for the development of higher eukaryotes, several unicellular fungi including the budding yeast Saccharomyces cerevisiae can survive without a functional CSN. Nevertheless, the budding yeast CSN is biochemically active and deletion mutants of each of its subunits exhibit deficiency in cullins deneddylation, although the biological context of this activity is still unknown in this organism. To further characterize CSN function in budding yeast, we present here a transcriptomic and proteomic analysis of a S. cerevisiae strain deleted in the CSN5/RRI1 gene (hereafter referred to as CSN5), coding for the only canonical subunit of the complex. We show that Csn5 is involved in modulation of the genes controlling amino acid and lipid metabolism and especially ergosterol biosynthesis. These alterations in gene expression correlate with the lower ergosterol levels and increased intracellular zinc content which we observed in csn5 null mutant cells. We show that some of these regulatory effects of Csn5, in particular the control of isoprenoid biosynthesis, are conserved through evolution, since similar transcriptomic and/or proteomic effects of csn5 mutation were previously observed in other eukaryotic organisms such as Aspergillus nidulans, Arabidopsis thaliana and Drosophila melanogaster. Our results suggest that the diverged budding yeast CSN is more conserved than was previously thought., (© 2013 FEBS.)

The COP9 signalosome (CSN) is a eukaryotic protein complex, which regulates a wide range of biological processes mainly through modulating the cullin ubiquitin E3 ligases in the ubiquitin-proteasome pathway. The CSN possesses a highly conserved deneddylase activity that centers at the JAMM motif of the Csn5 subunit but requires other subunits in a complex assembly. The classic CSN is composed of 8 subunits (Csn1-8), yet in several Ascomycota, the complex is smaller and lacks orthologs for a few CSN subunits, but nevertheless contains a conserved Csn5. This feature makes yeast a powerful model to determine the minimal assemblage required for deneddylation activity. Here we report, that Csi1, a diverged S. cerevisiae CSN subunit, displays significant homology with the carboxyl terminal domain of the canonical Csn6, but lacks the amino terminal MPN(-) domain. Through the comparative and experimental analyses of the budding yeast and the mammalian CSNs, we demonstrate that the MPN(-) domain of the canonical mouse Csn6 is not part of the CSN deneddylase core. We also show that the carboxyl domain of Csn6 has an indispensable role in maintaining the integrity of the CSN complex. The CSN complex assembled with the carboxyl fragment of Csn6, despite its lack of an MPN(-) domain, is fully active in deneddylation of cullins. We propose that the budding yeast Csi1 is a functional equivalent of the canonical Csn6, and thus the composition of the CSN across phyla is more conserved than hitherto appreciated.