Auxin Responses in Mutants of the Arabidopsis CONSTITUTIVE PHOTOMORPHOGENIC9 Signalosome      

Abstract

The CONSTITUTIVE PHOTOMORPHOGENIC9 (COP9) signalosome (CSN) is an evolutionarily conserved multiprotein complex that interacts with cullin-RING type E3 ubiquitin ligases (CRLs). CSN subunit 5 (CSN5), which, when incorporated into CSN, can deconjugate the NEDD8 modification from the cullin subunit of CRLs, is essential for CSN's role in controlling CRL activity. Whether the CSN5 monomer, which is maintained in csn mutants such as csn3 or csn4, has a functional role, remains to be established. We performed a comparative gene expression-profiling experiment with Arabidopsis (Arabidopsis thaliana) csn3, csn4, and csn5 mutants, and we show here that these mutants cannot be distinguished at the transcriptional level. Furthermore, we show that csn3 csn5 mutants are morphologically indistinguishable from csn3 or csn5 mutants. Taken together, these data suggest that the CSN5 monomer does not have a function that leads to transcriptional or morphological changes in the csn mutants. We further examined auxin responses in csn mutants. Whereas CSN had previously been shown to be required for the auxin response-regulatory E3 complexes, specifically SCF TIR1 , the csn mutant phenotype suggests that CSN is not essential for auxin responses. We present physiological and genetic data that indicate that auxin responses are indeed only partially impaired in csn mutants and that this is not the result of maternally contributed CSN. Finally, we discuss these findings in the context of the current understanding of the role of neddylation and CSN-mediated deneddylation for CRL activity.

Fungal development and the COP9 signalosome

“…The proportion of psi factors derived from linoleic acid control the ratio of asexual and sexual development [42]. Plant csn mutants are impaired in gibberellic acid (GA) and SCF (SLY-1)-dependent germination and elongation [43][44][45]. Fungi and plants might share that CSN plays a key role in light response by controlling appropriate hormone production during development.…”

The COP9 signalosome is involved in the regulation of lipid metabolism and of transition metals uptake in Saccharomyces cerevisiae

“…Among them, we found the homocitrate synthase Lys20, which catalyzes the acetyl CoA-a-chetoglutarate condensation required for lysine biosynthesis. The A. thaliana orthologue gene, IMS2, is also strongly downregulated in a csn5 null mutant, suggesting that this can be a crucial step in the CSN control on lysine biosynthesis ( [44] and Table S4). With regard to genes involved in oxidation-reduction processes there is a striking parallelism in the regulation of the thiol peroxidase and hydroperoxide receptor Hyr1, which is one of the most overexpressed proteins in the ScΔcsn5 strain (Table 3), and its A. thaliana orthologue At4g11600 is strongly induced in a csn5 null mutant ( [44] and Table S4).…”

“…The A. thaliana orthologue gene, IMS2, is also strongly downregulated in a csn5 null mutant, suggesting that this can be a crucial step in the CSN control on lysine biosynthesis ( [44] and Table S4). With regard to genes involved in oxidation-reduction processes there is a striking parallelism in the regulation of the thiol peroxidase and hydroperoxide receptor Hyr1, which is one of the most overexpressed proteins in the ScΔcsn5 strain (Table 3), and its A. thaliana orthologue At4g11600 is strongly induced in a csn5 null mutant ( [44] and Table S4). Since this protein is an important sensor of lipid oxidation and is involved in the cellular response to oxidative stress, we think that this is a noteworthy feature of the CSN regulatory action.…”

Novel aspects of COP9 signalosome functions revealed through analysis of hypomorphiccsnmutants

“…20 In plants, however, no indications for CSN-independent roles of CSN5 have been provided so far and no differences between csn3, csn4 and csn5 mutants could be detected by transcriptional profiling. 21 The csn2-5 mutant and the csn5a-1 mutant (which carries a mutation in one of two Arabidopsis CSN5 loci) both accumulate reduced amounts of CSN, but while the CSN5 monomer is unaffected by csn2-5, it is barely detectable in csn5a mutant lines. 1,5,17 We compared these two mutants in different biochemical and physiological assays.…”