Characterization of the Last Subunit of the Arabidopsis COP9 Signalosome

The COP9 signalosome (CSN) is a multiple-subunit protein complex involved in protein degradation. The CSN was identified from constitutive photomorphogenic/deetiolated/fusca (cop/det/fus) mutants of Arabidopsis that mimic light-induced seedling development when grown in the dark (Wei et al., 1994). Subsequently, the CSN was found to be highly evolutionarily conserved (Seeger et al., 1998; Wei and Deng, 1998) and to play multiple roles in plant and animal development via its role in regulating specific proteasome-mediated degradation events. The CSN exhibits homol-ogy, both in the composition of the complex and subunit sequence, with the lid of the 26S proteasome, which performs degradation of ubiquitylated proteins and is responsible for the bulk of nonlysosomal protein degradation in eukaryotes. The lid of the proteasome functions in the recognition and subsequent deubiquitylation of ubiquitylated substrates, which then are passed on to the catalytic core of the proteasome, where they are degraded (reviewed by Voges et al., 1999). The precise function of the CSN and the relationship between the CSN and the lid of the proteasome are unclear, but recent work suggests that it is involved in the regulation of E3 ligases that promote the ubiquitylation of specific substrate proteins (Schwechheimer et al., 2001). Much of the seminal work on the CSN has come from Xing Wang Deng (Yale University, New Haven, CT) and colleagues. In this issue of The Plant Cell, Deng and colleagues (pages 719–731) report the cloning of AtCSN2, the last of the eight Arabidopsis CSN subunits to be characterized.

Serino et al. show that AtCSN2 corresponds to Arabidopsis FUS12 and is an integral component of the CSN. AtCSN2 is able to interact with the cullins AtCUL1 and AtCUL3, which are subunits of SCF^TIR1 E3 ubiquitin ligase and a non-SCF E3 ubiq-uitin ligase, respectively. The cullin components of E3 ligases are modified by binding of the ubiquitin-like protein RUB1/NEDD8 (a process that is called “neddylation”), which enhances E3 ligase activity. CSN activity has been found to be necessary for the cleavage (“deneddylation”) of RUB1/NEDD8 from CUL1 in plants and animals (Figure 1). These observations are consistent with a role for the CSN in regulating the activity of cullin-based E3 ligases and suggest that the CSN and the lid of the proteasome have the distinct but analogous functions of deneddylation (which regulates cullin-based E3 ligase activity) and deubiquitylation (which regulates the entry of ubiquitylated substrates into the proteasome). This hypothesis is supported by the report that similar metalloprotease motifs called JAMMs in the Rpn11 subunit of the proteasome lid and the CSN5 subunit are involved in deubiquitylation and deneddylation, respectively (Berndt et al., 2002). The completion of the cloning and characterization of all eight Arabidopsis CSN subunits should facilitate further investigations aimed at determining the precise function and activity of the CSN in relation to the 26S proteasome.

Figure 1.

Model of CSN Function.

The CSN is thought to interact with CUL1 of SCF^TIR1 E3 ubiquitin ligase (and with cullin subunits of other E3 ligases) and to regulate E3 ligase activity via deneddylation, or removal of the ubiquitin-like molecule RUB1/NEDD8 (orange circles). SCF^TIR1 E3 is made up of four subunits: RBX1, CUL1, ASK1, and the F-box protein TIR1, which is responsible for E3 ligase substrate recognition and the transfer of ubiquitin (gray circles) from the E2 ubiquitin-conjugating enzyme to the substrate. E2 is conjugated to ubiquitin that is activated by the E1 ubiquitin-activating enzyme. Also shown in this model is the interaction of AUX/IAA proteins, which are transcriptional regulators of auxin-responsive genes, with SCF^TIR1. Auxin induces the transcription of AUX/IAA proteins and also enhances their degradation via SCF^TIR1-mediated interactions, thus modulating the expression of downstream auxin-responsive genes in a complex interplay of protein synthesis and degradation (reviewed by Kepinsky and Leyser, 2002). AXR1 and ECR1 are subunits of an E1-like RUB1-activating enzyme that activates RUB1/NEDD8, which subsequently binds to the E2-like protein RCE1, and promotes the neddylation of CUL1. 2, 5, and 6 designate respective CSN subunits. (Figure courtesy of G. Serino and X.W. Deng.)