Figure 4.   Hypothetical role of the COP9 signalosome in protein degradation. A, Comparison of the Arabidopsis COP9 signalosome and the 19S proteasome lid subcomplex of S. cerevisiae. Signalosome subunits and their corresponding counterparts in the 19S regulatory particle are opposite one another and their primary sequence similarity is indicated. Previous names of loci or genes corresponding to signalosome subunits are indicated if applicable. The revised nomenclature for the signalosome is indicated on the left (CSN1-CSN8). B, Model for the hypothetical role of the signalosome as an alternative lid subcomplex in the 19S regulatory particle of the 26S proteasome. The regular proteasome processes substrates that are selected by the lid subcomplex. The lid specifically recognizes substrates ubiquitinated by a set of E3 ubiquitin ligases such as an unknown E3-A, which targets the factor X for degradation. In an alternative scenario, the lid is replaced by the signalosome. The signalosome recognizes a different subset of proteasome substrates targeted for degradation by a different subset of E3 ligases such as a factor Y ubiquitinated by a ligase E3-B. In this subset of E3 ligases, a ligase involving COP1 is included, which targets HY5 for degradation (see Fig. 3). Note that the specificity of the lid and the signalosome could also be provided by direct association with the specific E3 ligase complexes, rather than substrate recognition (not indicated).

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