antibody-coupled agarose beads (9E10AC, Santa Cruz Biotech- nology). The precipitates of each sample were analyzed by SDS- PAGE (4%–20% acrylamide), which was subsequently silver stained following the manufacturer’s instructions (ProteoSilver Plus, Sigma). Specific bands in the Myc-His-CSN-5 purified products or in the Myc-His-CSN-5H127A purified products were excised and subjected to tryptic digestion and LC-MS/MS. Gel filtration chromatography of Myc-His-CSN-5 or mutant CSN-5s in csn-5 mutant The protocol of gel filtration chromatography was the same as described previously [16,21]. Briefly, purified proteins (400 mg) were loaded onto a SuperdexTM 200 (GE) gel filtration column that was equilibrated with 25 mL (150 mM NaCl, 20 mM Tris Cl pH 7.4). The proteins were eluted in the same buffer at a flow rate of 0.3 mL/min. Fractions of 0.4 mL were collected starting from the onset of the column void volume (8.0 mL) and finishing at 18 mL (25 fractions). 20 mL of each fraction were prepared in 20 mL of 26 SDS loading buffer, separated by 7.5% SDS-PAGE, and then examined by Western blot analysis using c-Myc antibody (9E10, Santa Cruz Biotechnology). Protein analyses Protein extraction, quantification, western blot analysis, protein degradation assays, and immunoprecipitation assays were per- formed as described previously [24,56]. Western blot analyses using a monoclonal c-Myc antibody (9E10, Santa Cruz Biotech- References 1. Wei N, Deng XW (2003) The COP9 signalosome. Annual review of cell and developmental biology 19: 261–286. 2. Wei N, Deng XW (1992) COP9: A new genetic locus involved in light-regulated development and gene expression in Arabidopsis. The Plant Cell Online 4: 1507. 3. 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