The COP9 signalosome is an evolutionary conserved multiprotein complex of unknown function that acts as a negative regulator of photomorphogenic seedling development in Arabidopsis . Here, we show that plants with reduced COP9 signalosome levels had decreased auxin response similar to loss-of-function mutants of the E3 ubiquitin ligase SCF TIR1 . Furthermore, we found that the COP9 signalosome and SCF TIR1 interacted in vivo and that the COP9 signalosome was required for efficient degradation of PSIAA6, a candidate substrate of SCF TIR1 . Thus, the COP9 signalosome may play an important role in mediating E3 ubiquitin ligase–mediated responses.