Interactions of the COP9 Signalosome with the E3 Ubiquitin Ligase SCF TIR1 in Mediating Auxin Response

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.