Interactions of the COP9 Signalosome with the E3

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 SCFTIR1. Furthermore, we found that the COP9 signalosome and SCFTIR1 interacted in vivo and that the COP9 signalosome was required for efÞcient degradation of PSIAA6, a candidate substrate of SCFTIR1. Thus, the COP9 signalosome may play an important role in mediating E3 ubiquitin ligaseÐmediated responses. The COP9 signalosome is a multiprotein complex that was discovered during the char-acterization of the photomorphogenic cop/ det/fus mutants from Arabidopsis (1, 2). The COP9 signalosome is required for the protea-some-mediated degradation of HY5, a posi