Light can signal photomorphogenic changes in Arabidopsis development that depend on the relative amounts of the two antagonistic proteins HY5 and COP1. HY5 is a transcription factor that activates light-dependent genes, whereas COP1, whose nuclear accumulation is inhibited by light, blocks HY5 activity. HY5 is much more abundant in seedlings grown in light than those grown in the dark; however, when light-grown plants are transferred to dark conditions, HY5 protein levels decrease dramatically. On the other hand, COP1 levels decrease when plants are grown in light. Osterlund et al. investigate the interaction between HY5 and COP1 and identify a role for COP1 in targeting HY5 for proteasomal degradation. Cycloheximide treatment of plants demonstrated that reduction of HY5 levels required preexisting proteins; further experiments showed that HY5 degradation was proteosome-dependent. Transgenic plants expressing HY5 and COP1 mutants revealed that HY5 and COP1 must associate to allow dark-dependent degradation of HY5. Because the COP1 WD40 domain binds HY5 and the COP1 RING finger domain may bind an E2-ubiquitin complex, it is suggested that COP1 may act as an E3 ubiquitin-protein ligase that targets HY5 for proteosomal degradation.
Osterlund, M.T., Hardtke, C.S., Wei, N., and Deng, X.W. (2000) Targeted destabilization of HY5 during light-regulated development of Arabidopsis. Nature 405: 462-466. [Online Journal]