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]