Plant biology

Light, Phosphorylation, and Degradation

Science's STKE  26 Sep 2000:
Vol. 2000, Issue 51, pp. tw7
DOI: 10.1126/stke.2000.51.tw7

Plant seedlings respond quickly to light. In Arabidopsis, the activity of the transcription factor HY5 is increased in response to light. The level of HY5 is controlled by the photomorphogenic repressor COP1, which accumulates in the nucleus in the dark and promotes HY5 degradation by the proteasome. Hardtke et al. show that HY5 activity is also controlled by light-activated phosphorylation, most likely catalyzed by casein kinase II (CKII). Phosphorylated HY5 was detected in vivo, and the phosphorylated form was absent in HY5 null plants expressing HY5 with the CKII consensus serine mutated (HY5S36A). In vitro assays with bacterially expressed HY5 showed that unphosphorylated HY5 has a higher affinity for COP1. In plant extracts from plants grown in the dark, the unphosphorylated form is not detectable and accumulates under assay conditions in which the proteasome is pharmacologically inhibited. Finally, in vitro promoter binding assays suggested that the unphosphorylated form of HY5 interacted more strongly with target promoters, and HY5S36A plants exhibit phenotypes characteristic of hyperactive HY5 activity, confirming that the unphosphorylated form is more active. Their results suggest a mechanism by which a rapidly activatable pool of HY5 can be maintained even in the dark, allowing rapid response to light.

Hardtke, C.S., Gohda, K., Osterlund, M.T., Oyama, T., Okada, K., and Deng, X.W. (2000) HY5 stability and activity in Arabidopsis is regulated by phosphorylation in its COP1 binding domain. EMBO J. 19: 4997-5006. [Abstract] [Full Text]