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ARS Home » Pacific West Area » Albany, California » Plant Gene Expression Center » Research » Publications at this Location » Publication #218383
Title: CUL1 Regulates TOC1 Protein Stability in the Arabidopsis Circadian Clock

Author
item Harmon, Frank
item IMAIZUMI, TAKATO - UC SAN DIEGO, LA JOLLA CA
item GRAY, WILLIAM - UNIV MINN ST. PAUL MN

Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2008
Publication Date: 8/1/2008
Citation: Harmon, F.G., Imaizumi, T., Gray, W.M. 2008. CUL1 Regulates TOC1 Protein Stability in the Arabidopsis Circadian Clock. Plant Journal. 55(4):568-579.

Interpretive Summary: The circadian clock is the endogenous biological timer that produces 24 hour rhythmic behavior in many aspects of plant physiology. The rhythms produced by the clock are critical to optimize plant responses to daily and seasonal environmental changes. The molecular components of the plant circadian oscillator and their activities are incompletely understood. In the model plant Arabidopsis, maintenance of proper length circadian period length involves targeted degradation of TIMING OF CAB EXPRESSION 1 (TOC1) by the 26S proteasome. The 26S proteasome is responsible for degradation of proteins in a controlled manner; after the target protein has been modified by the addition of the small protein ubiquitin by the action of a ubiquitin ligase. ZEITLUPE (ZTL) is the F-box protein that associates with the SCF (for Skp/Cullin/F-box) E3 ubiquitin ligase that is responsible for marking TOC1 for turnover by the proteasome. CULLIN1 (CUL1) is a core component of SCF complexes and is involved in multiple signaling pathways. Circadian rhythms were examined in the recessive, temperature-sensitive CUL1 allele axr6-3, to assess the contribution of CUL1-based SCF complexes to signaling within the plant oscillator, Circadian rhythms in axr6-3 were comparable to those observed in ztl null mutants; namely, a longer circadian period, altered expression of core oscillator genes, and limited degradation of TOC1. In addition, treatment of seedlings with exogenous auxin did not alter TOC1 stability. These results demonstrate that CUL1 is required for TOC1 degradation and further suggest that this protein is the functional cullin for the SCFZTL complex. which had not been known previously. This work also underscores the importance of post-translational regulatory mechanisms in generating and maintaining circadian rhythms in plants.

Technical Abstract: The circadian clock is the endogenous timer that coordinates physiological processes with daily and seasonal environmental changes. In Arabidopsis thaliana, establishment of the circadian period relies on targeted degradation of TIMING OF CAB EXPRESSION 1 (TOC1) by the 26S proteasome. ZEITLUPE (ZTL) is the F-box protein that associates with the SCF (for Skp/Cullin/F-box) E3 ubiquitin ligase that is responsible for marking TOC1 for turnover. CULLIN1 (CUL1) is a core component of SCF complexes and is involved in multiple signaling pathways. To assess the contribution of CUL1-containing SCF complexes to signaling within the plant oscillator, circadian rhythms were examined in the recessive, temperature-sensitive CUL1 allele axr6-3. The activity of CUL1 in this mutant declines progressively with increasing ambient temperature, resulting in more severe defects in CUL1-dependent activities at elevated temperature. Examination of circadian rhythms in axr6-3 revealed circadian phenotypes comparable to those observed in ztl null mutants; namely, a lengthened circadian period, altered expression of core oscillator genes, and limited degradation of TOC1. In addition, treatment of seedlings with exogenous auxin did not alter TOC1 stability. These results demonstrate that CUL1 is required for TOC1 degradation and further suggest that this protein is the functional cullin for the SCFZTL complex.