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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Analysis of the Subcellular Localization, Function, and Proteolytic Control of the Arabidopsis Cyclin-Dependent Kinase Inhibitor ICK1/KRP1^1,[W]

University group at the Max-Planck-Institute for Plant Breeding, Max-Delbrück-Laboratorium, Department of Botany III, University of Cologne, 50829 Cologne, Germany (M.J.J., C.W., S.P., S.J.H.K., N.D., A.S.); and Department of Genomic Research, University of Bielefeld, V6-122, 33594 Bielefeld, Germany (T.M.)

Recent studies have shown that cyclin-dependent kinase (CDK) inhibitors can have a tremendous impact on cell cycle progression in plants. In animals, CDK inhibitors are tightly regulated, especially by posttranslational mechanisms of which control of nuclear access and regulation of protein turnover are particularly important. Here we address the posttranslational regulation of INHIBITOR/INTERACTOR OF CDK 1 (ICK1)/KIP RELATED PROTEIN 1 (KRP1), an Arabidopsis (Arabidopsis thaliana) CDK inhibitor. We show that ICK1/KRP1 exerts its function in the nucleus and its presence in the nucleus is controlled by multiple nuclear localization signals as well as by nuclear export. In addition, we show that ICK1/KRP1 localizes to different subnuclear domains, i.e. in the nucleoplasm and to the chromocenters, hinting at specific actions within the nuclear compartment. Localization to the chromocenters is mediated by an N-terminal domain, in addition we find that this domain may be involved in cyclin binding. Further we demonstrate that ICK1/KRP1 is an unstable protein and degraded by the 26S proteasome in the nucleus. This degradation is mediated by at least two domains indicating the presence of at least two different pathways impinging on ICK1/KRP1 protein stability.

² These authors contributed equally to the paper.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Arp Schnittger (schnitt{at}mpiz-koeln.mpg.de).

^[W] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.081406.

^* Corresponding author; e-mail schnitt{at}mpiz-koeln.mpg.de; fax 49–0–221–5062–113.

Received April 7, 2006; returned for revision May 27, 2006; accepted May 30, 2006.

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