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Targeted Degradation of the Cyclin-Dependent Kinase Inhibitor ICK4/KRP6 by RING-Type E3 Ligases Is Essential for Mitotic Cell Cycle Progression during Arabidopsis Gametogenesis^[W],[OA]

^a National Laboratory for Protein Engineering and Plant Genetic Engineering, Peking-Yale Joint Research Center for Plant Molecular Genetics and AgroBiotechnology, College of Life Sciences, Peking University, Beijing 100871, People's Republic of China
^b Institute of Genetics and Developmental Biology, Beijing 100101, People's Republic of China
^c Institute for Chemical Research, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan
^d Graduate School of Biological Sciences, Nara Institute of Science and Technology, Takayama 8916-5 Ikoma, Nara 630-0101, Japan
^e National Plant Gene Research Center (Beijing), Beijing 100101, People's Republic of China

¹ Address correspondence to qulj{at}pku.edu.cn.

Following meiosis, plant gametophytes develop through two or three rounds of mitosis. Although the ontogeny of gametophyte development has been defined in Arabidopsis thaliana, the molecular mechanisms regulating mitotic cell cycle progression are not well understood. Here, we report that RING-H2 group F 1a (RHF1a) and RHF2a, two RING-finger E3 ligases, play an important role in Arabidopsis gametogenesis. The rhf1a rhf2a double mutants are defective in the formation of male and female gametophytes due to interphase arrest of the mitotic cell cycle at the microspore stage of pollen development and at female gametophyte stage 1 of embryo sac development. We demonstrate that RHF1a directly interacts with and targets a cyclin-dependent kinase inhibitor ICK4/KRP6 (for Interactors of Cdc2 Kinase 4/Kip-related protein 6) for proteasome-mediated degradation. Inactivation of the two redundant RHF genes leads to the accumulation of ICK4/KRP6, and reduction of ICK4/KRP6 expression largely rescues the gametophytic defects in rhf1a rhf2a double mutants, indicating that ICK4/KRP6 is a substrate of the RHF E3 ligases. Interestingly, in situ hybridization showed that ICK4/KRP6 was predominantly expressed in sporophytes during meiosis. Our findings indicate that RHF1a/2a-mediated degradation of the meiosis-accumulated ICK4/KRP6 is essential to ensure the progression of subsequent mitoses to form gametophytes in Arabidopsis.

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				J. Mach Ubiquitin Ligation RINGs Twice: Redundant Control of Plant Processes by E3 Ubiquitin Ligases PLANT CELL, June 1, 2008; 20(6): 1424 - 1424. [Full Text] [PDF]