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Plant Cell Advance Online Publication
Published on July 25, 2002; 10.1105/tpc.002550

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Received February 21, 2002
Accepted April 9, 2002

Phosphorylation of Retinoblastoma-Related Protein by the Cyclin D/Cyclin-Dependent Kinase Complex Is Activated at the G1/S-Phase Transition in Tobacco

Hirofumi Nakagami 1, Kazue Kawamura 1, Keiko Sugisaka 1, Masami Sekine 1*, and Atsuhiko Shinmyo 1

1 Graduate School of Biological Sciences, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101, Japan

* To whom correspondence should be addressed. E-mail: sekine{at}bs.aist-nara.ac.jp.

In mammals, D-type cyclin-associated kinases mainly regulate the G1/S transition by phosphorylating the retinoblastoma (Rb) protein. We previously demonstrated that in tobacco, cyclin D (Nicta; CycD3;3) is complexed with the PSTAIRE-containing cyclin-dependent kinase (CDKA) from tobacco. Here, we show that Nicta; CycD3;3--associated kinases phosphorylate both the tobacco Rb-related protein (NtRb1) and histone H1. Although NtRb1 kinase activity was detected only during the middle G1- to early S-phase, histone H1 kinase activity was observed as two peaks in G1- to S-phase and G2/M- to M-phase. Importantly, we show that the proportion of cells in the G1-phase was reduced in transgenic Bright Yellow-2 cells overexpressing Nicta; CycD3;3-GFP . Mutational analyses revealed that phosphorylation of Thr-191 in Nicta; CycD3;3 possibly is required for both full kinase activity and localization predominantly to the nucleus. These data suggest that Nicta; CycD3;3 acts as a rate-limiting regulator in the G1/S transition by forming active complexes with CDKA or its related kinases to phosphorylate Rb-related protein and potentially plays a novel role during G2/M and mitosis.






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