ICK1, A CYCLIN-DEPENDENT PROTEIN-KINASE INHIBITOR FROM ARABIDOPSIS-THALIANA INTERACTS WITH BOTH CDC2A AND CYCD3, AND ITS EXPRESSION IS INDUCED BY ABSCISIC-ACID

Cyclin-dependent kinase (CDK) inhibitor genes encode low molecular wei ght proteins which have important functions in cell cycle regulation, development and perhaps also in tumorigenesis. The first plant CDK inh ibitor gene ICK1 was recently identified from Arabidopsis thaliana. Al though the C-terminal domain of ICK1 contained an important consensus sequence with the mammalian CDK inhibitor p27(Kip1), the remainder of the deduced ICK1 sequence showed little similarity to any known CDK in hibitors. In vitro assays showed that recombinant ICK1 exhibited uniqu e kinase inhibitory properties. In the present study we characterized ICK1 in terms of its gene structure, its interaction with both A. thal iana Cdc2a and CycD3, and its induction by the plant growth regulator, abscisic acid (ABA). ICK1 was expressed at a relatively low level in the tissues surveyed. However, ICK1 was induced by ABA, and along with ICK1 induction there was a decrease in Cdc2-like histone H1 kinase ac tivity. These results suggest a molecular mechanism by which plant cel l division might be inhibited by ABA. ICK1 clones were also identified from independent yeast two-hybrid screens using the CycD3 construct. The implication that ICK1 protein could interact with both Cdc2a and C ycD3 was confirmed by in vitro binding assays. Furthermore, deletion a nalysis indicated that different regions of ICK1 are required for the interactions with Cdc2a and CycD3. These results provide a mechanistic basis for understanding the role of CDK inhibitors in cell cycle regu lation in plant cells.