The Arabidopsis Cdc2a-interacting protein ICK2 is structurally related to ICK1 and is a potent inhibitor of cyclin-dependent kinase activity in vitro

Cyclin-dependent kinases (CDKs) are important regulators of the eukaryotic cell division cycle. To study protein-protein interactions involving plant CDKs, the Arabidopsis thaliana Cdc2aAt was used as bait in the yeast two-hy brid system. Here we report on the isolation of ICK2, and show that it inte racts with Cdc2aAt, but not with a second CDK from Arabidopsis, Cdc2bAt. IC K2 contains a carboxy-terminal domain related to that of ICK1, a previously described CDK inhibitor from Arabidopsis, and to the CDK-binding domain of the mammalian inhibitor p27(Kip1.) Outside of this domain, ICK2 is distinc t from ICK1, p27(Kip1), and other proteins. At nanogram levels (8 nM), puri fied recombinant ICK2 inhibits p13(Suc1)-associated histone H1 kinase activ ity from Arabidopsis tissue extracts, demonstrating that it is a potent inh ibitor of plant CDK activity in vitro. ICK2 mRNA was present in all tissues analysed by Northern hybridization, and its distribution was distinct from that of ICK1. These results demonstrate that plants possess a family of di fferentially regulated CDK inhibitors that contain a conserved carboxy term inal but with distinct amino terminal regions.