Functions, regulation and cellular localization of plant cyclin-dependent kinase inhibitors.

The cell cycle is regulated by the cyclin-dependent kinase (CDK), and CDK inhibitors can bind to CDKs and inhibit their activities. This review examines plant CDK inhibitors, with particular emphasis on their molecular and cellular functions, regulation and cellular localization. In plants, a family of ICK/KRP CDK inhibitors represented by ICK1 is known and another type of CDK inhibitor represented by the SIMESE (SIM) has recently been reported. Considerable understanding has been gained with the ICK/KRP CDK inhibitors. These plant CDK inhibitors share only limited sequence similarity in the C-terminal region with the KIP/CIP family of mammalian CDK inhibitors. The ICK/KRP CDK inhibitors thus provide good tools to understand the basic machinery as well as the unique aspects of the plant cell cycle. The ICK/KRP CDK inhibitors interact with D-type cyclins or A-type CDKs or both. Several functional regions and motifs have been identified in ICK1 for CDK inhibition, nuclear localization and protein instability. Clear evidence shows that ICK/KRP proteins are important for the cell cycle and endoreduplication. Preliminary evidence suggests that they may also be involved in cell differentiation and cell death. Results so far show that plant CDK inhibitors are exclusively localized in the nucleus. The molecular sequences regulating the localization and functional significance will be discussed.