SCF SKP2B - AND KPC1-DEPENDENT DEGRADATION OF CYCLIN-DEPENDENT KINASE INHIBITOR KRP1 AND CELL CYCLE REGULATION IN ARABIDOPSIS THALIANA

Abstract

In animals and fungi, cyclin-dependent kinase inhibitors play a key role in cell cycle regulation by inhibiting the activities of cyclin-dependent kinase/cyclin complexes. However, little is known about the role of this group of proteins in plant cell cycle regulation. To gain insight into the mechanisms by which the plant cell cycle is regulated, I studied the cyclin-dependent kinase inhibitor KRP1 in Arabidopsis. The role of KRP1 in pericycle activation during lateral root initiation and in cell cycle regulation and how ubiquitin-mediated protein degradation regulates KRP1 protein turnover were investigated. My results show that KRP1 plays an important role in the regulation of pericycle activation during lateral root initiation. KRP1 interacts with the CDKA;1/CYCD2;1 complex in planta and functions in the G1-S transition of the cell cycle. KRP1 is an unstable protein in planta and its degradation depends on the 26S proteasome. Further, an SCF complex composed of CUL1 and SKP2b regulates KRP1 degradation. These results suggest that SCF SKP2B targets KRP1 for degradation by the 26S proteasome to regulate the G1-S transition of the cell cycle. In addition to SCF SKP2B-mediated KRP1 degradation, KRP1 degradation is also regulated by the RING finger ubiquitin ligase KPC1. To further understand the mechanisms of KRP1 degradation and identify novel proteins that regulate KRP1 degradation, I performed a genetic screen for mutations that stabilize KRP1 protein. Three mutants called msk (mutant stabilizes KRP1) caused by a recessive mutation were identified. These three msk mutants define three distinct genetic loci. The results of these studies reveal a novel function of KRP1 in the regulation of pericycle activation during lateral root initiation and provide new insight into the mechanisms by which plant cell cycle is regulated by ubiquitin-mediated protein degradation.