60 W.-H. Shen
2
The G1/S Cell Cycle Machinery
The universal drivers of cell-cycle transitions are cyclin-dependent kinases
(CDKs), which are conserved from yeast to humans. The kinase activity
of CDKs is accomplished by their association with cyclins and the cyclin-
CDK complex is negatively regulated by binding with proteins named CKIs
(CDK Kinase Inhibitors) in animals and ICKs (Inhibitor of CDK Kinases) or
KRPs (KIP-Related Proteins) in plants. Specific classes of CDKs, cyclins and
ICK/KRPs appear to be involved in the regulation of the G1/S transition in
plants (Fig. 1).
Fig. 1Schematic view of G1/S transition in the plant cell cycle. Upon stimulation by en-
vironmental and developmental signals (mitogenic signals), D-type cyclins (CYCDs) and
then A-type cyclins (CYCAs) are produced and associate with the A-type CDK (CDKA).
The CYCD-CDKA and CYCA-CDKA are negatively regulated by the binding of a CDK
inhibitor (KRP) whose production is induced by stress. Degradation of KRP by ubiquitin-
dependent proteolysis allows re-activation of the CYCD-CDKA and CYCACDKA kinases.
The CYCD-CDKA and CYCA-CDKA kinases activate the downstream Rb- E2F pathway by
phosphorylation on Rb, which dissociates Rb from the Rb-E2F-DP complex and then the
released E2F-DP activates transcription of E2F-target genes
2.1
CDKs
On the basis of sequence homology, plant CDKs have been classified into six
types, namely, CDKA to CDKF (Vandepoele et al. 2002). CDKA is the pro-