Handbook of Plant and Crop Physiology

clins [42]. Cyclins are involved in activation of Cdks and are probably involved in substrate specificity of

Cdks [43–45].

Fourteen different types of cyclins have been identified in vertebrates (Table 2). The defining feature

of all cyclins is a conserved 100-amino-acid domain called the cyclin box, which contains the Cdk-bind-

ing site [46]. Cyclins have been generally divided into two groups—mitotic cyclins, which are involved

in the G 2 /M transition, and G 1 cyclins, involved at Start and the G 1 /S transition. Mitotic cyclins have the

motif RXXL(X)2–4N, called a destruction box, that has been implicated in their destruction through the

ubiquitin pathway [47]. G 1 cyclins do not have this motif but alternatively have PEST (proline, glutamic

acid, serine, and threonine-rich) motifs that are associated with protein instability and are thought to al-

low protein levels to closely parallel messenger RNA (mRNA) abundance [48]. The mitotic and G 1 cy-

clins also differ in their overall structure. Mitotic cyclins have approximately 200 amino acids (including

the destruction box) N-terminal to the cyclin box, whereas G 1 cyclins mostly extend C-terminal (includ-

ing PEST sequences) [49]. Cyclins A and B are considered mitotic cyclins. Cyclin A is involved in S

CELL CYCLE REGULATION IN PLANTS 231

TABLE 1 Cyclin-Dependent Kinases and Their Regulatory Subunits Identified in Vertebrates

Amino acid
identity to
cdc2 kinase Major phosphorylation sites
Name PSTAIRE motif domain (%) Regulatory subunit TYa Tb

cdc (Cdk1) PSTAIRE 100 Cyclin A, B types GEGTYGV RVYTHEV
Cdk2 PSTAIRE 65 Cyclin A, D types GEGTYGV RTYTHEV
Cdk3 PSTAIRE 66 Unknown GEGTYGV RTYTHEV
Cdk4 PV/ISTVRE 44 Cyclin D types GVGAYGT MALTPVV
Cdk5 PSSALRE 57 p35,cCyclin D types GEGTYGT RCYSAEV
Cdk6 PLSTIRE 47 Cyclin D types GEGAYGK MALTSVV
Cdk7 NRTALRE 40 Cyclin H, p36d GEGQFAT RAYTHQV
Cdk8 SACRE 36 e Cyclin C GRGTYGH Missing
Cdk9 PITALRE 42 Cyclin K, T types GQGTFGEV NRYTNRV
aT-14, Y-15 in p34cdc2.
bT-161 in p34cdc2.
cp35 is a brain-specific activator of Cdk5 and is not similar to cyclins.
dp36 is not structurally similar to cyclins.
eOverall sequence identity.

TABLE 2 Cyclins in Vertebrates

Name Cdk partner Functiona Expressionb

A Cdk1/Cdk2 S and G 2 /M Peak at G 2 /M
B1 Cdk1 G 2 /M Peak at G 2 /M
B2 Cdk1 G 2 /M Peak at G 2 /M
B3 Cdk1/Cdk2 G 2 /M (and S?) Peak at G 2 /M
C Cdk8 G 1 Peak early G 1
D1 Cdk4/6 and Cdk2/5c G 1 Predominantly G 1
D2 Cdk4 G 1 Constant
D3 Cdk4/6 G 1 Peak at G 1 /S
E Cdk2 G 1 /S Peak at G 1 /S
F?d? Peaks at G 2
G?? Responds to growth stimuli
H Cdk7 Multiple phases Constant
I?? Constant
K Cdk9 Transcription
aPhase of cell cycle functions in.
bRefers to protein levels.
cCdk4/6 appear to be the primary partners of cyclin D1.
d?, not known.