Handbook of Plant and Crop Physiology

CELL CYCLE REGULATION IN PLANTS 233

Figure 1 Activation and inactivation of vertebrate MPF (cyclinB/Cdk1). Cyclin B and Cdk1 association

leads to phosphorylation of Thr 161 by CAK (Cdk activating kinase) and Tyr 15 and Thr 14 and WEE1/MYT1

kinases, respectively. Phosphorylation of Tyr 15 and Thr 14 is inactivating. Dephosphorylation of Tyr15 and

Thr14 by the dual-action phosphatase CDC25 activates the complex. Cyclin is degraded through the ubiquitin-

mediated pathway. Cdk1 is inactivated and presumably dephosphorylated.

TABLE 3 Some Known Substrates of Cdk/Cyclin Complexes

G 2 /M and M phase G 1 /S and S phase

Substrate Protein type Substrate Protein type

Histone H1 Chromatin-associated protein p53 Transcription factor
HMG I, Y, P1 Chromatin-associated protein E2F Transcription factor
NO38, nucleolin Chromatin-associated protein PH04 Transcription factor
Nuclear lamins Cytoskeletal protein p110Rb Regulator of transcription
factors
Caldesmon Cytoskeletal protein p107 Regulator of transcription
factors
Vimentin Cytoskeletal protein RNA polymerase II Transcription enzyme
Neurofilament H Cytoskeletal protein Simian virus 40 T Protein implicated in replication
antigen
Myosin regulatory Cytoskeletal protein DNA polymerases Protein implicated in replication
light chain Ribonuclotide reductase Protein implicated in replication
SW1, SW15 Transcription factor Replication protein A Protein implicated in replication
c-myb Transcription factor Subunits of yeast origin Protein implicated in replication
EF-1 beta Translation factor recognition complex
EF-1 gamma Translation factor
CENP-E Motor protein
MAP4/MAP1B Microtubule-associated
protein
p60c-src Protein kinase
CKII (alpha/beta) Protein kinase
p150ab1 Protein kinase
Cyclin B Cyclin
Cdc 25 Protein phosphatase
PtP1b Protein phosphatase
Rab1Ap/Rab4Ap GTP-binding proteins
Rap1GAP G protein