85Plx1 Xenopus Polo-like kinase 1
PP1 Protein phosphatase type 1
PP2A Protein phosphatase type 2A
PPMs Metal-dependent protein phosphatases
PPPs Phosphoprotein phosphatases
PPs Protein phosphatases
PRC1 Protein regulator of cytokinesis 1
Repo-Man Recruits PP1 onto mitotic chromatin at anaphase
RING Really interesting new gene
Rsk2 Ribosomal S6-kinase family member 2
S-phase Synthesis phase
SAC Spindle assembly checkpoint
SCF Skp1, Cullin and F-box protein
SCP Small CTD phosphatases
Sds22 Suppressor of dis2
siRNA Small-interfering ribonucleic acid
T-loop Activation loop
Ubc Ubiquitin-conjugating enzyme
UTR Untranslated region
UV-light Ultraviolet light
Wee1 Wee1-like protein kinase
XErp1 Xenopus Emi1-related protein 1/Emi2
ZGA Zygotic genome activation
3.1 Introduction
Entry into and exit from the different cell cycle stages are driven by changes in the
phosphorylation state and level of cell cycle proteins (Fig. 3.1). In this book chapter,
we primarily focus on the transition from interphase into M-phase and exit from
M-phase. Entry into M-phase is mediated by active Cdk1 associated with cyclin-
B, two key components of the maturation-promoting factor (MPF) (Masui and
Markert 1971 ). Exit from M-phase requires the inactivation of MPF. For a long
time, Cdk1 was in the limelight of cell cycle research and in particular the question
of how the switch-like activation of MPF is accomplished. However, to achieve the
mitotic phosphorylation state, activation of kinases is not sufficient, but also requires
the inactivation of antagonising protein phosphatases (PPs) (Fig. 3.2). Resetting the
system to interphase, on the other hand, depends on the reactivation of PPs in addi-
tion to MPF inactivation. As outlined here, the MPF and PPs modules are controlled
in a reciprocal manner such that the activity of one module suppresses the activity
of the other one. The selective destruction of proteins via the ubiquitin-proteasome
system presents another regulatory element feeding into the control of MPF and PPs
activity. Depending on the developmental stage, the different control mechanisms
have varying impact on the transitions between interphase and M-phase.
3 Regulation of Cell Division