93its counteracting phosphatase? The answer to this question started to emerge when
the precise role of an AGC-type kinase called Greatwall (Gwl) was examined (Fig.
3.5). Gwl was first identified in a complementation screen for cell cycle regulators
in Drosophila, and it was shown that a hyperactive Gwl variant called ‘Scant’
induces mitotic defects (White-Cooper et al. 1996 ). Almost 10 years later, key
insights into the function of Gwl were provided by studies in Xenopus laevis egg
extract. These studies revealed that Gwl contributes to Cdk1 activation by stimulat-
ing the amplification loop (Yu et al. 2006 ). However, further studies suggested that
Gwl does not act directly on activation loop components but rather by inactivating
an okadaic acid-sensitive phosphatase that negatively regulates Cdc25 (Yu et al.
2006 ; Zhao et al. 2008 ). This phosphatase turned out to be PP2A-B55, and the
mechanism of Gwl-mediated PP2A-B55 inactivation—which will be discussed in
detail below—affects not only the phosphorylation state of Cdc25 but applies in
general for PP2A-B55 substrates (Vigneron et al. 2009 ; Castilho et al. 2009 ).
Dysfunction of Gwl and, hence, deregulated PP2A-B55 activity affect the phos-
phorylation state of Cdk1 substrates by two means: first, it alters Cdk1 activity itself
by acting on Cdc25, and second, it acts directly on Cdk1 substrates, which ulti-
mately results in severe mitotic defects (Burgess et al. 2010 ). Cells of Gwl knockout
(KO) mice enter mitosis with normal kinetics, but most of them arrest in prometa-
phase and show severe defects in chromosome condensation and segregation
(Alvarez-Fernandez et al. 2013 ).
GwlGwlPP2AB55Fcp1
Arpp19CDK1
CycBPPPPSubstrate SubstrateArpp19P
Arpp19CPP2AB55 CFig. 3.5 Cdk1/cyclin-B activity results in the inhibition of PP2A-B55. PP2A-B55 is a major
Cdk1/cyclin-B-antagonising phosphatase. Active Cdk1/cyclin-B phosphorylates and thereby acti-
vates Gwl kinase, which in turn phosphorylates the small protein Arpp19 (Ensa). Phosphorylated
Arpp19 is a high-affinity inhibitor of PP2A-B55. The reactivation of PP2A-B55 at mitotic exit
involves dephosphorylation of Gwl by PP2A-B55 and Arpp19 by PP2A-B55 and Fcp1
3 Regulation of Cell Division